Dielectric property testing system and method based on free space terminal short circuit method under temperature gradient

Abstract

The application provides a dielectric property testing system and method based on a free space terminal short circuit method under a temperature gradient, and belongs to the technical field of microwave testing. The system innovatively designs a heating mode and a cooling mode based on the free space short circuit method, thereby constructing a temperature gradient on a microwave material to be tested, so that extraction of electromagnetic parameters under the action of the temperature gradient is realized. Meanwhile, an electromagnetic parameter extraction algorithm is designed. The system has the advantages of high automation, wide test frequency band, simple detection mode, high accuracy and the like, and can realize dielectric constant testing of a microwave frequency band under the action of a temperature gradient of 1000 DEG C on one side and -60 DEG C on the other side.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of microwave testing, and particularly relates to a dielectric property testing system and method based on a free space terminal short circuit method under a temperature gradient. BACKGROUND

[0002] Under the background of rapid development of science and technology, microwave materials have been widely used in many fields such as aerospace, military equipment, medical equipment, etc. For example, aerospace, satellite communication, radar navigation, infrared remote sensing, etc. When the aircraft, missile, rocket and other aerospace equipment fly at high speed, the friction between the high-speed airflow formed by the air and the head of the equipment increases, which leads to a sharp rise in the temperature of the head material, while the temperature inside the equipment is relatively low. At this time, the microwave material is in a temperature gradient environment. The dielectric parameters of the microwave material show nonlinear changes under the temperature gradient environment, which will have a significant impact on the signal transmission and reception of the aircraft antenna. Therefore, it is of great practical significance to accurately test and analyze the changes of the dielectric parameters of these microwave materials under the temperature gradient environment.

[0003] In view of the demand for extracting electromagnetic parameters of microwave materials under variable temperature, Li Jianqiao's paper "Research on Variable Temperature Testing Technology of Material Dielectric Properties Based on Free Space Terminal Short Circuit Method" uses the free space terminal short circuit method to test the microwave material under variable temperature. By designing a new metal ellipsoid reflecting surface, a feed antenna and a metal reflecting plate, a high-temperature testing system is built, which can test the microwave band of 15GHz-17GHz and the measurable temperature range is room temperature-1000℃. Cai Linhong's paper "Dielectric Property Testing Technology of Absorbing Materials under Temperature Field and Pressure Field" realizes the extraction of electromagnetic parameters of microwave materials under the combined action of high temperature and high pressure in a wide frequency band. The system can realize the testing under the combined action of room temperature-1000℃ and pressure of 0-12.5kPa.

[0004] However, the above methods are only for extracting electromagnetic parameters of microwave materials under a single temperature field, and there is no related technical solution to extract electromagnetic parameters under a temperature gradient field. Therefore, how to realize the extraction of electromagnetic parameters of microwave materials under a temperature gradient has important practical significance for current microwave material testing. SUMMARY

[0005] In view of the problems existing in the background art, the purpose of the present application is to provide a dielectric property testing system and method based on a free space terminal short circuit method under a temperature gradient. The system innovatively designs the heating and cooling methods based on the free space short circuit method, thereby constructing a temperature gradient on the microwave material to be tested, so as to realize the extraction of electromagnetic parameters under the action of a temperature gradient. At the same time, an electromagnetic parameter extraction algorithm is designed. The system of the present application has the advantages of high automation, wide test frequency band, simple detection method, high accuracy, etc.

[0006] To achieve the above object, the technical scheme of the present application is as follows:

[0007] The temperature gradient-based dielectric property testing system based on the free space terminal short circuit method comprises a temperature control unit, a liquid cooling unit, a support, a metal short circuit plate, a heating unit, a metal ellipsoidal reflecting surface, a transceiving antenna, a data processing unit, a vector network analyzer, a first temperature sensor and a second temperature sensor.

[0008] The metal short circuit plate is arranged on the support, and the microwave material to be measured is arranged on the metal short circuit plate, and the heating unit is arranged on the microwave material to be measured; the temperature control unit is connected with the heating unit, and is used for controlling the heating unit to heat, so that the upper surface of the microwave material to be measured is kept at a first temperature; the liquid cooling tank is arranged in the metal short circuit plate, and the liquid cooling tank is connected with the liquid cooling unit, and the cooling liquid in the liquid cooling unit circulates in the liquid cooling tank, and is used for keeping the lower surface of the microwave material to be measured at a second temperature; the first temperature sensor is used for measuring the temperature of the upper surface of the microwave material to be measured, and the second temperature sensor is used for measuring the temperature of the lower surface of the microwave material to be measured.

