Measuring device and measuring method

Abstract

The present disclosure relates to a measuring device and a measuring method which make it possible to reduce the size of a part that comes into contact with soil to be measured. Provided is a measuring device comprising a probe that comes into contact with soil to be measured, and a measuring unit that measures an index representing a characteristic of the soil, wherein: the probe includes a substrate having an outer layer surface on which a transmission line is formed; and the measuring unit transmits a high-frequency signal to the transmission line to measure the index. The present disclosure can be applied to measuring devices for measuring soil.

Description

Measuring device and measuring method

[0001] The present disclosure relates to a measurement device and a measurement method, and more particularly to a measurement device and a measurement method that enable miniaturization of the portion that comes into contact with soil to be measured.

[0002] Conventionally, devices have been proposed for measuring soil moisture content, water potential, etc. For example, Patent Document 1 discloses a device that includes a water-permeable housing containing a volume-changing material configured to increase or decrease its volume in response to a change in the water potential of a water-containing medium, and a compressible insert, and that brings about a change in the water potential of the water-containing medium.

[0003] Another known device is one that has a porous ceramic body with a known water retention curve mounted around a dielectric sensor, and outputs a water potential based on the known water retention curve in response to the measurement value of the dielectric sensor inside the porous ceramic body with soil attached.

[0004] Special table 2020-528546 publication

[0005] In this type of device, it is required to miniaturize the part that comes into contact with the soil to be measured.

[0006] The present disclosure has been made in consideration of such circumstances, and aims to make it possible to reduce the size of the part that comes into contact with the soil to be measured.

[0007] A measurement device according to one aspect of the present disclosure includes a probe that contacts the soil to be measured and a measurement unit that measures an index representing the characteristics of the soil, the probe having a substrate on the surface of which a transmission line is formed, and the measurement unit transmitting a high-frequency signal to the transmission line to measure the index.

[0008] A measurement method according to one aspect of the present disclosure is a measurement method in which a measurement device transmits a high-frequency signal to a transmission line formed on the surface of a substrate of a probe that contacts the soil to be measured, and measures an index representing the characteristics of the soil.

[0009] In one aspect of the measurement device and measurement method of the present disclosure, a probe that contacts the soil to be measured and a measurement unit that measures an index representing the characteristics of the soil are provided, and the probe has a substrate on whose surface a transmission line is formed, and the measurement unit transmits a high-frequency signal to the transmission line to measure the index.

[0010] The measurement device according to one aspect of the present disclosure may be an independent device or an internal block constituting a single device.

[0011] It is a block diagram showing a configuration example of a measuring device to which the present disclosure is applied. It is a block diagram showing another configuration example of a measuring device to which the present disclosure is applied. It is a diagram showing a configuration example of a probe in a measuring device to which the present disclosure is applied. It is a diagram showing a configuration example of a probe in a measuring device to which the present disclosure is applied. It is a diagram showing an example of a method for assembling the probe of FIG. 3. It is a diagram for explaining the principle of an existing capacitance method. It is a diagram showing the configuration of a metal part. It is a diagram showing the configuration of an existing capacitance type sensor. It is a diagram showing a detailed configuration example of a measuring device to which the present disclosure is applied. It is a cross-sectional view showing a detailed configuration example of the probe of FIG. 9. It is a plan view showing a configuration example of a substrate in the probe of FIG. 9. It is a cross-sectional view showing a configuration example of a substrate and a porous ceramic in the probe of FIG. 9. It is a diagram showing a first example of a measurement sequence. It is a diagram showing a modification example of the first example of the measurement sequence. It is a diagram showing a second example of a measurement sequence. It is a diagram showing a modification example of the second example of the measurement sequence. It is a cross-sectional view showing a configuration example of the probe of FIG. 9. It is a plan view showing a detailed configuration example of a substrate in the probe of FIG. 9. It is a plan view showing a configuration example of a through hole formed in the substrate of the probe of FIG. 9. It is a cross-sectional view showing a configuration example of a through hole formed in the substrate of the probe of FIG. 9. It is a diagram showing an example of a method for assembling the probe of FIG. 10. It is a cross-sectional view showing another example of the detailed configuration of the probe of FIG. 9. It is a plan view showing another example of the detailed configuration of the probe of FIG. 9. It is a diagram showing an example of a method for assembling the probe of FIG. 22. It is a diagram showing a configuration example of a tip circuit of a transmission line in the substrate of the probe of FIG. 9. It is a diagram showing a VNA calibration plane in the case where the tip circuit is open and short-circuited. It is a diagram showing an example of a time waveform in the case where the tip circuit is open and short-circuited. It is a diagram showing a VNA calibration plane in the case where the tip circuit is terminated. It is a diagram showing an example of a time waveform in the case where the tip circuit is terminated. It is a plan view showing the position of a cross section in the substrate of the probe of FIG. 9. It is a diagram for explaining an existing processing method. It is a diagram for explaining a processing method to which the present disclosure is applied. It is a diagram for explaining a processing method of applying an ultrathin film coating agent. It is a diagram showing a detailed configuration example of a measuring device to which the present disclosure is applied. It is a cross-sectional view showing a detailed configuration example of the probe of FIG. 34. It is a diagram showing a configuration example of a jig for making dumplings. It is a diagram for explaining the usage method of the jig for making dumplings.48 is a diagram showing the pore size distribution of the porous ceramic of an existing MP sensor. 49 is a diagram showing a moisture retention curve of the porous ceramic of an existing MP sensor. 50 is a diagram showing an example of the pore size distribution of the porous ceramic of FIG. 9. 51 is a diagram showing an example of the moisture retention curve of the porous ceramic of FIG. 9. 52 is a diagram showing an example of the moisture retention curve range of the porous ceramic of FIG. 9. 53 is a plan view showing an example of the configuration of the porous ceramic of FIG. 9. 54 is a cross-sectional view showing an example of the configuration of the porous ceramic of FIG. 9. 55 is a cross-sectional view showing another example of the configuration of the porous ceramic of FIG. 9. 56 is a diagram showing an existing configuration for measuring soil moisture and matric potential. 57 is a diagram showing an example of the detailed configuration of a measurement device to which the present disclosure is applied. 58 is a cross-sectional view showing an example of the configuration of a substrate having a substrate A surface and a substrate B surface of FIG. 48. 59 is a plan view showing an example of the configuration of the substrate A surface of FIG. 49. 59 is a plan view showing an example of the configuration of the substrate B surface of FIG. 49. 59 is a diagram showing another example of the detailed configuration of a measurement device to which the present disclosure is applied. 59 is a diagram showing a first example of a measurement sequence. 59 is a diagram showing a second example of a measurement sequence. 59 is a diagram showing a first example of the detailed configuration of a measurement device having a branch switchless configuration. 55. is a diagram showing a time waveform of reflection generated from the reflection coefficient (S11) measured by the measurement apparatus of FIG. 55. is a diagram showing a second example of the detailed configuration of a measurement apparatus with a branch switchless configuration. is a diagram showing a time waveform of reflection generated from the reflection coefficient (S11) measured by the measurement apparatus of FIG. 57. is a diagram showing a time waveform of transmission generated from the transmission coefficient (S21) measured by the measurement apparatus of FIG. 57. is a diagram explaining the relationship between the amplitude of the reflected wave of 2*t1 and the wave of t2. is a diagram showing an example of the shape of a time waveform. is a diagram showing an example of a time waveform when the center position of the waveform at t2 is assumed. is a diagram showing a schematic diagram of the distance that an electromagnetic wave travels back and forth between the front surface reflection position and the rear surface reflection position of a transmission line. is a diagram showing a first example of the detailed configuration of a measurement apparatus capable of measuring the round trip time of a transmission line. is a diagram showing a second example of the detailed configuration of a measurement apparatus capable of measuring the round trip time of a transmission line. is a diagram showing a first example of the detailed configuration of a measurement apparatus equipped with a probe having a VNA calibration standard. is a diagram showing a second example of the detailed configuration of a measurement apparatus equipped with a probe having a VNA calibration standard.FIG. 10 is a diagram showing a third example of the detailed configuration of a measurement device equipped with a probe having a VNA calibration standard. FIG. 11 is a diagram showing a fourth example of the detailed configuration of a measurement device equipped with a probe having a VNA calibration standard. FIG. 12 is a diagram showing a first example of a configuration in which a VNA and a probe are integrated. FIG. 13 is a diagram showing a second example of a configuration in which a VNA and a probe are integrated. FIG. 14 is a diagram showing a third example of a configuration in which a VNA and a probe are integrated. FIG. 15 is a diagram showing a fourth example of a configuration in which a VNA and a probe are integrated. FIG. 16 is a diagram showing a first example of a probe configuration in which measurements are made using an MP sensor and a moisture sensor on one side. FIG. 17 is a diagram showing a second example of a probe configuration in which measurements are made using an MP sensor and a moisture sensor on one side.

[0012] <Device Configuration> Fig. 1 is a block diagram showing an example configuration of a measurement device to which the present disclosure is applied. In Fig. 1, the measurement device 1 is composed of a VNA 11 and a probe 12 electrically connected to the VNA 11 via an RF cable 13.

[0013] The VNA 11 is a vector network analyzer (VNA) that measures high-frequency characteristics. The probe 12 is a device that extracts signals from the object under test. The probe 12 is inserted (buried) into the soil to be measured during measurement, thereby coming into contact with the soil.

[0014] The VNA 11 has a measurement unit 11A configured as an electrical circuit. The measurement unit 11A measures S-parameters related to the soil under measurement by applying a radio frequency signal (RF signal) to a specific port and transmitting it via a transmission line in the probe 12. The measurement unit 11A calculates an index representing the characteristics of the soil under measurement based on the measured S-parameters. In FIG. 1 , the VNA 11 has one port (Port 1), and by burying the probe 12 electrically connected to the Port 1 terminal via an RF cable 13 in the soil, the VNA 11 can measure the reflection coefficient (S11) as an S-parameter.

[0015] The number of ports of the VNA 11 is not limited to one, but may be multiple. For example, in the measurement device 1 of Fig. 2, the VNA 11 is configured with two ports (Port1 and Port2), and probes 12 electrically connected to the Port1 terminal and the Port2 terminal via RF cables 13 and 14 are buried in the soil, whereby the VNA 11 can measure the transmission coefficient (S21) as an S-parameter.

[0016] Indicators representing soil characteristics include the amount of water contained in the soil to be measured (volumetric water content) and the matric potential of the soil to be measured. Matric potential is the potential due to the tension (matric) of the soil. The probe 12 can be used as at least one of a moisture sensor for measuring the amount of water in the soil to be measured and a matric potential sensor (hereinafter also referred to as an MP sensor) for measuring the matric potential of the soil to be measured.

[0017] First Embodiment A planar transmission line method can be adopted in the measurement device 1. The planar transmission line method is a method in which a transmission line is formed on the surface of a substrate of the probe 12, and an RF signal from the VNA 11 is transmitted via an RF cable.

[0018] <Probe Configuration> Figures 3 and 4 are diagrams showing an example configuration of the probe 12 in the measurement device 1 of Figure 1. In Figures 3 and 4, the x-axis, y-axis, and z-axis directions are perpendicular to one another. In Figure 3, the x-axis corresponds to the left-right direction, the y-axis corresponds to the front-back direction, and the z-axis corresponds to the up-down direction, and the configuration of the components of the probe 12 when viewed from the y-direction (front-back direction) is shown. In Figure 4, the x-axis corresponds to the left-right direction, the y-axis corresponds to the up-down direction, and the z-axis corresponds to the front-back direction, and the configuration of the components of the probe 12 when viewed from the z-direction (front-back direction) is shown. The relationships between the x-direction, y-direction, and z-direction are the same in other figures described below.

[0019] 3 and 4, the probe 12 is connected to the Port 1 terminal of the VNA 11 via an RF cable 13 that is electrically connected to an RF connector 21. As shown in Fig. 4, in the probe 12, a transmission line 31 is formed (printed) on the surface of a substrate 22, and one end of a signal line of the transmission line 31 is electrically connected to the RF connector 21. The transmission line 31 formed on the surface of the substrate 22 is formed (wired) so that the signal line has a predetermined shape (e.g., a U-shape) on the surface in a plan view. The other end (tip) of the signal line of the transmission line 31 can be terminated (loaded), open, or shorted.

[0020] In the probe 12, soil 23 having a wide pore size distribution is placed (loaded) on the top of the surface of the substrate 22. The surface of the substrate 22, except for the area where the soil 23 is placed, is filled with resin to form a resin portion 24. When the soil 23 is placed on the surface of the substrate 22, a wall portion 25 is provided in a location corresponding to the periphery of the transmission line 31 to prevent the soil from flowing out, and the soil 23 can be buried inside the wall portion 25. Furthermore, to prevent soil from flowing out, the top of the soil 23 placed on the surface of the substrate 22 may be covered with a fine mesh net (filter). Note that the substrate 22 should not have any through holes or the like to prevent soil from flowing out.

[0021] The soil 23 is an example of a substance having a wide pore size distribution, and an artificial substance such as glass beads may also be used. For example, when glass beads having a wide pore size distribution are placed on the upper surface of the substrate 22, the wall portion 25 may not be provided.

[0022] During measurement, the probe 12 is buried in the soil to be measured, and measurements are performed by the VNA 11. The probe 12 can be used as an MP sensor by placing a material, such as soil 23, with a wide pore size distribution on the top surface of the substrate 22 on which the transmission line 31 is formed. That is, the volumetric water content of the material placed on the top surface of the substrate 22 changes depending on the matric potential of the soil to be measured, and the dielectric constant of the material changes depending on the volumetric water content. The VNA 11 applies an RF signal via the RF cable 13, measures the round-trip propagation time of the transmission line 31 formed on the top surface of the substrate 22, and measures the change in the dielectric constant of the material placed on the top surface of the substrate 22, thereby determining the matric potential of the soil to be measured.

[0023] <Assembly Method> Fig. 5 illustrates, in plan view in the z direction, an example of a method for assembling the probe 12 in Fig. 3. As shown in A of Fig. 5, the process of assembling the probe 12 from components constituted by the housing 20 and the substrate 22 will be described.

[0024] Here, although the previous steps are omitted, the housing 20 is made of, for example, resin and is processed to have a concave shape so that the plate-shaped substrate 22 fits into the recess, and the concave shape forms the wall portion 25. The substrate 22 has a signal line printed on its surface, and one end of the signal line is electrically connected to the RF connector 21. Note that the shape of the housing 20 is merely an example, and other shapes corresponding to the shape of the substrate 22 may be adopted. Furthermore, as shown in FIG. 4 , the shape of the substrate 22 may have a curved shape corresponding to the shape (U-shape) of the transmission line 31 in a plan view, or may be another shape such as a rectangular shape. The housing 20 and the wall portion 25 need to be processed to have a shape corresponding to the shape of the substrate 22.

[0025] First, as shown in Fig. 5B, the substrate 22 is placed in the recessed portion of the housing 20, and the portion of the surface of the substrate 22 excluding the area where the soil 23 is to be placed is filled with resin to form the resin portion 24. As a result, the portion of the surface of the substrate 22 where the soil 23 is to be placed is surrounded by the resin portion 24 and the wall portion 25. Next, as shown in Fig. 5C, the soil 23 is placed in the portion surrounded by the resin portion 24 and the wall portion 25, so that the soil 23 is placed on top of the transmission line 31 formed on the surface of the substrate 22.

[0026] The probe 12 assembled as described above can be used as an MP sensor. While Figures 3 to 5 show a configuration in which a substance such as soil 23 is placed on the surface of the substrate 22 and the probe 12 is used as an MP sensor, the probe 12 may also be used as a moisture sensor by configuring the probe 12 so that no substance such as soil 23 is placed on the surface of the substrate 22. That is, when using the probe 12 as a moisture sensor, the probe 12 is buried in the soil to be measured, so that the soil to be measured comes into contact with the surface of the substrate 22. The VNA 11 applies an RF signal, measures the round-trip propagation time of the transmission line 31 on the surface, and measures the change in the dielectric constant of the soil to be measured, thereby determining the volume content from the change in dielectric constant.

[0027] As described above, the planar transmission line method is adopted in the measuring device 1, and the probe 12 can be used as an MP sensor or a moisture sensor. The probe 12 can be used as an MP sensor by using soil 23 or the like as a substance (substance with a wide pore size distribution) placed on the upper surface of the substrate 22. Therefore, the sensor can be manufactured more inexpensively than when a substance such as porous ceramics is used.

[0028] Existing moisture sensors that use the TDR (Time Domain Reflectometry) or TDT (Time Domain Transmission) methods perform measurements using low-frequency, sweeping electromagnetic waves, requiring a rod length of approximately 10 to 15 cm, making the sensors large. As a result, they cannot be inserted into seedling pods, which are approximately 7 to 8 cm high. Furthermore, with the TDR and TDT methods, there is a risk of a gap (air gap) between the soil and the sensor, which can cause errors. Furthermore, sensors that use the SFDT (Spatial Frequency Domain Transmissometry) method are affected by reflections from the container, which means that the sphere of influence is large and accurate measurements may not be possible if they are placed in a small pot (such as a seedling pod).

[0029] In contrast, by configuring the probe 12 so that soil 23 is not placed on the surface of the substrate 22, it can be used as a compact moisture sensor capable of measuring even small pots such as seedling pods. In other words, by using broadband electromagnetic waves, for example, 1-6 GHz or 1-9 GHz, the measurement device 1 can reduce the rod length to a few centimeters, allowing the probe 12 to be placed in a seedling pod approximately 7-8 cm long. Furthermore, due to its compact size (due to the small probe dimensions), the probe 12 can be surrounded by wet soil, like a mud ball, eliminating any air gaps. Hereinafter, the wet soil surrounding the probe will also be referred to as a mud ball.

[0030] As described above, existing moisture sensors have large probe dimensions, making it difficult to form a mud ball, but with the probe 12 in the measuring device 1, the entire probe can be covered with a mud ball, preventing the formation of gaps (voids) between the soil and the sensor that are unique to the TDR and TDT methods. In other words, the probe 12 in the measuring device 1 can prevent the formation of gaps (voids) between the soil and the sensor that occur when using existing moisture sensors that employ the TDR or TDT methods.

[0031] <<Second Embodiment>> When the planar transmission line method is adopted in the measurement device 1, measurements can be performed using an MP sensor using porous ceramics. Porous ceramics are ceramics that have many pores (fine pores) inside.

[0032] For comparison, a conventional capacitance-based sensor will be described with reference to FIGS. 6 to 8. In the conventional capacitance-based sensor, a metal component is required to uniformize the electric field inside the porous ceramic. Specifically, as shown in FIG. 6, the conventional capacitance-based sensor includes a substrate 901, porous ceramics 902 and 903 formed on the upper and lower layers of the substrate 901, and metal components 904 and 905 sandwiching the substrate 901. As shown in FIG. 7, the metal components 904 and 905 are made of circular metal disks, and the porous ceramics 902 and 903 are covered by the metal disks to form a uniform and stable electric field.

[0033] However, the metal parts 904 and 905 prevent contact between the soil to be measured and the porous ceramics 902 and 903, reducing the contact rate (aperture rate) of the porous ceramics 902 and 903. In existing capacitive sensors, when the aperture rate on the substrate side of the porous ceramic is 0% and the aperture rate on the metal part side is 40%, the effective aperture rate is approximately 25%. This low contact rate (aperture rate) reduces permeability and drainage, resulting in reduced responsiveness. Furthermore, the thickness of each porous ceramic is approximately 3 mm, and the larger the volume of the porous ceramic, the lower the responsiveness.

[0034] Existing capacitance-based sensors require the probe containing the porous ceramics to be integrated (closely located) with the electrical circuit, resulting in a large sensor body. Figure 8 shows the configuration of an existing capacitance-based sensor. As shown in Figure 8, the existing capacitance-based sensor is configured by integrating an electrical circuit 911 and a probe 912 containing the porous ceramics. Furthermore, existing capacitance-based sensors have poor insertion properties because the sensor body must be buried in the soil during measurement. In this case, the sensor body may restrict the flow of water in the soil, or a water path may form along the sensor body, disrupting the moisture distribution in the soil. Thus, existing capacitance-based sensors have the problems of a large sensor body and poor soil insertion properties.

[0035] <Apparatus Configuration> FIG. 9 is a diagram showing a detailed configuration example of the measurement apparatus 1 shown in FIG. 1. As shown in FIG. 9, the measurement apparatus 1 is composed of a VNA 11 and a probe 12. The probe 12 is connected to a port (Port 1 terminal) of the VNA 11 via an RF cable 13 electrically connected to an RF connector 21. In the probe 12, a transmission line 31 is formed (printed) on the surface of a substrate 22, and a porous ceramic 26 is placed (loaded) on top of the surface. One end of the signal line of the transmission line 31 is electrically connected to the RF connector 21, and the other end of the signal line is terminated by a resistor. Note that the other end (tip) of the signal line of the transmission line 31 is not limited to being terminated (loaded), but may be open or short.

[0036] The measuring device 1 shown in Figure 9 employs a planar transmission line system. That is, in the probe 12, the transmission line 31 formed on the surface of the substrate 22 is formed so that the signal line has a predetermined shape (e.g., U-shape) on the surface in a plan view, and the porous ceramic 26 is placed on top of this surface. However, the U-shape does not necessarily have to be a strict "U" shape, and it may be any shape in which the signal line formed from one end in a first direction is folded back at a certain position in a second direction opposite the first direction, and is formed in the second direction to reach the other end.

[0037] During measurement, the probe 12 is buried in the soil to be measured, and the VNA 11 measures the soil in contact with the porous ceramic 26. At this time, the volumetric water content of the porous ceramic 26 changes depending on the matrix potential of the soil to be measured. The relative dielectric constant of the porous ceramic 26 also changes depending on the volumetric water content. The VNA 11 can measure the change in the relative dielectric constant of the porous ceramic 26 by measuring the round-trip propagation time (hereinafter also referred to as the round-trip time) of the transmission line 31 formed on the surface of the substrate 22.

[0038] Detailed configuration examples of the probe 12 in FIG. 9 are shown in FIGS. 10 to 12. In FIGS. 10 to 12, the relationship between the x, y, and z axes is the same as in FIG. 3 and the like. FIG. 10 illustrates the components of the probe 12 in FIG. 9 in a cross-sectional view in the y direction. As shown in FIG. 10, a porous ceramic 26 is placed on the upper surface of the substrate 22. Two screw holes are formed in the substrate 22 and the porous ceramic 26 at corresponding positions. Screws 41-1 and 41-2 are inserted into the two screw holes and fastened with nuts 42-1 and 42-2, thereby fixing the porous ceramic 26 to the upper surface of the substrate 22. A resin portion 24 is formed by filling the surface of the substrate 22 with resin in the area corresponding to the area excluding the area where the porous ceramic 26 is placed. In addition, although Figure 10 shows a structure in which screws 41-1 and 41-2 are inserted from the upper surface side of the porous ceramic 26 and fastened with nuts 42-1 and 42-2 on the lower surface side of the substrate 22, a structure in which screws 41-1 and 41-2 are inserted from the lower surface side of the substrate 22 and fastened with nuts 42-1 and 42-2 on the upper surface side of the porous ceramic 26 may also be used.

[0039] FIG. 11 illustrates the substrate 22 of FIG. 10 in a plan view in the z direction. FIG. 12 illustrates a cross-sectional view of the A-A' cross section of the surface of the substrate 22 of FIG. 11, together with a layer of porous ceramics 26 placed on top of the surface of the substrate 22. In FIG. 12, the signal line of the transmission line 31 formed on the surface of the substrate 22 is represented by SIG, and the electric field due to the high-frequency electromagnetic wave in the signal line (SIG) is represented by a thick arrow. An electric field E generated by transmitting an RF signal from the VNA 11 through the signal line (SIG) of the transmission line 31 is directed toward the ground (GND) formed on the upper and lower layers of the substrate 22. At this time, the electric field E from the signal line (SIG) of the transmission line 31 passes through the porous ceramics 26, and the VNA 11 measures the electric field E passing through the porous ceramics 26.

[0040] <Measurement Method> In the measurement device 1 of Figure 9, the following three steps must be performed as preparations for measurement. First, the relationship between the round-trip propagation time of the transmission line 31 and the volumetric water content of the porous ceramic when the probe 12 is used as an MP sensor is calibrated in advance. Second, the relationship between the volumetric water content of the porous ceramic and the matric potential (pF) is calibrated in advance. Third, based on the measurement results from the above calibration, a table correlating the round-trip propagation time with the matric potential (pF) is written to the memory in the measurement unit 11A. The table may be stored in an external memory and retrieved as needed.

[0041] During measurement, the probe 12 is buried in the soil to be measured, and soil measurement is performed. In the VNA 11, the measurement unit 11A measures the reflection coefficient (S11) over a wide band (e.g., 1 to 9 GHz or 1 to 6 GHz). The measurement unit 11A can calculate the round-trip propagation time on the transmission line 31 by performing an inverse Fourier transform on the measured reflection coefficient (S11). The measurement unit 11A can convert the round-trip propagation time into matric potential (pF) using a table pre-stored in memory. Thus, in the measurement device 1 shown in FIG. 9, when the VNA 11 and the probe 12 are connected via one port (Port 1), the planar transmission line method is employed to measure the reflection coefficient (S11) as an S-parameter, thereby measuring the matric potential of the soil to be measured.

[0042] In the measuring device 1 that performs the above-described measurements, the use of a planar transmission line system eliminates the need for metal components in the probe 12, thereby improving the contact rate between the soil to be measured and the porous ceramics 26. Improving the contact rate between the soil and the porous ceramics 26 improves responsiveness.

[0043] Furthermore, in the measurement device 1, the VNA 11 can be separated from the probe 12, which allows the volume of the probe 12, where the soil is inserted, to be reduced. This allows the probe to be miniaturized, improving soil insertion performance. Furthermore, since the measurement device 1 measures the impedance of the porous ceramics rather than the capacitance of the porous ceramics, the measurement area depends on the height of the electric field of the transmission line 31 (i.e., the thickness of the porous ceramics). Therefore, the narrower the line width of the transmission line 31, the lower the height of the electric field of the transmission line 31, and the smaller the volume (thickness) of the porous ceramics 26 can be.

[0044] <Measurement Sequence> The sequence of the measurement method using the measurement device 1 in Fig. 9 will be described. Fig. 13 is a diagram showing a first example of a measurement sequence using the VNA 11. In Fig. 13, when the number of frequency channels is n and the number of averaging times is m, the measurement order from start to finish is shown in order from left to right in the diagram. That is, when measurements of one channel (one frequency) are repeated m times to measure channels 1 to n, when measurements of channel i have been performed m times, the measurement moves to the next channel i+1, and measurements of each channel are repeated in order.

[0045] In Fig. 13, the frequency of each frequency channel is arbitrary. For example, measurement may be started from the lowest frequency, gradually increased to the next frequency, and ended at the highest frequency. Alternatively, measurement may be started from the highest frequency, gradually decreased to the next frequency, and ended at the lowest frequency. Measurement may be performed while randomly selecting frequencies, and the measurement may end when measurements at all frequencies have been completed.

[0046] 14 is a diagram showing a modification of the first example of the measurement sequence by the VNA 11. In FIG. 14, the measurement sequence from start to finish is shown in order from left to right in the diagram, assuming that the number of frequency channels is n and the number of averaging times for each channel is a, b, c, .... That is, when measuring from ch1 to chn, the measurement of ch1 is repeated a times, the measurement of ch2 is repeated b times, and the measurement of chn is repeated c times.

[0047] In this way, if a sufficient S / N ratio can be ensured in the measurement of each frequency channel, it is not necessary to set the number of averaging operations to the same for each frequency channel, and the number of averaging operations may be variable for each frequency channel. For example, the number of averaging operations a, b, and c in Figure 14 may be less than the number of averaging operations m in Figure 13, such as m > a, m > b, and m > c. In the sequence of Figure 14, the number of averaging operations for each frequency channel can be reduced compared to the sequence of Figure 13, thereby achieving lower power consumption.

[0048] 14, as in FIG. 13, the frequency of each frequency channel is arbitrary, and for example, measurement can be started from the lowest frequency, gradually increased to the next frequency, and finished at the highest frequency.

[0049] 15 is a diagram showing a second example of a measurement sequence using the VNA 11. In FIG. 15, the measurement sequence from start to finish is shown in order from left to right in the diagram, assuming that the number of frequency channels is n and the number of averaging times is m. That is, a measurement set in which measurements are taken once for each of ch1 to chn is repeated m times.

[0050] In Fig. 15, the frequency of each frequency channel is arbitrary. For example, measurement may be started from the lowest frequency, gradually increased to the next frequency, and ended at the highest frequency. Alternatively, measurement may be started from the highest frequency, gradually decreased to the next frequency, and ended at the lowest frequency. Measurement may be performed while randomly selecting frequencies, and the measurement may end when measurements at all frequencies have been completed.

[0051] FIG. 16 is a diagram showing a modified example of the second example of the measurement sequence using the VNA 11. In FIG. 16, the measurement sequence from start to finish is shown in order from left to right in the diagram, assuming that the number of frequency channels is n and the number of averaging times for each channel is a, b, c, .... That is, the first time after measurement starts, all channels from ch1 to chn are measured once as a trial to confirm the S / N ratio, and the optimal number of averaging times for each channel is determined. From the second time onwards, the measurement is repeated the optimal number of averaging times for each channel. Specifically, depending on the optimal number of averaging times for each channel determined in the trial measurement, the measurement of ch1 is repeated a number of times, the measurement of ch2 is repeated b number of times, and the measurement of chn is repeated c number of times.

[0052] In this way, after starting measurement, a single trial measurement may be performed first, the S / N ratio for each frequency channel may be checked to determine the optimal number of averaging operations, and thereafter measurements may be performed according to the optimal number of averaging operations for each frequency channel. For example, the number of averaging operations a, b, and c in Figure 16 may be less than the number of averaging operations m in Figure 15, such as m > a, m > b, and m > c. In the sequence in Figure 16, the number of averaging operations for each frequency channel can be reduced compared to the sequence in Figure 15, thereby achieving lower power consumption.

[0053] 16, as in FIG. 15, the frequency of each frequency channel is arbitrary, and for example, measurement can be started from the lowest frequency, gradually increased to the next frequency, and finished at the highest frequency.

[0054] <Configuration of Substrate> A detailed configuration of the substrate 22 of the probe 12 in the measurement device 1 in Fig. 9 will be described. Fig. 17 illustrates the components of the probe 12 in Fig. 9 in a cross-sectional view in the y direction. Fig. 18 illustrates the surface of the substrate 22 in the probe 12 in Fig. 17 in a plan view when viewed from the direction of arrow A in Fig. 17.

[0055] 18, in the probe 12, the porous ceramic 26 is placed on top of the surface of the substrate 22, on which the signal line of the transmission line 31 is formed in a U-shape, and screws 41-1 and 41-2 are inserted into the screw holes 32-1 and 32-2 and fastened with nuts 42-1 and 42-2, thereby fixing the porous ceramic 26. By folding the signal line of the transmission line 31 back in a U-shape and running it along the surface of the substrate 22, the transmission line 31 can be arranged in a space-saving manner.

[0056] By folding the signal line back in a U-shape, the RF connector 21 electrically connected to one end of the transmission line 31 and the resistor 34 electrically connected to the other end can be concentrated in the same area (the area on the left side of FIG. 18), and the waterproofing areas of the electrical circuit section using the resin section 24 can be concentrated in one location. Furthermore, by folding the signal line back in a U-shape, screw holes 32-1 and 32-2 can be formed in two locations in the center of the substrate 22 and screwed in, thereby reducing the number of screwing locations.

[0057] Drainage holes 33-1 to 33-3 are formed in the center of the substrate 22, penetrating the substrate 22. By forming the drainage holes 33-1 to 33-3, soil from the back side of the substrate 22 (the surface opposite the surface) passes through the drainage holes (lightening holes) and comes into contact with the back side of the porous ceramic 26. This improves the contact rate (opening rate) between the soil to be measured and the porous ceramic 26. The shape and number of drainage holes 33 are not particularly limited, and any number of drainage holes can be formed, for example, in a shape corresponding to the area excluding the screw holes in the center of the substrate 22.