[0009] The metal ellipsoidal reflecting surface is arranged directly above the wave-transparent material plate; the transceiving antenna is connected with the vector network analyzer through a coaxial cable, and the metal ellipsoidal reflecting surface is used for focusing the electromagnetic waves transmitted and received by the transceiving antenna, and making the transmitted electromagnetic waves on the surface of the microwave material to be measured approximate to a uniform plane wave, and the plane where the focus of the reflected electromagnetic waves is located coincides with the upper surface of the microwave material to be measured; the vector network analyzer is used for measuring the parameters of the microwave material to be measured in the free space, and transmitting the S parameters to the data processing unit for processing, so as to obtain the electromagnetic parameters of the microwave material to be measured.

[0010] Further, the transceiving antenna is a dual-mode horn antenna or a corrugated horn antenna, which can realize the testing in a super wide band range.

[0011] Further, the heating unit adopts metal heating grid lines, the arrangement direction of the grid lines is perpendicular to the E plane direction of the transceiving antenna, and the influence on the testing by using the free space method is reduced.

[0012] Further, the metal heating grid lines are obtained by arranging a plurality of heating rod arrays on the microwave material to be measured, the metal heating grid lines are not in direct contact with the surface of the microwave material to be measured, and the heating of the microwave material to be measured is more uniform; wherein the spacing between the adjacent two heating rods is determined according to the temperature uniformity and the influence on the measurement accuracy of the free space method, and is preferably 15 mm, the radius of the heating rod is preferably 1 mm, and the material of the heating rod is preferably silicon molybdenum material.

[0013] Further, the dielectric property testing system further comprises a first temperature display instrument and a second temperature display instrument 14, the first temperature display instrument 13 is connected with the first temperature sensor 11, the second temperature display instrument 14 is connected with the second temperature sensor 12, and the temperature display instrument is used for displaying the temperature value measured by the temperature sensor.

[0014] Further, the surface of the microwave material to be measured should be smooth and flat, so as to meet the condition that the propagation direction of the uniform plane wave is perpendicular to the surface of the microwave material to be measured.

[0015] Further, the metal short-circuit plate 4 and the support 3 are made of high-temperature metal, so as to ensure that the microwave material to be measured is not oxidized when being heated to a target high temperature, and the repeatability of the test is ensured; and the material of the metal elliptical reflecting surface 7 is preferably brass.

[0016] Further, the cooling liquid is alcohol or liquid nitrogen, etc.

[0017] Further, the first temperature is above 150 DEG C, and the second temperature is below -20 DEG C.

[0018] The application further provides a method for testing the dielectric constant based on the dielectric property testing system under the temperature gradient of the free space terminal short-circuit method.

[0019] Step 1, the metal calibration plate is placed on the plane where the focus of the electromagnetic wave reflected by the metal ellipsoidal reflecting surface is located, meanwhile, the placement direction of the transceiving antenna is adjusted, so that the E face direction of the antenna is the same as the arrangement direction of the metal grid line, and the spatial position of the antenna is adjusted, so that the reflection parameter S11 of the antenna is higher than the set threshold value.

[0020] Step 2, the parameters such as the test frequency bandwidth and the intermediate frequency bandwidth are set in the vector network analyzer, and the whole free terminal short-circuit system is calibrated at normal temperature SOL.

[0021] Step 3, the calibration reflecting plate is placed on the focal plane of the transceiving antenna, the upper surface of the calibration reflecting plate is heated to the first temperature by the heating unit, meanwhile, the lower surface of the calibration reflecting plate is cooled to the second temperature by the liquid cooling unit, after the temperature is stable, the single-port short-circuit reflection parameter under the required temperature gradient is measured by the vector network analyzer.

[0022] Step 4, the calibration reflecting plate is replaced by the microwave material to be measured, the thickness of the microwave material to be measured is the same as that of the calibration reflecting plate, the upper surface of the microwave material to be measured is heated to the first temperature by the heating unit, meanwhile, the lower surface of the microwave material to be measured is cooled to the second temperature by the liquid cooling unit, after the temperature is stable, the single-port short-circuit reflection parameter under the required temperature gradient is measured by the vector network analyzer.