[0058] In the substrate 22, the through holes for blocking electromagnetic waves are also used for drainage, achieving both electromagnetic wave blocking and drainage. FIG. 19 illustrates the through holes formed in the substrate 22 in a plan view in the z direction. FIG. 20 illustrates the through holes formed in the substrate 22 in a cross-sectional view in the x direction. In FIG. 19, the through holes 35 are holes that penetrate the substrate 22 and are represented by numerous white circles arranged along both sides of the transmission line 31, which is a signal line formed in a U-shape on the surface of the substrate 22. The numerous through holes 35 formed along both sides of the transmission line 31 block electromagnetic waves from the signal line. The through holes 35 also drain excess water from the surface of the substrate 22 to the back side, preventing water from accumulating in minute irregularities on the surface of the substrate 22.

[0059] <Assembly Method> Figure 21 illustrates, in plan view in the z direction, an example of a method for assembling the probe 12 of Figure 10. As shown in A of Figure 21, the process of assembling the probe 12 from components made up of the housing 20, the substrate 22, and the porous ceramics 26 will be described.

[0060] Here, the previous steps are omitted, but the housing 20 is made of, for example, resin, and is processed to have a concave shape so that a portion of the plate-like shape of the substrate 22 fits into the concave. The substrate 22 has a U-shaped signal line printed on its surface, and one end of the signal line is electrically connected to the RF connector 21. The central portion of the substrate 22 is also processed to form two screw holes, three drainage holes, and the like. The porous ceramic 26 has a shape corresponding to a portion of the substrate 22, and is processed to form two screw holes. Note that the shape of the housing 20 is one example, and other shapes corresponding to the shape of the substrate 22 may be used.

[0061] 21B, porous ceramic 26 is placed on top of the surface of substrate 22, and is fixed by inserting screws 41-1 and 41-2 into two screw holes that penetrate substrate 22 and porous ceramic 26 and fastening them with nuts 42-1 and 42-2. Next, as shown in Fig. 21C, a part of substrate 22 with porous ceramic 26 fixed to its surface (the part on which porous ceramic 26 is not placed) is placed in a recessed part of housing 20, and the top and bottom of housing 20 are filled with resin to form resin part 24.

[0062] The probe 12 assembled as described above has a porous ceramic 26 fixed to the upper surface of the substrate 22 on which the transmission line 31 having a U-shaped signal line is formed, and can be used as an MP sensor.

[0063] <Other Configurations of Substrate> Another example of the detailed configuration of the probe 12 in Fig. 9 will be described. Fig. 22 illustrates the components of the probe 12 in Fig. 9 in a cross-sectional view in the y direction. Fig. 23 illustrates the surface of the substrate 22 in the probe 12 in Fig. 22 in a plan view when viewed from the direction of arrow A in Fig. 22.

[0064] 23 , in the probe 12, the signal line of the transmission line 31 is formed linearly on the surface of the substrate 22. One end of the transmission line 31, which is a linear signal line, is electrically connected to the RF connector 21, and the other end is electrically connected to a resistor 34. When the signal line of the transmission line 31 is formed linearly, screw holes that penetrate the substrate 22 and the porous ceramic 26 placed on the upper part of the surface of the substrate 22 are formed near the four corners of the porous ceramic 26, which has a rectangular shape in a plan view.

[0065] The porous ceramics 26 can be fixed by placing the porous ceramics 26 on the upper surface of the substrate 22, inserting screws 41-1 to 41-4 into the screw holes 32-1 to 32-4, and fastening them with nuts 42-1 to 42-4. In the configurations of FIGS. 22 and 23 , the porous ceramics 26 is fastened with screws at four locations on the four corners, but any location can be used, such as two diagonal locations. The tip circuit of the signal line of the transmission line 31 does not have to be terminated, but may be open or short-circuited. When the signal line of the transmission line 31 is formed linearly, the electrical circuit portion is waterproofed by resin in two locations: the resin portion 24-1 at one end of the transmission line 31 (the RF connector 21 side) and the resin portion 24-2 at the other end (the resistor 34 side).

[0066] <Assembly Method> Figure 24 illustrates an example of a method for assembling the probe 12 of Figure 22 in a plan view in the z direction. Similar to Figure 21, Figure 24 describes the process of assembling the probe 12 from components consisting of the housing 20, the substrate 22, and the porous ceramic 26. In Figure 24A, the substrate 22 has a signal line printed in a straight line on its surface, and one end of the signal line is electrically connected to the RF connector 21. The substrate 22 and the porous ceramic 26 have four screw holes formed at corresponding positions.

[0067] First, as shown in Figure 24B, porous ceramic 26 is placed on top of the surface of substrate 22, and screws 41-1 to 41-4 are inserted into four screw holes that penetrate substrate 22 and porous ceramic 26, and each screw is fastened with a nut to secure it in place. Next, as shown in Figure 24C, a portion of substrate 22 with porous ceramic 26 fixed to its surface (the portion on the RF connector 21 side where porous ceramic 26 is not placed) is placed in a recess in housing 20, and the top and bottom of housing 20 are filled with resin to form resin part 24-1. Furthermore, resin part 24-2 is formed by filling the surface and back of another portion of substrate 22 (the portion on the resistor 34 side where porous ceramic 26 is not placed) with resin.

[0068] The probe 12 assembled as described above has a porous ceramic 26 fixed to the upper surface of the substrate 22 on which the transmission line 31, in which the signal line is linear, is formed, and can be used as an MP sensor.

[0069] <Configuration of the Tip Circuit of the Transmission Line> Figure 25 is a diagram showing an example of the configuration of the tip circuit of the transmission line 31 on the substrate 22 of the probe 12 of Figure 9. When measuring the round-trip propagation time (round-trip time) in the measurement device 1 of Figure 9, the tip circuit of the transmission line 31 formed on the surface of the substrate 22 can be configured as either an open, short, or load. The tip circuit is configured at the end of the signal line of the transmission line 31 opposite the end connected to the RF connector 21.

[0070] Fig. 25A is a diagram showing a configuration example when the tip circuit is open. As shown in Fig. 25A, when the tip circuit is open, nothing is electrically connected to the tip of the transmission line 31. Fig. 25B is a diagram showing a configuration example when the tip circuit is short. As shown in Fig. 25B, when the tip circuit is short, the tip of the transmission line 31 is connected to ground (GND).

[0071] Fig. 25C shows a configuration example in which the tip circuit is a termination (load). As shown in Fig. 25C, when the tip circuit is a termination, a resistor 34 having a resistance value close to the value of the characteristic impedance of the transmission line 31 is electrically connected to the tip of the transmission line 31. For example, if the target value of the characteristic impedance of the VNA 11 and the transmission line 31 is 50Ω, the resistance value of the resistor 34 can be set to 50Ω.

[0072] Details of the case where the tip circuit is open or shorted will be described with reference to Figures 26 and 27. Figure 26 is a diagram showing the VNA calibration plane when the tip circuit is open and shorted. Figure 27 is a diagram showing examples of time waveforms when the tip circuit is open and shorted.

[0073] In Fig. 26, VNA calibration plane C indicated by the dashed dotted line is the calibration plane of the VNA 11, and is the starting point (0 s) of the time waveform calculated from the reflection coefficient (S11). In Fig. 26, the impedance of the load on the right side (the side indicated by the arrow in the figure) viewed from the VNA calibration plane C does not match the characteristic impedance of the VNA 11, so the electromagnetic wave is reflected at VNA calibration plane C. Fig. 27 shows an example of a time waveform with time (unit: seconds (s)) on the horizontal axis and amplitude (unit: dB) on the vertical axis.

[0074] The case where the tip circuit is the load will be described in detail with reference to Figures 28 and 29. Figure 28 is a diagram showing the VNA calibration plane when the tip circuit is the load. Figure 29 is a diagram showing an example of a time waveform when the tip circuit is the load.

[0075] In Fig. 28, VNA calibration plane C indicated by the dashed line is the start point (0 s) of the time waveform calculated from the reflection coefficient (S11), as in Fig. 26. In Fig. 28, by electrically connecting resistor 34 to the tip of transmission line 31, the impedance of the load on the right side as viewed from VNA calibration plane C is close to the characteristic impedance of VNA 11, and therefore the reflection of the electromagnetic wave at the tip is smaller than in the open and short-circuit cases of Fig. 26.

[0076] As with Fig. 27, Fig. 29 shows an example of a time waveform with the horizontal axis representing time and the vertical axis representing amplitude. Comparing the waveform in Fig. 29 with the waveform in Fig. 27, the amplitude at time 0 s is the same, but the amplitude of the desired wave indicated by arrow A is more attenuated in the waveform in Fig. 29 than in the waveform in Fig. 27. Thus, although the amplitude of the desired wave is attenuated in the waveform in Fig. 29, this is not a particular problem because it is at a level sufficiently higher than the noise level of the signal processor.

[0077] Furthermore, if the tip circuit is open or short-circuited, the characteristic impedance of the transmission line 31 and the impedance of the tip circuit do not match, which may result in unnecessary radiation of electromagnetic waves into space and cause radio wave interference with other communication systems. On the other hand, if the tip circuit is terminated and a resistor 34 is connected, impedance mismatch can be minimized, thereby suppressing radiation of electromagnetic waves into space. Therefore, the tip circuit of the transmission line 31 on the substrate 22 can be configured as either open, short, or terminated (load), but configuring it as terminated (load) particularly can suppress radiation of electromagnetic waves into space.

[0078] If the impedance of the transmission line 31 and the resistor 34 were perfectly matched, then not only would electromagnetic waves not be radiated into space, but reflection on the transmission line 31 would also not occur. Strictly speaking, the resistor 34 is not perfectly 50 Ω, and the transmission line 31 is not perfectly 50 Ω either, so the impedances of the transmission line 31 and the resistor 34 do not perfectly match, but are slightly different. Therefore, in this disclosure, it is assumed that the resistor 34 of the leading-edge circuit is set to a value that maintains the reflected waves of the transmission line 31 while not radiating electromagnetic waves into space.

[0079] <Substrate Processing Method> A solder resist is formed on the surface layer of the substrate 22 of the probe 12 in Fig. 9, covering the surface layer of the substrate and protecting the circuit pattern. A method for processing the solder resist applied to the substrate 22 will be described. Here, as shown in Fig. 30, the description will be given using as an example the A-A' cross section of the substrate 22 in a plan view in the z direction. For comparison, a conventionally used existing processing method will be described with reference to Fig. 31, and then a processing method to which the present disclosure is applied will be described with reference to Figs. 32 and 33.

[0080] As shown in A of Figure 31, in the substrate 22, a pattern of the transmission line 31 is printed on the upper surface of the base material 51, and a conductor layer 52 made of copper foil is formed. In the conductor layer 52, the areas where copper foil is present and areas where it is not, depending on whether the pattern is present or not, form steps, and grooves are formed between the copper foil. In the conductor layer 52, the square marked SIG represents the copper foil for the signal line, and the squares on both sides marked GND represent the copper foil for the ground (GND). A conductor layer made of copper foil is formed on the lower surface of the base material 51.

[0081] 31B, in the existing processing method, solder resist 53 is applied to the upper surface of base material 51 on which conductor layer 52 is formed, resulting in grooves on the surface of substrate 22 due to differences in level caused by the presence or absence of transmission line 31 patterns (SIG, GND). Because solder resist 53 is applied to the copper foil of conductor layer 52, the differences in level caused by the presence or absence of transmission line 31 patterns are not eliminated. In probe 12, porous ceramic 26 is placed and fixed on the surface of substrate 22. However, if there are differences in level caused by the presence or absence of transmission line 31 patterns formed on the surface of substrate 22 during measurement, the dielectric constant may change if water gets into the gap (or accumulates), potentially causing the characteristics to deviate from the desired values.

[0082] Therefore, it is desirable that the surface layer of the substrate 22 be as flat and thin as possible to make it waterproof. Fig. 32 is a diagram illustrating a processing method to which the present disclosure is applied. As shown in Fig. 32A, in the processing method of the present disclosure, when solder resist 53 is applied to the upper surface of the base material 51 on which the conductor layer 52 is formed, the solder resist 53 is applied thickly. In Fig. 32A, compared to Fig. 31B, by applying the solder resist 53 thickly, grooves on the surface caused by steps depending on the presence or absence of the pattern of the transmission line 31 are filled, and the grooves on the surface are eliminated.

[0083] Next, as shown in B of FIG. 32 , the solder resist 53 is thinned by polishing. For example, by polishing with a buff, the thickly applied solder resist 53 is polished to reduce the surface roughness. This processing flattens the surface of the substrate 22 (the surface in contact with the porous ceramic 26) and reduces the thickness of the solder resist 53. If the solder resist 53 is too thick, there is a risk that the dielectric constant of the solder resist 53 will be measured instead of the dielectric constant of the porous ceramic 26 fixed to the surface of the substrate 22. However, the processing method of the present disclosure can reduce the thickness of the solder resist 53, thereby ensuring that the dielectric constant of the porous ceramic 26 can be measured reliably.

[0084] The substance that is thickly coated on the upper surface of the substrate 51 on which the conductor layer 52 is formed is not limited to solder resist, and may be another substance (e.g., an insulating film) that can protect the pattern of the transmission line 31. For example, a through-hole filling resin can be used as the other insulating film. When using a through-hole filling resin, applying the resin to fill the holes allows the resin to flow onto the surface layer and form a film on the surface layer. This fills the through-holes, but the resin formed on the surface layer is harder than the solder resist and is therefore easier to process (polish). Furthermore, the above-mentioned processing method is one example, and other techniques that can apply a substance such as solder resist thinly and evenly may be used.

[0085] 33, an ultra-thin film coating agent 54 is applied to the upper surface of the thinned solder resist 53. For example, the ultra-thin film coating agent 54 can be a waterproof coating agent that is ultra-thin (about 10 to 20 μm) thinner than polyurethane-based coating agents (100 μm).

[0086] Generally, from the viewpoint of waterproofing, the thicker the waterproof coating, the more effective the waterproofing effect. However, since the measurement device 1 employs a planar transmission line system and the electric field generated by the transmission line 31 in the probe 12 is low, it is preferable to use a thinner waterproof coating. That is, for example, if a polyurethane-based coating (100 μm) is used, the influence of the coating's dielectric constant during measurement becomes significant. However, the processing method disclosed herein uses an ultra-thin coating (approximately 10 to 20 μm), which reduces the influence of the coating during measurement while maintaining waterproofing, thereby increasing the contribution of the dielectric constant of the porous ceramic 26.

[0087] In this way, the processing method of the present disclosure reduces the thickness of the solder resist 53 and further uses the ultra-thin film coating agent 54 as the waterproof coating agent, thereby making the surface layer of the substrate 22 as flat and thin as possible to waterproof it. If the thickness of the material used for waterproofing or flattening is large, there is a risk that the dielectric constant of the solder resist 53 or the waterproof coating agent will be measured instead of the dielectric constant of the porous ceramic 26, but the measuring device 1 can avoid such erroneous measurements.

[0088] <Apparatus Configuration> Figure 34 is a diagram showing a detailed configuration example of the measurement apparatus 1 of Figure 2. As shown in Figure 34, the measurement apparatus 1 is composed of a VNA 11 and a probe 12. In the probe 12, a transmission line 31 is formed on the surface of a substrate 22, and a porous ceramic 26 is placed on top of the surface of the transmission line 31. One end of the transmission line 31 is connected to the Port 1 terminal of the VNA 11 via an RF cable 13 connected to an RF connector 21-1, and the other end is connected to the Port 2 terminal of the VNA 11 via an RF cable 14 connected to an RF connector 21-2.

[0089] In the measurement device 1 shown in Figure 34, the VNA 11 and the probe 12 are connected via two ports (Port1, Port2), but a planar transmission line system is used. That is, in the probe 12, the transmission line 31 formed on the surface of the substrate 22 is formed so that the signal line has a predetermined shape (such as a straight line) on the surface in a planar view, and the porous ceramic 26 is placed on top of this surface. However, the straight line does not have to be a strict straight line, and it may be a shape that extends from the RF connector 21-1 provided on the first side of the substrate 22, which has a plate-like shape in a planar view, to the RF connector 21-2 provided on the second side opposite the first side.

[0090] During measurement, the probe 12 is buried in the soil to be measured, and the VNA 11 measures the soil in contact with the porous ceramic 26. At this time, the volumetric water content of the porous ceramic 26 changes depending on the matrix potential of the soil to be measured. The relative dielectric constant of the porous ceramic 26 also changes depending on the volumetric water content. The VNA 11 can measure the change in the relative dielectric constant of the porous ceramic 26 by measuring the transmission time through the transmission line 31 formed on the surface layer of the substrate 22.

[0091] Figure 35 shows the components of the probe 12 in Figure 34 in a cross section taken in the y direction. In Figure 35, the relationship between the x, y, and z axes is the same as in Figure 3 described above. As shown in Figure 35, an RF connector 21-1 is fixed to one side of a substrate 22, and an RF connector 21-2 is fixed to the other side, and the signal line of a transmission line 31 formed on the surface of the substrate 22 is electrically connected to the RF connector 21-1 and the RF connector 21-2.

[0092] A porous ceramic 26 is placed on the top of the surface of the substrate 22. Four screw holes are formed in the substrate 22 and the porous ceramic 26 at corresponding positions, and the porous ceramic 26 is fixed to the top of the surface of the substrate 22 by inserting screws into the four screw holes and fastening each with a nut. Resin portions 24-1 and 24-2 are formed by filling resin in the portions of the surface of the substrate 22 corresponding to the area excluding the area where the porous ceramic 26 is placed. Note that while Figure 35 shows an example where the porous ceramic 26 is fastened with screws at four locations, the porous ceramic 26 may be fastened with screws at different locations (two locations, four or more locations, etc.) depending on the shape of the transmission line 31 formed on the surface of the substrate 22.

[0093] <Measurement Method> In the measurement device 1 of Fig. 34, the following three steps must be performed as preparation for measurement. First, the relationship between the transmission time of the transmission line 31 and the volumetric water content of the porous ceramic when the probe 12 is used as an MP sensor is calibrated in advance. Second, the relationship between the volumetric water content of the porous ceramic and the matric potential (pF) is calibrated in advance. Furthermore, third, based on the measurement results from the above calibration, a table correlating transmission time with matric potential (pF) is written to the memory of the measurement unit 11A.

[0094] During measurement, the probe 12 is buried in the soil to be measured, and soil measurements are performed. In the VNA 11, the measurement unit 11A measures the transmission coefficient (S21) over a wide band (e.g., 1 to 9 GHz or 1 to 6 GHz). The measurement unit 11A performs an inverse Fourier transform on the measured transmission coefficient (S21) to determine the propagation time through the transmission line 31. The measurement unit 11A can convert the propagation time into matric potential (pF) using a table pre-stored in memory. Thus, in the measurement device of FIG. 34, when the VNA 11 and the probe 12 are connected via two ports (Port 1 and Port 2), the planar transmission line method is employed to measure the transmission coefficient (S21) as an S-parameter, thereby measuring the matric potential of the soil to be measured.

[0095] The measurement device 1 of FIG. 34 , like the measurement device 1 of FIG. 9 , employs a planar transmission line system, eliminating the need for metal components in the probe 12, thereby improving the contact rate between the soil to be measured and the porous ceramics 26. Furthermore, in the measurement device of FIG. 34 , the VNA 11 can be separated from the probe 12, thereby reducing the volume of the probe 12, which is the soil insertion point. Furthermore, the measurement device 1 of FIG. 34 measures the impedance of the porous ceramics, rather than the capacitance of the porous ceramics, so the measurement area depends on the electric field height of the transmission line 31. Therefore, the narrower the line width of the transmission line 31, the lower the electric field height of the transmission line 31, allowing the volume of the porous ceramics 26 to be reduced.

[0096] <Tool for Making Mud Balls> When performing measurements with an MP sensor using a probe 12 having porous ceramics 26, the probe 12 needs to be covered with water-soaked soil and buried in the soil. Currently, no tool exists for making this type of mud ball (water-soaked soil that covers the probe 12), so users have to make the mud balls by hand. If the user makes the mud balls, errors may occur in the measurement results of the MP sensor depending on the accuracy of the mud balls they make. In addition, the user's hands get dirty when making the mud balls.

[0097] This disclosure proposes a jig for making mud balls. FIG. 36 is a diagram showing an example of the configuration of a jig for making mud balls. FIG. 36A shows the configuration of a probe 12 that is covered with mud balls in a plan view in the z direction. FIG. 36B shows the configuration of a jig 60 for making mud balls in a plan view in the z direction. The jig 60 has a shape that covers the entire periphery of the probe 12 (for example, a circumference of about 1 cm). For example, the jig 60 can be formed using a material such as synthetic resin.

[0098] In FIG. 36B, the jig 60 is composed of an upper jig portion 61 and a lower jig portion 62. The jig 60 is composed of a rectangular parallelepiped member with one side open and an internal space. The upper and lower portions 61 and 62 are the divided members obtained by dividing the jig 60 into two parts, an upper portion and a lower portion, in the xy plane. The bottom surface of the upper jig portion 61 corresponds to the top surface of the jig 60 and has a concave shape when viewed cross-sectionally in the z direction. The bottom surface of the lower jig portion 62 corresponds to the bottom surface of the jig 60 and has a concave shape when viewed cross-sectionally in the z direction. Lines are drawn on the inner walls of the upper jig portion 61 and the lower jig portion 62 in the x or y direction to serve as a guide for filling the soil to be measured. The upper jig portion 61 and the lower jig portion 62 may be configured separately and independently, or may be configured as an integrated unit with corresponding side surfaces movably connected.

[0099] FIG. 37 is a diagram illustrating how to use the jig 60. First, as shown in A of FIG. 37 , the user fills the upper jig 61 with water-wet soil 63 along the line of the inner wall. The user similarly fills the lower jig 62 with water-wet soil 63. After filling the upper and lower jig 61 and 62 with soil 63, the user places the probe 12 on the soil filled in the lower jig 62, as shown in B of FIG. 37 . At this time, the housing 20 of the probe 12 has a shape corresponding to the lower jig 62, and the portion where the substrate 22 and porous ceramic 26 are laminated is placed on the soil 63.

[0100] After placing the probe 12 on the lower jig 62, as shown in FIG. 37C, the user aligns the surface of the lower jig 62 on which the probe 12 is placed with the surface of the upper jig 61 on which the soil is filled. That is, by placing the probe 12 on top of the soil filled in the lower jig 62 and covering it with the upper jig 61 filled with soil, the layered portion of the probe 12's substrate 22 and porous ceramic 26 can be covered from above and below with water-wet soil. Then, by removing the upper jig 61 and the lower jig 62, a mud ball can be created in which the probe 12 is surrounded by soil. Slits may be formed at predetermined positions in the upper jig 61 and the lower jig 62, allowing excess soil to be removed from the slits.

[0101] In the above description, an example was shown in which the jig 60 has a shape that covers the entire circumference of the probe 12 by approximately 1 cm. However, by preparing jigs 60 of various sizes, mud balls of any size can be made. For example, if a jig 60 with a circumference of n cm is manufactured, a mud ball with a thickness of approximately n cm can be made. In this way, using the mud ball making jig 60 allows mud balls to be made stably and accurately. When a user makes mud balls by hand, it is difficult to make them accurately, and there is a risk of errors in the measurement results depending on the accuracy of the mud balls. However, using the jig 60 avoids such situations. Furthermore, by using the jig 60, the user can make mud balls without getting their hands dirty.

[0102] <<Third Embodiment>> In the measurement device 1, when the planar transmission line method is adopted, the porous ceramic 26 placed on the upper surface of the substrate 22 has an appropriate pore size distribution, making it possible to measure various types of soil.

[0103] For comparison, the porous ceramics of conventionally used existing MP sensors will be described with reference to Figures 38 and 39. Figure 38 is a diagram showing the pore size distribution of the porous ceramics of an existing MP sensor. Figure 38 shows the pore size distribution of the porous ceramics, with the horizontal axis representing pore size (unit: μm) and the vertical axis representing frequency. However, the pore size on the horizontal axis is a logarithmic axis, and the frequency on the vertical axis represents the Log differential pore volume distribution.

[0104] As shown by the curve in Figure 38, the pore size distribution of porous ceramics used in existing MP sensors peaks at around 20 μm. The particle size of soil used in agriculture is generally classified as follows: clay is 2 μm or less, silt is 2 to 20 μm, and fine sand is 20 to 200 μm. For example, when using an existing MP sensor to measure fine sand (particle size: 20 to 200 μm), it is difficult to measure because the pore size distribution does not match that of porous ceramics.

[0105] The shape of the water retention curve, which indicates water retention and drainage, depends on the pore size distribution of the porous ceramic. Figure 39 shows the water retention curve of the porous ceramic of an existing MP sensor. In Figure 39, the horizontal axis is pF and the vertical axis is volumetric water content (unit: m 3 ・m －3 ) shows the moisture retention curve of porous ceramics. The units of matric potential are pressure P [kPa], pressure head h [cm], pF, etc. Here, since matric potential is a negative pressure, pressure P [kPa] is P≦0, and pressure head h [cm] is h≦0. pF is the logarithm of pressure head h, and pF = log 10 There is a relationship of |h|. Furthermore, there is a relationship between pressure P [kPa] and pressure head h [cm], P [kPa] = h [cm] x 0.098. pF on the horizontal axis is the logarithmic notation of matric potential (pressure), and it is pF0 = -0.098 kPa, pF1 = -0.98 kPa, pF2 = -9.8 kPa, pF3 = -98 kPa, pF4 = -980 kPa.

[0106] As shown by the curves in Figure 39, water drains steeply in the range of pF2 to pF3, which corresponds to pore diameters of several tens of micrometers, while water is retained without drainage in the range of pF1 to pF2, which corresponds to less frequent pore diameters, or above pF3, resulting in a poor balance between drainage and water retention. In other words, there is a relationship between the pore diameter of porous ceramics and the pressure (pF) on the water retention curve, and the larger the pore diameter in the porous ceramic, the smaller the negative pressure (smaller pF) required for drainage. On the other hand, the smaller the pore diameter in the porous ceramic, the greater the negative pressure (larger pF) required for drainage.

[0107] Next, the porous ceramic 26 placed on the upper surface of the substrate 22 of the probe 12 in the measuring device 1 will be described with reference to Figures 40 to 43. Figure 40 is a diagram showing an example of the pore size distribution of the porous ceramic 26. Similar to Figure 38, Figure 40 shows the pore size distribution of the porous ceramic with the horizontal axis representing the pore size and the vertical axis representing the frequency.

[0108] As shown by the curve in Figure 40, the porous ceramic 26 has a uniform pore size distribution in the range of 0.1 to 100 μm. That is, the porous ceramic 26 has a pore size distribution in the range of 0.1 to 100 μm, and this distribution is made as flat as possible. As mentioned above, the particle size of soil used in agriculture is classified into clay (2 μm or less), silt (2 to 20 μm), and fine sand (20 to 200 μm). When measuring with the MP sensor of the present disclosure, the pore size distribution of the porous ceramic 26 is a wide range from 0.1 to 100 μm, making it possible to measure the matric potential of a wide range of soil, from clay to fine sand.

[0109] The pore size distribution of the porous ceramics 26 can be measured by, for example, mercury intrusion porosimetry. The porous ceramics 26 has open holes, most of which are connected. If the holes are closed, water cannot be drained even if there is a distribution of pore sizes, but with the porous ceramics 26, water can be drained through the open holes.

[0110] The shape of the water retention curve, which indicates water retention and drainage, depends on the pore size distribution of the porous ceramic 26. Figure 41 shows an example of the water retention curve for the porous ceramic 26. Similar to Figure 39, Figure 41 shows the water retention curve for the porous ceramic 26, with pF on the horizontal axis and volumetric water content on the vertical axis. In Figure 41, pF on the horizontal axis is also a logarithmic representation of the matric potential (pressure), with pF0 = -0.098 kPa, pF1 = -0.98 kPa, pF2 = -9.8 kPa, pF3 = -98 kPa, and pF4 = -980 kPa. As shown in Figure 41, the water retention curve for the porous ceramic 26 maintains a linear shape in the range of pF1 to pF4.

[0111] Here, the details of the moisture retention curve of the porous ceramic 26 in Fig. 9 will be described with reference to Fig. 42 and Fig. 43. Fig. 42 shows the moisture retention curve of the porous ceramic 26, with the horizontal axis representing pF and the vertical axis representing the volumetric water content θ. Fig. 42 shows the moisture retention curve of the porous ceramic 26 approximated by the linear equations shown in the following equations (1) and (2), and the linear equations and the parameters a, b, c, d, e, and f have the following relationship:

[0112] ...(1)

[0113] ...(2)

[0114] As shown in Figure 42, a is the intersection of lines expressed by two linear equations with different slopes, and for example, falls within the range of 0.5 ≦ a ≦ 1.5. In the range of 0 ≦ pF ≦ a, pF and θ are expressed by the above formula (1). However, in formula (1), a is the intersection of the two lines, c is the volumetric water content at that time, and d is the saturated volumetric water content. For example, c and d fall within the ranges of 0.2 ≦ c ≦ d and 0.3 ≦ d < 1.

[0115] In addition, in the range of a < pF < b, pF and θ are expressed by the above formula (2), where 0.5 ≦ a ≦ 1.5, b = 4.2. Here, from the slope shown in Figure 42, the relationship of the following formula (3) holds, and e is expressed by the following formula (4).

[0116] ...(3)

[0117] ...(4)

[0118] Therefore, the following equation (5) can be obtained from the above equations (2) and (4). In equation (5), b = 4.2 is the pressure at the permanent wilting point, and f is the residual volumetric moisture content. For example, f is in the range of 0 ≦ f < 0.2.

[0119] ...(5)

[0120] In this way, the porous ceramic 26 is characterized in that, when the moisture retention curve is approximated by the linear equations shown in the above formulas (1) and (2), the function shapes shown in the above formulas (1) and (5) and the parameters a, b, c, d, and f fall within the following ranges: For example, a, b, c, d, and f can be in the ranges of 0.5≦a≦1.5, b=4.2, 0.2≦c≦d, 0.3≦d<1, 0≦f<0.2.

[0121] Fig. 43 is a diagram showing the range of the moisture retention curve according to formulas (1) and (5). Fig. 43 shows the moisture retention curve of the porous ceramic 26 when the horizontal axis is pF and the vertical axis is volumetric water content θ, and the area inside the thick line L is the range that formulas (1) and (5) can take.

[0122] As described above, the porous ceramic 26, possessing the characteristics shown in Figures 40 to 43, provides an improved balance between water retention and drainage, and can be used in a variety of soils with pore sizes ranging from 0.1 to 100 μm, ranging from pF1 to pF4. Therefore, measurements can be performed on a variety of soil types. The porous ceramic 26 has a pore size distribution ranging from 0.1 to 100 μm, which is generally flat. However, the upper and lower limits of this range may be set to other values, such as 1 to 50 μm, as long as they are within the range of 0.1 to 100 μm. For example, even if the pore size distribution of the porous ceramic 26 is set to the range of 1 to 50 μm, as shown in Figure 40, the distribution remains generally flat within that range, enabling measurement of a wider variety of soils than existing MP sensors. Note that the ranges and values ​​of the parameters a, b, c, d, and f described above are merely examples and are not limited to the above ranges and values.

[0123] <<Fourth Embodiment>> When the planar transmission line system is adopted in the measurement device 1, a concave-convex structure can be provided on the surface (top surface) of the porous ceramic 26 placed on top of the surface of the substrate 22, the surface that comes into contact with the soil to be measured. Fig. 44 shows a partial configuration of the porous ceramic 26 of Fig. 9 in a plan view in the z direction. Fig. 45 shows the configuration of the porous ceramic 26 of Fig. 9 in a cross-sectional view in the y direction. Although not shown, other regions of the porous ceramic 26 are configured in a similar manner.

[0124] Fig. 44 shows a partial region on the top surface of the porous ceramic 26, and the surface that comes into contact with the soil to be measured has a lattice pattern in which convex portions 26A and concave portions 26B are alternately arranged. As shown in Fig. 45, the convex portions 26A and concave portions 26B are periodically repeated on the top surface of the porous ceramic 26, forming an uneven structure and increasing the surface area.

[0125] Increasing the surface area of ​​the upper surface of the porous ceramics 26 increases the surface area of ​​the surface that comes into contact with the soil to be measured, thereby improving the contact rate between the porous ceramics 26 and the soil. The surface (lower surface) of the porous ceramics 26 that comes into contact with the substrate 22 is polished flat to prevent the generation of a water film due to gaps.

[0126] In Figure 45, the upper surface of the porous ceramic 26 has an uneven structure with convex portions 26A and concave portions 26B. However, other structures may be used as long as the surface height of the upper surface is not uniform and the surface area of ​​the surface in contact with the soil to be measured is large. Figure 46 illustrates another configuration of the porous ceramic 26 of Figure 9 in a cross-sectional view in the y direction. In Figure 46, the upper surface of the porous ceramic 26 has a curved shape. That is, in Figure 45, the upper surface of the porous ceramic 26 has a rectangular wave-like shape in cross-section, but in Figure 46, the upper surface of the porous ceramic 26 has a sinusoidal wave-like shape in cross-section. As long as the surface height of the upper surface of the porous ceramic 26 is not uniform, the uneven pattern does not need to be uniform.

[0127] As described above, by forming the upper surface of the porous ceramic 26 into a rectangular wave or sine wave shape when viewed in cross section, and intentionally forming an uneven structure rather than a uniform surface height, the surface area of ​​the surface of the porous ceramic 26 that comes into contact with the soil is increased, thereby improving the contact rate between the soil to be measured and the porous ceramic 26.

[0128] <<Fifth Embodiment>> In the measurement device 1, when a planar transmission line system is adopted, a two-branch switch is provided to electrically connect to transmission lines formed on both surfaces A and B of the substrate 22, making it possible to perform measurements on both surfaces of the substrate 22. Hereinafter, one surface of the plate-shaped substrate 22 will be referred to as surface A, and the other surface will be referred to as surface B. For example, the upper surface (top surface) of the substrate 22 in a cross-sectional view will be referred to as surface A, and the lower surface (bottom surface) opposite the upper surface will be referred to as surface B, and measurements can be performed using surface A as an MP sensor and surface B as a moisture sensor.

[0129] For comparison, an existing configuration for measuring soil moisture content and matric potential will be described with reference to FIG. 47. As shown in FIG. 47, in the existing configuration, matric potential is measured using a capacitance measurement circuit 922 and a capacitance probe 923, and soil moisture content is measured using a TDT measurement circuit 924 and a TDT probe, with the two measurements controlled individually by a control circuit 921. The capacitance probe 923 is an MP sensor with a capacitance structure that uses porous ceramics to measure matric potential. The TDT probe is a moisture sensor with a transmission line that measures soil moisture content.

[0130] In other words, the existing configuration integrates an MP sensor and a moisture sensor, but because the matric potential is measured using a capacitance method and the soil moisture is measured using a TDT method, the individual measurement circuits are separate, making it difficult to miniaturize the measurement device and reduce power consumption. This type of existing configuration is disclosed, for example, in JP 2022-121360 A.

[0131] <Device Configuration> FIG. 48 is a diagram showing a detailed configuration example of the measurement device 1 of FIG. 1. In FIG. 48, the measurement device 1 is composed of a VNA 11 and a probe 12, and further includes a control circuit 10 that controls the VNA 11 and the probe 12. The control circuit 10 is composed of a microcontroller or the like. In FIG. 48, a substrate A-side 22A represents the A-side of the substrate 22, and a branch switch 71 and an MP sensor 72 are provided on the substrate A-side 22A. Furthermore, a substrate B-side 22B represents the B-side of the substrate 22, and a moisture sensor 75 is provided on the substrate B-side 22B. The ends of the transmission lines formed on the substrate A-side 22A and the substrate B-side 22B are terminated by resistors. Note that the ends of the transmission lines are not limited to being terminated (loaded), but may also be open or shorted.

[0132] The detailed configurations of the substrate A surface 22A and the substrate B surface 22B will be described with reference to Figures 49 to 51. Figure 49 illustrates the probe 12 in a cross-sectional view in the y direction. As shown in Figure 49, the upper surface of the substrate 22 having the porous ceramic 26 fixed to the upper surface is the substrate A surface 22A, and the lower surface is the substrate B surface 22B. Figure 50 illustrates the substrate A surface 22A in a plan view in the z direction. Figure 51 illustrates the substrate B surface 22B in a plan view in the z direction.

[0133] 50 , on the substrate A surface 22A, an RF connector 21, which is electrically connected to a port (Port 1 terminal) of the VNA 11 via an RF cable 13, is electrically connected via a signal line to a terminal of a branch switch 71. The branch switch 71 is configured, for example, by an SPDT (Single-Pole Double-Throw) RF switch, and switches the connection destination in accordance with a control signal from the control circuit 10.

[0134] 50 and 51 , a through-hole 74 is formed in the substrate 22, and a signal line from the branch switch 71 on the substrate A-side 22A is electrically connected to the transmission line 31B on the substrate B-side 22B via the through-hole 74 that penetrates the substrate 22. This allows the branch switch 71 to switch the connection destination of the RF cable 13 electrically connected to the RF connector 21 to the transmission line 31A formed on the substrate A-side 22A or the transmission line 31B formed on the substrate B-side 22B in accordance with a control signal from the control circuit 10.

[0135] Porous ceramics 26 is placed on the substrate A-side 22A, and an RF signal from the VNA 11 is transmitted to a transmission line 31A via an RF cable 13, thereby enabling use as an MP sensor 72. On the substrate A-side 22A, a resistor 73 is connected to the tip of the transmission line 31A, forming a termination. On the other hand, no porous ceramics is placed on the substrate B-side 22B, and an RF signal from the VNA 11 is transmitted to a transmission line 31B via an RF cable 13, thereby enabling use as a moisture sensor 75. On the substrate B-side 22B, a resistor 76 is connected to the tip of the transmission line 31B, forming a termination.

[0136] In the probe 12, the branch switch 71 switches the connection destination of the RF cable 13, and when it is connected to the signal line side of the transmission line 31A formed on the substrate A surface 22A, it is used as an MP sensor 72, and when it is connected to the signal line side of the transmission line 31B formed on the substrate B surface 22B, it is used as a moisture sensor 75.

[0137] In this way, in the measurement device 1, by configuring the substrate 22 of the probe 12 as a double-sided transmission line substrate, it is possible to measure both the matric potential and the soil moisture content almost simultaneously using only one electrical circuit (the measurement unit 11A of the VNA 11) by switching the connection destination of the VNA 11 (the RF cable 13 connected to its Port 1 terminal) using the branch switch 71. Therefore, the measurement device 1 can make the probe (sensor) more compact and achieve lower power consumption than the existing configuration (Figure 47) in which the two measurement circuits are separate circuits.

[0138] 48 to 51 show a configuration in which the substrate A surface 22A of the probe 12 is used as the MP sensor 72 and the substrate B surface 22B is used as the moisture sensor 75, but other configurations may be adopted. For example, by not placing porous ceramics on both surfaces of the substrate A surface 22A and the substrate B surface 22B, both surfaces of the substrate A surface 22A and the substrate B surface 22B may be used as moisture sensors. Alternatively, by placing porous ceramics on both surfaces of the substrate A surface 22A and the substrate B surface 22B, both surfaces of the substrate A surface 22A and the substrate B surface 22B may be used as MP sensors. However, when both surfaces of the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, it is desirable that the porous ceramics placed on each surface (surface A or surface B) have different characteristics.

[0139] <Other Configuration Examples> In the configuration example of Fig. 48, a configuration is shown in which a branch switch 71 is used to switch between the transmission line 31A formed on the substrate A surface 22A and the transmission line 31B formed on the substrate B surface 22B, but a configuration in which a VNA is provided for each transmission line without providing the branch switch 71 may also be used. Fig. 52 is a diagram showing another example of the detailed configuration of the measuring device 1.

[0140] 52, the measurement device 1 can be configured to include VNAs 11-1 and 11-2, with the port (Port1 terminal) of VNA 11-1 electrically connected via an RF cable to a transmission line 31A formed on the substrate A surface 22A, and the port (Port1 terminal) of VNA 11-2 electrically connected via an RF cable to a transmission line 31B formed on the substrate B surface 22B. VNAs 11-1 and 11-2 are controlled by a control circuit 10.

[0141] 52, the substrate A side 22A has porous ceramic 26 placed thereon and transmits an RF signal from VNA 11-1 to transmission line 31A via an RF cable, thereby being used as an MP sensor 72. The substrate B side 22B does not have porous ceramic 26 placed thereon, and transmits an RF signal from VNA 11-2 to transmission line 31B via an RF cable, thereby being used as a moisture sensor 75.

[0142] In this way, by electrically connecting VNA11-1 and VNA11-2 to the substrate A surface 22A and the substrate B surface 22B, respectively, on the substrate 22 of the probe 12, measurements can be performed on both surfaces (surfaces A and B) of the substrate 22 without providing a branch switch.

[0143] <Measurement Sequence> A description will be given of the sequence of the measurement method using the measurement apparatus 1 in Fig. 48. Fig. 53 is a diagram showing a first example of a measurement sequence when measurements are performed on both surfaces (side A and side B) of the substrate 22. In Fig. 53, the measurement order from start to finish when measurements are performed by switching between substrate side A 22A and substrate side B 22B is shown in order from left to right in the figure.

[0144] As shown in FIG. 53, first, the branch switch 71 switches the RF signal path to the signal line side of the board A surface 22A in accordance with a control signal from the control circuit 10 (S111), thereby measuring all channels on the board A surface 22A (S112). Measurement of all channels on the board A surface 22A can be performed using any of the four measurement sequences shown in FIGS. 13 to 16. When measurement of the board A surface 22A is completed, the branch switch 71 switches the RF signal path to the signal line side of the board B surface 22B in accordance with a control signal from the control circuit 10 (S113), thereby measuring all channels on the board B surface 22B (S114). Measurement of all channels on the board B surface 22B can be performed using any of the four measurement sequences shown in FIGS. 13 to 16. When measurement of the board B surface 22B is completed, the series of measurements is completed.

[0145] In this way, when measurements are made on both sides (sides A and B) of the substrate 22, a measurement method can be used in which, after measuring the substrate A side 22A, the measurement is switched from the substrate A side 22A side to the substrate B side 22B side, and the measurement is made on the substrate B side 22B. This measurement method has the advantage of reducing the number of times the path needs to be switched by the branch switch 71, and is one of the appropriate measurement methods.

[0146] Fig. 54 is a diagram showing a second example of a measurement sequence when measurements are made on both surfaces (side A and side B) of the substrate 22. In Fig. 54, when the number of frequency channels is n, the measurement order from start to finish when measurements are made while alternately switching between the substrate side A 22A and the substrate side B 22B is shown in two stages from left to right in the diagram.

[0147] 54 , first, the branch switch 71 switches the path of the RF signal to the signal line side of the board A surface 22A in accordance with a control signal from the control circuit 10, thereby measuring channel 1 on the board A surface 22A (S121, S122). When the measurement of channel 1 on the board A surface 22A is completed, the branch switch 71 switches the path of the RF signal to the signal line side of the board B surface 22B in accordance with a control signal from the control circuit 10, thereby measuring channel 1 on the board B surface 22B (S123, S124). When the measurement of channel 1 on the board B surface 22B is completed, the branch switch 71 switches the path, thereby measuring channel 2 on the board A surface 22A (S125, S126). When the measurement of channel 2 on the board A surface 22A is completed, the branch switch 71 switches the path, thereby measuring channel 2 on the board B surface 22B (S127, S128). Thereafter, measurements are similarly performed on both sides while switching between the substrate A side 22A and the substrate B side 22B for each channel measurement, and measurements are repeated from ch1 to chn.

[0148] Note that the measurement method for measuring both sides (side A and side B) of the substrate 22 is not limited to the measurement sequences shown in Figures 53 and 54, and other measurement methods may be used. For example, when a trigger signal is input, the branch switch 71 may switch between measuring the substrate side A 22A and measuring the substrate side B 22B. More specifically, based on the measurement data of one side (side A or side B), it may be determined whether to measure the other side (side B or side A). Furthermore, the side to be measured (side A or side B) may be determined based on external information (e.g., information obtained by an external device such as another sensor, or information such as weather), or measurement may be performed on only one side (side A or side B).

[0149] 48 shows a configuration in which a branch switch 71 is provided when the VNA 11 and the probe 12 are connected via one port (Port 1), but a configuration in which a through hole is formed through the substrate 22 to electrically connect the transmission line 31A formed on the substrate A surface 22A to the transmission line 31B formed on the substrate B surface 22B may also be employed without providing the branch switch 71. With such a configuration, the propagation time (transmission time) between the substrate A surface 22A and the substrate B surface 22B can be measured from the time waveform of reflection by the through hole formed at the end of the transmission line 31A.

[0150] FIG. 55 is a diagram showing a first example of the detailed configuration of a measurement device 1 that is compatible with a branch switchless configuration. In FIG. 55, the measurement device 1 is composed of a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to one port (Port 1 terminal) of the VNA 11 via an RF cable 13. In the probe 12, a through-hole 78 that penetrates the substrate 22 is formed at the end (the end opposite the end connected to the RF cable 13) of a transmission line 31A formed on the substrate A surface 22A. The transmission line 31A is electrically connected to a transmission line 31B formed on the substrate B surface 22B via the through-hole 78. The end of the transmission line 31B is terminated by a resistor. Note that the end of the transmission line 31B is not limited to being terminated (loaded), but may also be open or shorted.

[0151] In the probe 12, a porous ceramic 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and an RF signal from the VNA 11 is transmitted to the transmission line 31A, thereby being used as an MP sensor 72. In addition, in the probe 12, a transmission line 31B is formed on the substrate B surface 22B, and an RF signal from the VNA 11 is transmitted to the transmission line 31B, thereby being used as a moisture sensor 75.

[0152] FIG. 56 is a diagram showing a time waveform of reflection generated from the reflection coefficient (S11) measured by the measuring device 1 of FIG. 55. In FIG. 56, the horizontal axis represents time (unit: seconds (s)) and the vertical axis represents amplitude (unit: dB). In the waveform of FIG. 56, if the time indicated by the first peak is round-trip time 1 (t1) and the time indicated by the second peak is round-trip time 2 (t2), the propagation time of the substrate A surface 22A is round-trip time 1, and the propagation time of the substrate B surface 22B is calculated from the difference between round-trip time 2 and round-trip time 1. That is, as shown in FIG. 55, the propagation time of the substrate A surface 22A is t1, and the propagation time of the substrate B surface 22B is Δt = t2 - t1.

[0153] As described above, in the measurement apparatus 1 of Figure 55, a through-hole 78 penetrating the substrate 22 is provided at the end of the transmission line 31A on the substrate A surface 22A, and the transmission line 31A is electrically connected to the transmission line 31B on the substrate B surface 22B. When an RF signal is transmitted from the VNA 11 during measurement, the propagation time t1 on the substrate A surface 22A and the propagation time Δt (t2 - t1) on the substrate B surface 22B can be measured based on the time waveform reflected by the through-hole 78. In the VNA 11, the measurement unit 11A measures the matric potential of the soil to be measured based on the propagation time t1 on the substrate A surface 22A. Furthermore, the measurement unit 11A measures the moisture content of the soil to be measured based on the propagation time Δt on the substrate B surface 22B.

[0154] In the measuring device 1 of Fig. 55, the measurement sequence can be any of the four measurement sequences shown in Fig. 13 to Fig. 16. In the measuring device 1 of Fig. 55, both sides (sides A and B) of the substrate 22 can be measured in one measurement without providing a branch switch, which reduces power consumption by reducing the number of measurements (measurement time) and makes it possible to miniaturize the probe (sensor) by reducing the area by reducing the number of parts.

[0155] 55 shows a configuration in which the substrate A surface 22A is used as the MP sensor 72 and the substrate B surface 22B is used as the moisture sensor 75, but it is also possible to use a configuration in which both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, or a configuration in which both surfaces are used as moisture sensors. When both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, it is possible to use porous ceramics with different properties for each surface.

[0156] FIG. 57 is a diagram showing a second example of the detailed configuration of a measurement device 1 that is compatible with a branch switchless configuration. In FIG. 57 , the measurement device 1 includes a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to two ports (Port1 terminal and Port2 terminal) of the VNA 11 via RF cables 13 and 14. In the probe 12, a through-hole 78 is formed at the end of a transmission line 31A formed on the substrate A-side 22A (the end opposite the end connected to the RF cable 13), and the transmission line 31A is electrically connected to a transmission line 31B formed on the substrate B-side 22B via the through-hole 78. In addition, in the probe 12, a through-hole 79 is formed at the end of a transmission line 31B (the end opposite the end connected to the transmission line 31A), and the transmission line 31B is electrically connected to the RF cable 14 via the through-hole 79.

[0157] In the probe 12, a porous ceramic 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and an RF signal from the VNA 11 is transmitted to the transmission line 31A, thereby being used as an MP sensor 72. In addition, in the probe 12, a transmission line 31B is formed on the substrate B surface 22B, and an RF signal from the VNA 11 is transmitted to the transmission line 31B, thereby being used as a moisture sensor 75.

[0158] Figure 58 is a diagram showing a time waveform of reflection generated from the reflection coefficient (S11) measured by the measuring device 1 of Figure 57. Figure 58 shows a waveform with time (unit: seconds (s)) on the horizontal axis and amplitude (unit: dB) on the vertical axis. In the waveform of Figure 58, as with the waveform of Figure 56, if the time indicated by the first peak is round-trip time 1 (t1) and the time indicated by the second peak is round-trip time 2 (t2), the propagation time of the substrate A surface 22A is round-trip time 1. That is, as shown in Figure 57, the propagation time of the substrate A surface 22A is t1.

[0159] Here, we propose a method for determining the propagation time on the substrate B surface 22B without using round-trip time 2 (t2), because the peak in the waveform indicating round-trip time 2 has a poor S / N ratio and may reduce measurement accuracy. Figure 59 shows a transmission time waveform generated from the transmission coefficient (S21) measured by the measurement device 1 in Figure 57. Figure 59 shows a waveform with time (unit: seconds (s)) on the horizontal axis and amplitude (unit: dB) on the vertical axis. In the waveform in Figure 59, if the time indicated by the first peak is transmission time 1 (t3), the propagation time on the substrate B surface 22B can be determined using transmission time 1 (t3). That is, as shown in Figure 57, the propagation time on the substrate B surface 22B is Δt = 2*t3 - t1. Here, "*" indicates multiplication.

[0160] In this way, in the measurement apparatus 1 of Figure 57, when the VNA 11 and probe 12 are connected via two ports (Port 1, Port 2), the propagation time Δt (2*t3-t1) on the substrate B surface 22B can be calculated from the round-trip time 1 (t1) measured from the time waveform of reflection generated from the reflection coefficient (S11) and the transmission time 1 (t3) measured from the time waveform of transmission generated from the transmission coefficient (S21). In the VNA 11, the measurement unit 11A measures the matric potential of the soil to be measured based on the propagation time t1 on the substrate A surface 22A. The measurement unit 11A also measures the moisture content of the soil to be measured based on the propagation time Δt on the substrate B surface 22B.

[0161] The measurement apparatus 1 of Figure 57 can use any of the four measurement sequences shown in Figures 13 to 16 as the measurement sequence. The measurement apparatus 1 of Figure 57 can measure both sides (sides A and B) of the substrate 22 in a single measurement without using a branch switch, thereby achieving low power consumption and a compact probe. Furthermore, the measurement apparatus 1 of Figure 57 can calculate the propagation time (Δt) for the substrate B side 22B from round trip time 1 (t1) and transmission time 1 (t3), which have a good S / N ratio (high S / N ratio), without using round trip time 2 (t2), which has a poor S / N ratio (low S / N ratio), thereby improving measurement accuracy.

[0162] 57 shows a configuration in which the substrate A surface 22A is used as the MP sensor 72 and the substrate B surface 22B is used as the moisture sensor 75, but it is also possible to use a configuration in which both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, or a configuration in which both surfaces are used as moisture sensors. When both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, it is possible to use porous ceramics with different properties for each surface.

[0163] In the measurement apparatus 1 of Figure 48, when the VNA 11 and the probe 12 are connected via one port (Port 1), the branch switch 71 is provided, which requires two measurements: one for the substrate A surface 22A and one for the substrate B surface 22B, as shown in the measurement sequences of Figures 53 and 54. On the other hand, the measurement apparatus 1 of Figure 55 does not require a branch switch, so it is only necessary to measure the reflection times of the substrate A surface 22A and the substrate B surface 22B once, thereby halving power consumption. In the branch switch-less configuration shown in Figure 55, as shown in Figure 56, two time waveforms, t1 and t2, must be observed in a single measurement, and it is necessary to accurately measure the round-trip time of the transmission line 31A in the MP sensor 72 of Figure 55 and the round-trip time of the transmission line 31B in the moisture sensor 75 of Figure 55.

[0164] Here, there are three conditions under which the reflected wave from the substrate A surface 22A and the reflected wave from the substrate B surface 22B can be measured. The first condition is that the time waveforms at t2 and t1 can be sufficiently separated without interference, that is, the path length must be long enough for Δt(t2-t1). The second condition is that the signal level (amplitude) of the reflected wave at t2 must be sufficiently higher than the noise level (sufficient S / N ratio) so that it can be demodulated by the circuitry and signal processing of the VNA 11. The third condition is that the signal level (amplitude) of the reflected wave at t1 must be sufficiently higher than the noise level (sufficient S / N ratio) so that it can be demodulated by the circuitry and signal processing of the VNA 11.

[0165] However, it is necessary that the multiple reflection components (2*t1, 3*t1, ...) of the reflection time at the substrate A surface 22A do not interfere with t2, and that the amplitude of the 2*t1 reflected wave is sufficiently small compared to the t2 wave. Here, Fig. 60 shows waveforms used to determine the propagation times at the substrate A surface 22A and the substrate B surface 22B, as in Fig. 56, with a dashed-dotted line indicating the noise level and a dotted line indicating the spectrum of the multiple reflections at t1 superimposed. In Fig. 60, the double-wave of t1, i.e., the amplitude of the reflected wave of 2*t1, is sufficiently small compared to the amplitude of the wave at t2, as indicated by the bidirectional arrow A.

[0166] The first condition can be met by finding the path length that satisfies it. On the other hand, with the configuration of the measuring device 1 in Figure 55, it may be difficult to satisfy the second and third conditions. Here, reflection occurs because the characteristic impedance of the transmission line differs from the impedance of the reflecting object (this is called impedance mismatch). Normally, when the characteristic impedance of the transmission line is 50 Ω, the impedance of the through hole is designed to be 50 Ω, so reflection does not occur. However, it is possible to cause reflection by adjusting the impedance of the through hole.

[0167] First, we will explain how to calculate the path length for separating the time waveform (t1) of reflection from the substrate A surface 22A (top surface: front surface) and the time waveform (t2) of reflection from the substrate B surface 22B (bottom surface: back surface) under the first condition. For example, when using a band of 1 to 9 GHz, the shape of the time waveform will be as shown in Figure 61. Figure 61 shows the spectrum of the time waveform obtained by performing an IFFT (Inverse Fast Fourier Transform) on a signal with a reflection coefficient (S11) from 1 to 9 GHz, and normalizes the peak intensity to 0 dB.

[0168] Here, when the time waveform shown in Figure 61 is separated into two time waveforms, for example, if the peak center of the waveform t2 is located at twice the time it takes for the waveform t1 to decay -40 dB from its peak, the waveforms t1 and t2 will not interfere with each other. Figure 62 shows time waveforms when the center of the waveform t1 is set to 0 s and the center of the waveform t2 is assumed to be located at twice the time it takes for the waveform t1 to decay -40 dB. In Figure 62, the waveform t1 is shown with a solid line and the waveform t2 with a dashed line, and the time between the peak centers of the two waveforms is set to Δt = 0.5 [ns].

[0169] As shown schematically in Fig. 63, if the distance that an electromagnetic wave travels back and forth between the front surface reflection position R1 and the rear surface reflection position R2 is L, and the speed of the electromagnetic wave on the transmission line is v, the condition under which L is the longest is when it is assumed that the space above the transmission line is air. In Fig. 63, the front surface reflection position R1 is the position where the electromagnetic wave is reflected by the transmission line 31A on the substrate A surface 22A. Furthermore, the rear surface reflection position R2 is the position where the electromagnetic wave is reflected by the transmission line 31B on the substrate B surface 22B. For example, if the relative dielectric constant of the substrate 22 is 3.6, the time required for the electromagnetic wave to travel per 1 cm on the substrate is 56 ps according to an electromagnetic field simulation, so v = 1.8 × 10 8 Furthermore, since it is sufficient that L > v × Δt, and v does not exceed the speed of light, the relationship in equation (6) below can be derived.

[0170] L > v × Δt = 1.8 × 10 8 [m / s] × 0.5[ns] = 0.09[m] ... (6)

[0171] Therefore, L should be 9 cm or more. As shown in FIG. 63, L corresponds to the round-trip distance of the electromagnetic wave, so the one-way distance (L / 2) should be 4.5 cm or more. Note that the wavelength of the electromagnetic wave shortens and v changes depending on the relative dielectric constant of the substrate 22, so it is necessary to consider these changing conditions. Furthermore, the path length can be adjusted by setting the frequency bandwidth and Δt of the VNA 11. For example, if Δt is set with a margin, the path length (L / 2) will be long, and if Δt is set with a strict margin, the path length (L / 2) will be short. Therefore, in the example described here, calculations are performed with a sufficient margin for Δt under the above assumptions.

[0172] Next, to satisfy the second condition, the amplitude of the reflected wave at t2 must be large, and the signal is reflected by opening or shorting the end circuit of the transmission line rather than using it as a termination (load). Alternatively, since reflection only needs to occur away from the impedance (50 Ω) of the transmission line, the reflected wave at t2 can be measured at a sufficient distance from the impedance (50 Ω) of the transmission line, even if it is not an open or short circuit. Furthermore, to satisfy the third condition, the reflected wave at t1 must have sufficient amplitude so that it can be demodulated by the circuitry and signal processing of the VNA 11.

[0173] Here, a resistor with a value different from the impedance (50 Ω) of the transmission line is inserted before or after the through-hole. Inserting a resistor with a value different from the characteristic impedance (50 Ω) of the transmission line generates a reflected wave at the resistor, allowing the time waveform at t1 to be demodulated. For example, a chip resistor can be used as the resistor. Alternatively, the width of the transmission line can be adjusted to create a resistance. The reflected wave at t1 can also be generated by adjusting the impedance of the through-hole. Meanwhile, to prevent the 2*t1 waveform of the multiple reflections of the reflected wave at t1 from interfering with the waveform at t2, it is necessary to suppress the amplitude of the time waveform due to multiple reflections. If the value of the resistor is significantly different from 50 Ω, the amplitude of the multiple reflected waves at the resistor will increase, potentially interfering with the waveform at t2. Therefore, it is desirable to set the resistance value to a value that does not deviate too much from 50 Ω (e.g., 56 Ω or 47 Ω).

[0174] Figure 64 is a diagram showing a first example of the detailed configuration of a measurement device 1 capable of measuring the round-trip time of a transmission line. In Figure 64, parts corresponding to those in Figure 55, etc. are given the same reference numerals, and their description will be omitted as appropriate. As shown in Figure 64, the measurement device 1 is composed of a control circuit 10, a VNA 11, and a probe 12. In the probe 12 of Figure 64, a transmission line 31A formed on the substrate A surface 22A and a transmission line 31B formed on the substrate B surface 22B are electrically connected via a through hole 78 that penetrates the substrate 22.

[0175] In the probe 12 of FIG. 64 , the substrate A surface 22A is used as the MP sensor 72, and the substrate B surface 22B is used as the moisture sensor 75. As described above, the transmission line 31A on the substrate A surface 22A used as the MP sensor 72 is a 50 Ω transmission line, and the transmission line 31B on the substrate B surface 22B used as the moisture sensor 75 is a 50 Ω transmission line. The tip circuit of the transmission line 31B is open or shorted. Alternatively, the tip circuit of the transmission line 31B may have a resistance other than 50 Ω. In addition, a resistor 91 is provided on the substrate A surface 22A side of the through hole 78. The resistor 91 is provided near the middle between the substrate A surface 22A and the substrate B surface 22B, and its resistance value can be set to a value that suppresses multiple reflections from the substrate A surface 22A and ensures a good S / N ratio. The resistor 91 may be a chip resistor, or may be formed by adjusting the width of a transmission line.

[0176] FIG. 65 shows a second example of the detailed configuration of a measurement device 1 capable of measuring the round-trip time of a transmission line. In FIG. 65, parts corresponding to those in FIG. 64 and other figures are assigned the same reference numerals, and a resistor 92 is provided on the board B-side 22B side of the through-hole 78. The resistor 92 is provided near the middle between the board A-side 22A and the board B-side 22B, and its resistance value can be set to a value that suppresses multiple reflections from the board A-side 22A and ensures a high S / N ratio. The resistor 92 may be a chip resistor, as with the resistor 91, or may be formed by adjusting the width of the transmission line. The measurement device 1 of FIG. 65 differs from the measurement device 1 of FIG. 64 in that it includes a resistor 92 instead of the resistor 91. However, the other configurations are similar, and therefore, a description thereof will be omitted.

[0177] 64 and 65, the characteristic impedance of the transmission line is not limited to 50 Ω and may be another value such as 100 Ω. Also, in Figures 64 and 65, substrate A surface 22A is configured as MP sensor 72 and substrate B surface 22B as moisture sensor 75, but substrate A surface 22A may be configured as a moisture sensor to measure the moisture amount (volumetric moisture content), and substrate B surface 22B may be configured as an MP sensor to measure the matric potential.

[0178] <Configuration with Calibration Standard Mounted> In the measurement apparatus 1, the probe 12 may be configured to include a calibration standard for the VNA 11. By providing the calibration standard on the substrate 22 in the probe 12, it is possible to perform calibration of the VNA 11 on the substrate 22, and to constantly correct for changes in the VNA 11 over time.

[0179] <First Configuration Example> Figure 66 is a diagram showing a first example of the detailed configuration of a measurement apparatus 1 equipped with a probe 12 having a VNA calibration standard. The measurement apparatus 1 in Figure 66 corresponds to the configuration of the measurement apparatus 1 in Figures 1 and 9. In Figure 66, the measurement apparatus 1 is made up of a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to one port (Port 1 terminal) of the VNA 11 via an RF cable 13.

[0180] In the probe 12, a branch switch 71, an MP sensor 72, and a calibration standard 81 are provided on the substrate A surface 22A. That is, porous ceramics 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and an RF signal from the VNA 11 is transmitted to the transmission line 31A, thereby being used as the MP sensor 72. The tip of the transmission line 31A is terminated (loaded) by a resistor. Alternatively, the tip of the transmission line 31A may be open or shorted. In the probe 12 of Figure 66, only one surface of the substrate 22 (substrate A surface 22A) is used as a sensor.

[0181] The calibration standard 81 is a standard used to calibrate measurements by the VNA 11. For example, the calibration standard 81 has three types of standards: short, open, and load. The branch switch 71 is composed of, for example, an SP4T RF switch. The branch switch 71 may be composed of a single switch or multiple switches. The branch switch 71 is electrically connected to the RF connector 21 that is connected to the RF cable 13, and switches the connection destination of the RF cable 13 between the transmission line 31A formed on the substrate A surface 22A or the calibration standard 81 based on a control signal from the control circuit 10.

[0182] Specifically, the branch switch 71 has, as switchable terminals, a first terminal connected to the short standard via a signal line, a second terminal connected to the open standard via a signal line, a third terminal connected to the terminator via a signal line, and a fourth terminal connected to the transmission line 31A. The branch switch 71 switches the connection destination of the RF cable 13, which is electrically connected to the Port1 terminal of the VNA 11, between the first terminal, the second terminal, the third terminal, and the fourth terminal, in accordance with a control signal from the control circuit 10.

[0183] Since a short standard (short), an open standard (open), and a terminator (load) can be used as calibration standards for the VNA 11, calibration of the VNA 11 can be performed on the board A surface 22A. In FIG. 66 , the VNA calibration plane C indicated by the dashed line is the calibration plane of the VNA 11, and the signal line between the branch switch 71 and the calibration standard 81 is the calibration plane. For example, when no measurement is being performed, the branch switch 71 switches the connection destination of the RF cable 13 to the third terminal, thereby enabling calibration of the VNA 11 using a terminator. During measurement, the branch switch 71 switches the connection destination of the RF cable 13 to the fourth terminal, thereby enabling measurement using the MP sensor 72. Calibration of the VNA 11 using the calibration standard 81 may be performed before each measurement, or may be performed after a predetermined period (such as a preset deadline) has elapsed.

[0184] In this way, in the measurement apparatus 1 of FIG. 66 , when the VNA 11 and probe 12 are connected via one port (Port 1 terminal) and the board A surface 22A is used as the MP sensor 72, by mounting the calibration standard 81 on the board A surface 22A, the VNA 11 can be calibrated on the board A surface 22A, allowing for constant correction of changes over time in the VNA 11. Furthermore, because calibration is typically performed by manually attaching the calibration standard to the circuit before shipping, calibration can essentially only be performed at the time of shipment. By using the measurement apparatus 1 of FIG. 66 , there is no need to manually attach and calibrate the VNA calibration standard at the time of shipment from the factory, and VNA calibration can be automated. Furthermore, after shipping, changes over time in the measurement apparatus 1 of FIG. 66 can be periodically calibrated.

[0185] 66 shows a configuration in which the substrate A surface 22A is used as an MP sensor 72, but a configuration in which the porous ceramic 26 is removed and the substrate is used as a moisture sensor may also be adopted. Similarly, in a configuration in which the substrate is used as a moisture sensor, the calibration standard 81 can be mounted on the substrate A surface 22A.

[0186] <Second Configuration Example> Figure 67 is a diagram showing a second example of the detailed configuration of a measurement apparatus 1 equipped with a probe 12 having a VNA calibration standard. The measurement apparatus 1 in Figure 67 corresponds to the configuration of the measurement apparatus 1 in Figures 1 and 48. In Figure 67, the measurement apparatus 1 is made up of a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to one port (Port 1 terminal) of the VNA 11 via an RF cable 13.

[0187] In the probe 12, a branch switch 71, an MP sensor 72, and a calibration standard 81 are provided on the substrate A surface 22A. That is, a porous ceramic 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and an RF signal from the VNA 11 is transmitted to the transmission line 31A, thereby being used as the MP sensor 72. The tip of the transmission line 31A is terminated (loaded) by a resistor. Alternatively, the tip of the transmission line 31A may be open or shorted.

[0188] Furthermore, a moisture sensor 75 is provided on the substrate B surface 22B of the probe 12. That is, a transmission line 31B is formed on the substrate B surface 22B, and is used as the moisture sensor 75 by transmitting an RF signal from the VNA 11. The tip of the transmission line 31B is terminated (loaded) by a resistor. Alternatively, the tip of the transmission line 31B may be open or shorted.

[0189] The calibration standard 81 is a standard used to calibrate the VNA 11, and includes, for example, three types of standards: short, open, and load. The branch switch 71 is electrically connected to the RF connector 21 that is connected to the RF cable 13, and switches the connection destination of the RF cable 13 to the transmission line 31A, the transmission line 31B, or the calibration standard 81 formed on the substrate A surface 22A based on a control signal from the control circuit 10.

[0190] Specifically, the branch switch 71 is configured by, for example, an SP5T RF switch or the like, and has, as switchable terminals, a first terminal connected to a short standard via a signal line, a second terminal connected to an open standard via a signal line, a third terminal connected to a terminator via a signal line, a fourth terminal connected to transmission line 31A, and a fifth terminal connected to transmission line 31B via a through hole 74 that penetrates the substrate 22. In accordance with a control signal from the control circuit 10, the branch switch 71 switches the connection destination of the RF cable 13 electrically connected to the Port 1 terminal of the VNA 11 to the first terminal, second terminal, third terminal, fourth terminal, or fifth terminal.

[0191] The short standard (short), open standard (open), and terminator (load) can be used as calibration standards for the VNA 11, so that the VNA 11 can be calibrated on the substrate A surface 22A. Calibration of the VNA 11 using the calibration standard 81 may be performed before each measurement, or may be performed after a predetermined period of time has elapsed.

[0192] In this way, in the measurement apparatus 1 of FIG. 67 , when the VNA 11 and probe 12 are connected via one port (Port 1 terminal), the substrate A surface 22A is used as the MP sensor 72, and the substrate B surface 22B is used as the moisture sensor 75, by mounting the calibration standard 81 on the substrate A surface 22A, the VNA 11 can be calibrated on the substrate A surface 22A. This allows for constant correction of changes over time in the VNA 11. Furthermore, by using the measurement apparatus 1 of FIG. 67 , it is no longer necessary to manually install and calibrate the VNA calibration standard at the time of shipment from the factory, and VNA calibration can be automated. Furthermore, after shipment of the measurement apparatus 1 of FIG. 67 , changes over time in the measurement apparatus 1 can be periodically calibrated.

[0193] 67 shows a configuration in which the substrate A surface 22A is used as the MP sensor 72 and the substrate B surface 22B is used as the moisture sensor 75, but it is also possible to use a configuration in which both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, or a configuration in which both surfaces are used as moisture sensors. When both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, it is possible to use porous ceramics with different properties for each surface.

[0194] <Third Configuration Example> Figure 68 is a diagram showing a third example of the detailed configuration of a measurement apparatus 1 equipped with a probe 12 having a VNA calibration standard. The measurement apparatus 1 in Figure 68 corresponds to the configuration of the measurement apparatus 1 in Figures 2 and 34. In Figure 68, the measurement apparatus 1 is composed of a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to two ports (Port1 terminal, Port2 terminal) of the VNA 11 via RF cables 13 and 14.

[0195] In the probe 12, branch switches 71-1 and 71-2, an MP sensor 72, and a calibration standard 81 are provided on the substrate A surface 22A. That is, porous ceramics 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and an RF signal from the VNA 11 is transmitted to the transmission line 31A, thereby being used as the MP sensor 72. In the probe 12 of Figure 68, only one surface of the substrate 22 (substrate A surface 22A) is used as a sensor.

[0196] The calibration standards 81 are standards used to calibrate the VNA 11, and include, for example, four types of standards: short, open, load, and through. The branch switch 71-1 is electrically connected to the RF connector 21-1 that is connected to the RF cable 13, and switches the connection destination of the RF cable 13 between the transmission line 31A formed on the substrate A surface 22A or the calibration standard 81 based on a control signal from the control circuit 10. The branch switch 71-2 is electrically connected to the RF connector 21-2 that is connected to the RF cable 14, and switches the connection destination of the RF cable 14 between the transmission line 31A or the calibration standard 81 based on a control signal from the control circuit 10.

[0197] Specifically, the branch switch 71-1 is configured by, for example, an SP5T RF switch or the like, and has, as switchable terminals, a first terminal connected to a short standard via a signal line, a second terminal connected to an open standard via a signal line, a third terminal connected to a terminator via a signal line, a fourth terminal connected to a transmittance device via a signal line, and a fifth terminal connected to (one end of) the transmission line 31A. The branch switch 71-1 switches the connection destination of the RF cable 13, which is electrically connected to the Port1 terminal of the VNA 11, to the first terminal, second terminal, third terminal, fourth terminal, or fifth terminal, in accordance with a control signal from the control circuit 10.

[0198] The branch switch 71-2 is configured by, for example, an SPDT RF switch, and has, as terminals to be switched, a sixth terminal connected to the transmittance device via a signal line and a seventh terminal connected to (the other end of) the transmission line 31A. The branch switch 71-2 switches the connection destination of the RF cable 14, which is electrically connected to the Port 2 terminal of the VNA 11, between the sixth terminal and the seventh terminal in accordance with a control signal from the control circuit 10.

[0199] For example, when no measurement is being performed, the branch switch 71-1 switches the connection destination of the RF cable 13 to the fourth terminal, and the branch switch 71-2 switches the connection destination of the RF cable 14 to the sixth terminal, thereby enabling calibration of the VNA 11 using a transmissive sensor as the calibration standard 81. During measurement, the branch switch 71-1 switches the connection destination of the RF cable 13 to the fifth terminal, and the branch switch 71-2 switches the connection destination of the RF cable 14 to the seventh terminal, thereby enabling measurement using the MP sensor 72. Calibration of the VNA 11 using the calibration standard 81 may be performed before each measurement, or may be performed after a predetermined period of time has elapsed. Note that a DP7T (Double-Pole 7 Throw) switch or the like may be used as a single switch that integrates the branch switches 71-1 and 71-2.

[0200] In this way, in the measurement apparatus 1 of FIG. 68 , when the VNA 11 and the probe 12 are connected via two ports (Port 1 terminal and Port 2 terminal) and the board A surface 22A is used as the MP sensor 72, the calibration standard 81 for the VNA 11 can be mounted on the board A surface 22A, thereby enabling calibration of the VNA 11 on the board A surface 22A. This allows for constant correction of changes over time in the VNA 11. Furthermore, by using the measurement apparatus 1 of FIG. 68 , it is no longer necessary to manually install and calibrate the VNA calibration standard at the time of shipment from the factory, and calibration of the VNA can be automated. Furthermore, after shipment of the measurement apparatus 1 of FIG. 68 , changes over time in the measurement apparatus 1 can be periodically calibrated.

[0201] 68 shows a configuration in which the substrate A surface 22A is used as an MP sensor 72, but a configuration in which the porous ceramic 26 is removed and the substrate is used as a moisture sensor may also be adopted. Similarly, in a configuration in which the substrate is used as a moisture sensor, the calibration standard 81 can be mounted on the substrate A surface 22A.

[0202] <Fourth Configuration Example> Figure 69 is a diagram showing a fourth example of the detailed configuration of a measurement apparatus 1 equipped with a probe 12 having a VNA calibration standard. The measurement apparatus 1 in Figure 69 corresponds to the configuration of the measurement apparatus 1 in Figures 2 and 57. In Figure 69, the measurement apparatus 1 is composed of a control circuit 10, a VNA 11, and a probe 12. The probe 12 is electrically connected to two ports (Port1 terminal, Port2 terminal) of the VNA 11 via RF cables 13 and 14.

[0203] In the probe 12, branch switches 71-1 and 71-2, an MP sensor 72, and a calibration standard 81 are provided on the substrate A surface 22A. That is, porous ceramics 26 is placed on the substrate A surface 22A on which the transmission line 31A is formed, and is used as the MP sensor 72 by transmitting an RF signal from the VNA 11 to the transmission line 31A. In the probe 12, a moisture sensor 75 is provided on the substrate B surface 22B. That is, a transmission line 31B is formed on the substrate B surface 22B, and is used as the moisture sensor 75 by transmitting an RF signal from the VNA 11.

[0204] In the probe 12, a through-hole 78 is formed at the tip of the transmission line 31A formed on the substrate A-side 22A, and the transmission line 31A and the transmission line 31B formed on the substrate B-side 22B are electrically connected via the through-hole 78. In addition, in the probe 12, a through-hole 79 is formed at the tip of the transmission line 31B, and the transmission line 31B and the branch switch 71-2 are electrically connected via the through-hole 79.

[0205] The calibration standards 81 are standards used to calibrate the VNA 11, and include, for example, four types of standards: short, open, load, and through. The branch switch 71-1 is electrically connected to the RF cable 13, and switches the connection destination of the RF cable 13 between the transmission line 31A formed on the substrate A surface 22A and the calibration standard 81 based on a control signal from the control circuit 10. The branch switch 71-2 is electrically connected to the RF cable 14, and switches the connection destination of the RF cable 14 between the transmission line 31B and the calibration standard 81 based on a control signal from the control circuit 10.

[0206] Specifically, the branch switch 71-1 is configured by, for example, an SP5T RF switch or the like, and has, as switchable terminals, a first terminal connected to a short standard via a signal line, a second terminal connected to an open standard via a signal line, a third terminal connected to a terminator via a signal line, a fourth terminal connected to a transmittance device via a signal line, and a fifth terminal connected to the transmission line 31A. The branch switch 71-1 switches the connection destination of the RF cable 13, which is electrically connected to the Port1 terminal of the VNA 11, to the first terminal, second terminal, third terminal, fourth terminal, or fifth terminal, in accordance with a control signal from the control circuit 10.

[0207] The branch switch 71-2 is configured by, for example, an SPDT RF switch, and has, as terminals to be switched, a sixth terminal connected to the transmittance device via a signal line, and a seventh terminal connected to the transmission line 31B via a through hole 79. In accordance with a control signal from the control circuit 10, the branch switch 71-2 switches the connection destination of the RF cable 14, which is electrically connected to the Port 2 terminal of the VNA 11, between the sixth terminal and the seventh terminal.

[0208] For example, when no measurement is being performed, the branch switch 71-1 switches the connection destination of the RF cable 13 to the fourth terminal, and the branch switch 71-2 switches the connection destination of the RF cable 14 to the sixth terminal, thereby enabling calibration of the VNA 11 using a transmittance sensor as the calibration standard 81. During measurement, the branch switch 71-1 switches the connection destination of the RF cable 13 to the fifth terminal, and the branch switch 71-2 switches the connection destination of the RF cable 14 to the seventh terminal, thereby enabling measurement using the MP sensor 72 and the moisture sensor 75, as shown in FIG. 57. Calibration of the VNA 11 using the calibration standard 81 may be performed before each measurement, or may be performed after a predetermined period of time has elapsed. Note that a DP7T switch or the like may be used as a single switch that integrates the branch switches 71-1 and 71-2.

[0209] In this way, in the measurement apparatus 1 of FIG. 69 , when the VNA 11 and the probe 12 are connected via two ports (Port 1 terminal and Port 2 terminal), the substrate A surface 22A is used as the MP sensor 72, and the substrate B surface 22B is used as the moisture sensor 75, by mounting the calibration standard 81 of the VNA 11 on the substrate A surface 22A, the VNA 11 can be calibrated on the substrate A surface 22A. This allows for constant correction of changes over time in the VNA 11. Furthermore, by using the measurement apparatus 1 of FIG. 69 , it is no longer necessary to manually install and calibrate the VNA calibration standard at the time of shipment from the factory, and VNA calibration can be automated. Furthermore, after shipment of the measurement apparatus 1 of FIG. 69 , changes over time in the measurement apparatus 1 can be periodically calibrated.

[0210] 69 shows a configuration in which the substrate A surface 22A is used as the MP sensor 72 and the substrate B surface 22B is used as the moisture sensor 75, but it is also possible to use a configuration in which both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, or a configuration in which both surfaces are used as moisture sensors. When both the substrate A surface 22A and the substrate B surface 22B are used as MP sensors, it is possible to use porous ceramics with different properties for each surface.

[0211] <<Modifications>> In the above-described embodiment, the measurement device 1 is configured such that the VNA 11 and the probe 12 are connected via the RF cable 13. However, an integrated configuration including circuits such as the VNA 11 on the probe 12 side may also be adopted.

[0212] Figure 70 is a diagram showing a first example of a configuration in which a VNA 11 and a probe 12 are integrated. In Figure 70, parts corresponding to those in Figure 9 and the like are given the same reference numerals, and their description will be omitted where appropriate. As shown in Figure 70, the probe 12 has a transmission line 31 formed on the surface of a substrate 22, and a porous ceramic 26 placed on top of that surface. In addition, the circuit of the VNA 11 is disposed on top of the surface of the substrate 22 and is electrically connected, and the circuit of the VNA 11 is integrated with the substrate 22 of the probe 12.

[0213] A control circuit 111 and a communication circuit 112 are connected to the probe 12 via a cable 121. The control circuit 111 is configured, for example, by a microcontroller including a CPU (Central Processing Unit), a memory, etc. The control circuit 111 can control the VNA 11 by supplying a control signal to the probe 12 via the cable 121. The communication circuit 112 can connect to a network such as the Internet according to a predetermined communication method and exchange data with devices such as a server. For example, under control of the control circuit 111, the communication circuit 112 can transmit measurement data measured by the probe 12 to a server via the network. The control circuit 111 and the communication circuit 112 are connected to a power supply 113 and can supply power to the probe 12 via the cable 121. The control circuit 111 and the communication circuit 112 may be configured in the same device or in different devices.

[0214] Figure 71 is a diagram showing a second example of a configuration in which the VNA 11 and the probe 12 are integrated. In Figure 71, compared to Figure 70, the control circuit 111 is arranged above the surface of the substrate 22, and the circuits of the VNA 11 and the control circuit 111 are integrated into the substrate 22 of the probe 12. In other words, the probe 12 is configured to include the VNA 11 and the control circuit 111. The control circuit 111 controls the VNA 11. In Figure 71, the communication circuit 112 is connected to a power source 113, and can supply power to the probe 12 via a cable 121.

[0215] Figure 72 is a diagram showing a third example of a configuration in which the VNA 11 and the probe 12 are integrated. In comparison with Figure 71 , Figure 72 shows that the communication circuit 112 is disposed on the upper surface of the substrate 22, and the circuits of the VNA 11, the control circuit 111, and the communication circuit 112 are integrated into the substrate 22 of the probe 12. That is, the probe 12 is configured to include the VNA 11, the control circuit 111, and the communication circuit 112. The communication circuit 112 can be connected to a network to exchange data with other devices. In Figure 72 , a power supply 113 can supply power to the probe 12 via a cable 121.

[0216] Figure 73 is a diagram showing a fourth example of a configuration in which the VNA 11 and the probe 12 are integrated. In Figure 73, compared to Figure 72, the circuit for the power supply 113 is arranged above the surface of the substrate 22, and the circuits for the VNA 11, control circuit 111, communication circuit 112, and power supply 113 are integrated into the substrate 22 of the probe 12. In other words, the probe 12 is configured to include the VNA 11, control circuit 111, communication circuit 112, and power supply 113. The power supply 113 can supply power to the VNA 11, control circuit 111, and communication circuit 112.

[0217] In the above-described embodiment, the probe 12 is configured to form transmission lines on both surfaces (surface A and surface B) of the substrate 22, so that measurements are performed on the upper surface (surface A) of the substrate 22 as an MP sensor and the lower surface (surface B) as a moisture sensor, but a configuration in which both the MP sensor and the moisture sensor are realized on one surface (single surface) may also be adopted.

[0218] Figure 74 shows a first example of the configuration of a probe 12 that performs measurements using an MP sensor and a moisture sensor on one surface. In Figure 74, parts corresponding to those in Figure 9 and other figures are given the same reference numerals, and their descriptions will be omitted as appropriate. As shown in Figure 74, the probe 12 has a transmission line 31 formed on the surface of a substrate 22, and a porous ceramic 26 and soil 141 placed on top of the surface. That is, in the probe 12 of Figure 74, a portion of the transmission line 31 on one surface (side A) of the substrate 22 is in contact with the soil 141 to be measured, and the porous ceramic 26 is placed on the remaining portion of the transmission line 31.

[0219] By adopting this configuration, a portion of the transmission line 31 of the substrate 22 comes into contact with the soil to be measured, thereby enabling measurement of the amount of moisture (volumetric water content) contained in the soil. By placing the porous ceramic 26 on the remaining portion of the transmission line 31, the matric potential of the soil to be measured can be measured. Therefore, with the probe 12 of FIG. 74, measurements can be simultaneously performed using the MP sensor and moisture sensor on one side (side A) of the substrate 22. In the configuration shown in FIG. 74, measurements are performed using the MP sensor and moisture sensor on one side (side A) of the substrate 22, eliminating the need for the through-hole 74 of FIG. 48, the through-hole 78 of FIG. 55, and the through-holes 78 and 79 of FIG. 57. While FIG. 74 illustrates a configuration in which the VNA 11 and the probe 12 are connected via an RF cable 13, the VNA 11 may be integrated with the probe 12, as shown in FIGS. 70 to 73.

[0220] FIG. 75 shows a second example of the configuration of a probe 12 that performs measurements using an MP sensor and a moisture sensor on one surface. In FIG. 75, parts corresponding to those in FIG. 34 and other figures are designated by the same reference numerals, and their description will be omitted where appropriate. In FIG. 75, the VNA 11 has two ports (Port 1 and Port 2), and the probe 12 is connected to the Port 1 and Port 2 terminals via RF cables 13 and 14 to measure the transmission coefficient (S21). Even with this configuration, as shown in FIG. 75, the probe 12 can be configured such that a portion of the transmission line 31 on one surface (Surface A) of the substrate 22 contacts the soil 141 to be measured, and porous ceramic 26 is placed on the remaining portion of the transmission line 31. By adopting this configuration, the probe 12 of FIG. 75 can simultaneously perform measurements using an MP sensor and a moisture sensor on one surface (Surface A) of the substrate 22. In addition, in Figures 74 and 75, the positions of the porous ceramics 26 and the soil 141 may be interchanged.

[0221] The embodiments of the present disclosure are not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure. For example, the embodiments of the present disclosure may be implemented singly or in combination with two or more embodiments. More specifically, in the measuring device 1 of the second embodiment (FIGS. 9, 34, etc.), the porous ceramic 26 may have the characteristics of the third embodiment (FIGS. 40 to 43) and the shape of the fourth embodiment (FIGS. 44 to 46). Furthermore, in the measuring device 1 of the fifth embodiment (FIGS. 48, 55, 57, etc.), the porous ceramic 26 may have the characteristics of the third embodiment (FIGS. 40 to 43) and the shape of the fourth embodiment (FIGS. 44 to 46). Note that the effects described herein are merely examples and are not limiting, and other effects may also be present.

[0222] In the embodiment of the present disclosure, the matric potential and the amount of water contained in the soil (volumetric water content) have been described as indicators of the properties of the soil to be measured, but other indicators may also be measured. Also, in the embodiment of the present disclosure, the probe 12 has been described as being used as a moisture sensor, but it is not limited to being used as a moisture sensor, and it may also be used as a sensor for measuring the dielectric constant of a liquid or the dielectric constant of a substance, for example.

[0223] The present disclosure can also be configured as follows.

[0224] (1) A measuring device comprising: a probe that contacts soil to be measured; and a measuring unit that measures an index representing a property of the soil, wherein the probe has a substrate with a transmission line formed on a surface thereof, and the measuring unit transmits a high-frequency signal to the transmission line to measure the index. (2) The measuring device according to (1), wherein the probe places a substance having a wide pore size distribution on top of the surface of the substrate, and the measuring unit measures the matric potential of the soil. (3) The measuring device according to (2), wherein the substance is porous ceramic. (4) The measuring device according to (3), wherein the pore size distribution of the porous ceramic is in the range of 0.1 to 100 μm and is a substantially flat distribution. (5) The moisture retention curve of the porous ceramic has a functional shape expressed by the following equation in the range 0≦pF≦a, where pF is pressure and θ is volumetric water content, and a, b, c, d, and f are parameters: In the range of a < pF < b, the function shape is expressed by the following equation: In the parameters, a is the intersection of the lines expressed by the above two equations, b is the pressure at the permanent wilting point, c is the volumetric water content, d is the saturated volumetric water content, and f is the residual volumetric water content. (6) The measuring device according to any of (3) to (5), wherein the porous ceramic has an uneven structure on the surface that contacts the soil. (7) The measuring device according to any of (3) to (6), wherein the substrate has drainage holes that allow the soil to contact the porous ceramic from the surface opposite to the surface, and through holes that drain excess water. (8) The measuring device according to any of (1) to (7), wherein the substrate has a signal line of the transmission line formed in a U-shape on the surface. (9) The measuring device according to any of (1) to (8), wherein the end circuit of the transmission line is a termination resistor. (10) The measuring device according to any one of (3) to (7), wherein the substrate has a flat contact surface with the porous ceramic placed on top of its surface. (11) The measuring device according to (10), wherein the substrate has an ultra-thin coating formed on the contact surface. (12) The measuring device according to (2), wherein the substance is soil or glass beads. (13) The measuring device according to (1), wherein the measuring unit measures the amount of moisture contained in the soil. (14) The measuring device according to any one of (1) to (11), wherein the probe is electrically connected to a first port of a VNA having the measuring unit via a first cable, and the measuring unit measures the index based on a reflection coefficient measured by transmitting the high-frequency signal to the transmission line. (15) The measuring device according to (14), wherein the probe further has a standard device on the surface of the substrate that can be electrically connected to the first cable and is used to calibrate the measurement, and a switch on the surface of the substrate that switches the connection destination of the first cable to the transmission line or the standard device.(16) The measurement device according to (15), wherein the standard device has a short-circuit standard device, an open-circuit standard device, and a terminator, and the switch switches the connection destination of the first cable to a first terminal connected to the short-circuit standard device, a second terminal connected to the open-circuit standard device, a third terminal connected to the terminator, or a fourth terminal connected to the transmission line based on a control signal from a control circuit. (17) The measurement device according to any of (1) to (11), wherein the probe is electrically connected to a first port of a VNA having the measurement unit via a first cable and to a second port of the VNA via a second cable, and the measurement unit measures the index based on a transmission coefficient measured by transmitting the high-frequency signal to the transmission line. (18) The measuring device described in (17), wherein the probe further has a standard on the surface of the substrate that is electrically connectable to the first cable and the second cable and that is used to calibrate the measurement, and the measuring device further has a first switch on the surface of the substrate that switches the connection destination of the first cable to the transmission line or the standard, and a second switch that switches the connection destination of the second cable to the transmission line or the standard, (19) The measurement device according to (18), wherein the standard device has a short-circuit standard device, an open-circuit standard device, a terminator, and a transmission device, and the first switch switches the connection destination of the first cable to a first terminal connected to the short-circuit standard device, a second terminal connected to the open-circuit standard device, a third terminal connected to the terminator, a fourth terminal connected to the transmission device, or a fifth terminal connected to one end of the transmission line based on a control signal from a control circuit, and the second switch switches the connection destination of the second cable to a sixth terminal connected to the transmission device or a seventh terminal connected to the other end of the transmission line based on a control signal from the control circuit. (20) The measurement device according to any of (1) to (19), wherein the measurement unit is electrically connected to the probe via a cable. (21) The measurement device according to any of (1) to (20), wherein the probe has the measurement unit.(22) A measurement method in which a measurement device transmits a high-frequency signal to a transmission line formed on the surface of a substrate of a probe that contacts the soil to be measured, and measures an index representing the characteristics of the soil.

[0225] 1 Measuring device, 10 Control circuit, 11 VNA, 11A Measuring unit, 12 Probe, 13 RF cable, 14 RF cable, 20 Housing, 21, 21-1, 21-2 RF connector, 22 Substrate, 22A Substrate A surface, 22B Substrate B surface, 23 Soil, 24, 24-1, 24-2 Resin part, 25 Wall part, 26 Porous ceramics, 26A Convex part, 26B Concave part, 31 Transmission line, 31A Transmission line, 31B Transmission line, 33-1 to 33-3 Drainage hole, 34 Resistor, 35 Through hole, 51 Substrate, 52 Conductor layer, 53 Solder resist, 54 Ultra-thin film coating agent, 60 Jig, 61 Upper part of jig, 62 Lower part of jig, 71 branch switch, 72 MP sensor, 73 resistor, 74 through hole, 75 moisture sensor, 76 resistor, 77, 77-1, 77-2 multi-branch switch, 78 through hole, 79 through hole, 81 calibration standard, 91 resistor, 92 resistor, 111 control circuit, 112 communication circuit, 113 power supply, 121 cable, 141 soil

Claims

1. A measuring device comprising a probe that contacts the soil to be measured and a measuring unit that measures an index representing the characteristics of the soil, wherein the probe has a substrate on which a transmission line is formed on the surface layer, and the measuring unit transmits a high-frequency signal to the transmission line and measures the index.

2. The measuring device according to claim 1, wherein the probe places a substance having a wide pore size distribution on the upper part of the surface layer of the substrate, and the measuring unit measures the matric potential of the soil.

3. The measuring device according to claim 2, wherein the substance is a porous ceramic.

4. The measuring device according to claim 3, wherein the pore size distribution of the porous ceramic is distributed in the range of 0.1 to 100 μm and has a substantially flat distribution.

5. When the pressure is pF and the volumetric water content is θ, the water retention curve of the porous ceramic is represented by a functional shape expressed by the following equation in the range of 0 ≦ pF ≦ a, with a, b, c, d, and f as parameters: In the range of a < pF < b, it has a functional shape expressed by the following equation: In the above parameters, a is the intersection point of the straight lines represented by the above two equations, b is the pressure at the permanent wilting point, c is the volumetric water content, d is the saturated volumetric water content, and f is the residual volumetric water content. The measuring device according to claim 3.

6. The measuring device according to claim 3, wherein the porous ceramic has an uneven structure on the surface in contact with the soil.

7. The measuring device according to claim 3, wherein the substrate is formed with a drainage hole for bringing the soil into contact with the porous ceramic from the surface on the opposite side of the surface layer and a through hole for draining excess water.

8. The measuring device according to claim 3, wherein the signal line of the transmission line is formed in a U shape on the surface layer.

9. The measuring device according to claim 3, wherein the tip circuit of the transmission line is a termination resistor.

10. The measuring device according to claim 3, wherein the contact surface of the substrate with the porous ceramic placed on the upper part of the surface layer is formed flat.

11. The measuring device according to claim 10, wherein an ultra-thin film coating agent is formed on the contact surface.

12. The measuring device according to claim 2, wherein the substance is soil or glass beads.

13. The measuring device according to claim 1, wherein the measuring unit measures the moisture content contained in the soil.

14. The measuring device according to claim 1, wherein the probe is electrically connected to the first port of a VNA having the measuring unit via a first cable, and the measuring unit measures the index based on the reflection coefficient measured by transmitting the high-frequency signal to the transmission line.

15. The probe is further provided with a standard device that can be electrically connected to the first cable on the surface layer of the substrate and is used for calibration of the measurement. The measurement device according to claim 14, further comprising a switch on the surface layer of the substrate for switching the connection destination of the first cable to the transmission line or the standard device.

16. The standard device includes a short-circuit standard device, an open-circuit standard device, and a terminator. The switch switches the connection destination of the first cable to a first terminal connected to the short-circuit standard device, a second terminal connected to the open-circuit standard device, a third terminal connected to the terminator, or a fourth terminal connected to the transmission line based on a control signal from a control circuit. The measurement device according to claim 15.

17. The probe is electrically connected to the first port of a VNA having the measurement unit via a first cable and to the second port of the VNA via a second cable. The measurement unit measures the index based on the transmission coefficient measured by transmitting the high-frequency signal to the transmission line. The measurement device according to claim 1.

18. The probe is further provided with a standard device that can be electrically connected to the first cable and the second cable on the surface layer of the substrate and is used for calibration of the measurement. The measurement device according to claim 17, further comprising a first switch on the surface layer of the substrate for switching the connection destination of the first cable to the transmission line or the standard device, and a second switch for switching the connection destination of the second cable to the transmission line or the standard device.

19. The standard device includes a short-circuit standard device, an open-circuit standard device, a terminator, and a through device. The first switch switches the connection destination of the first cable to a first terminal connected to the short-circuit standard device, a second terminal connected to the open-circuit standard device, a third terminal connected to the terminator, a fourth terminal connected to the through device, or a fifth terminal connected to one end of the transmission line based on a control signal from a control circuit. The second switch switches the connection destination of the second cable to a sixth terminal connected to the through device or a seventh terminal connected to the other end of the transmission line based on a control signal from a control circuit. The measurement device according to claim 18.

20. The measurement unit is electrically connected via the probe and the cable. The measurement device according to claim 1.

21. The measuring device according to claim 1, wherein the probe has the measuring unit.

22. A measuring method in which a high-frequency signal is transmitted to a transmission line formed on the surface layer of a substrate included in a probe that contacts soil to be measured, and an index representing the characteristics of the soil is measured.

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