[0023] Step 5. Using the calibration data obtained in step 3 and the test data obtained in step 4, the actual reflection parameters of the microwave material to be tested under the target temperature gradient are calculated, and the calculation method is as follows:

[0024]

[0025] Step 6. The dielectric constant of the microwave material to be tested under the target temperature gradient is obtained by inverting the real reflection coefficient of the microwave material to be tested obtained in step 5, and the specific process is as follows:

[0026]

[0027] Where T 1,2 and Γ 1,2 respectively represent the transmission coefficient and the reflection coefficient of region 1 to region 2, T 2,1 represents the transmission coefficient of region 2 to region 1, Γ 2,1 represents the reflection coefficient of region 2 to region 1, is the generalized reflection coefficient of region 1 to region 2, is the generalized reflection coefficient of region 2 to region 3, the subscript 1 represents region 1, which is an air region, the subscript 2 represents region 2, which is a material to be tested region, and the subscript 3 represents region 3, which is a metal short-circuit plate region, η1 is the wave impedance of air, η2 is the wave impedance of the material to be tested, η is the wave impedance of the material, ε r is the dielectric constant of the material, μ r is the magnetic permeability of the material, k1 is the wave number of the material to be tested, and d1 is the thickness of the material to be tested.

[0028] The surface of the material to be tested is partially reflective, and the surface of the metal reflecting plate is fully reflective. Because the surface of the metal reflecting plate is fully reflective, there is Therefore, the reflection coefficient Γ 1,2 of the surface of the material to be tested can be calculated; because the magnetic permeability μ r of the material to be tested is known, the dielectric constant ε r of the microwave material to be tested under the temperature gradient can be obtained.

[0029] In summary, due to the adoption of the above technical scheme, the beneficial effects of the present application are:

[0030] The application is based on free space terminal short circuit method, and by innovatively designing a test system, dielectric properties of a material to be tested under the action of temperature gradient can be measured. Compared with other resonance methods for measuring dielectric properties of microwave materials, the free space terminal short circuit method can more accurately measure microwave materials with large loss by calibrating the loss caused by the metal heating grid between the microwave material to be tested and the metal ellipsoidal reflecting surface in free space to zero before measurement. Meanwhile, the direction of the metal grid and the E plane direction of the antenna are the same during the test, which reduces the influence on the dielectric constant test based on the free space method. The test system of the application can realize dielectric constant test of microwave frequency under the action of temperature gradient of 1000 DEG C on one side and -60 DEG C on the other side. BRIEF DESCRIPTION OF DRAWINGS DETAILED DESCRIPTION OF THE INVENTION BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The figure is a structural schematic diagram of the dielectric property test system based on free space terminal short circuit method under temperature gradient of the application.

[0032] Figure 2 The figure is a structural schematic diagram of the metal heating grid in the dielectric property test system of the application.

[0033] Figure 3 The figure is a propagation path diagram of electromagnetic wave during the test of the application.

[0034] Figure 4 The figure is a test result diagram of example 1 of the application.

[0035] The figure is a structural schematic diagram of the dielectric property test system based on free space terminal short circuit method under temperature gradient of the application. DETAILED DESCRIPTION OF THE INVENTION

[0036] In order to make the purpose, technical scheme and advantages of the application more clear, the application is described in further detail below in combination with examples and drawings.

[0037] The figure is a structural schematic diagram of the dielectric property test system based on free space terminal short circuit method under temperature gradient of the application. Figure 1 The figure is a structural schematic diagram of the dielectric property test system based on free space terminal short circuit method under temperature gradient of the application.

[0038] The metal support 3 is two vertically and parallel arranged rectangular metal supports, the metal short circuit plate 4 is arranged on the metal support 3, the microwave material 5 to be measured is arranged on the metal short circuit plate, the metal heating grid line 6 is arranged on the microwave material 5 to be measured through a clamp, the microwave material to be measured is heated through heat convection, the temperature of the surface of the microwave material to be measured is uniform, the arrangement direction of the metal heating grid line is the same as the E plane of the transceiving antenna 8, the silicon molybdenum rod is arranged on the surface of the microwave material 5 to be measured uniformly, and the structural diagram of the metal heating grid line is shown in Figure 2 The temperature control cabinet 1 is connected with the metal heating grid line 6 and is used for controlling the metal heating grid line to heat, so that the upper surface of the microwave material 5 to be measured is kept at a first temperature; the liquid cooling tank is arranged in the metal short circuit plate 4, the liquid cooling tank is connected with the liquid cooling cabinet 2 through a pipeline, the cooling liquid in the liquid cooling cabinet circulates in the liquid cooling tank, and the lower surface of the microwave material 5 to be measured is kept at a second temperature; the first temperature sensor 11 is used for measuring the temperature of the upper surface of the microwave material 5 to be measured, and is connected with the first temperature display instrument 13 through a wire, so as to measure and read the temperature of the upper surface of the microwave material to be measured in real time; the second temperature sensor 12 is used for measuring the temperature of the lower surface of the microwave material 5 to be measured, and is connected with the second temperature display instrument 14 through a wire, so as to measure and read the temperature of the lower surface of the microwave material to be measured in real time;

[0039] The metal elliptical reflecting surface 7 is arranged directly above the microwave material plate 5 to be measured; the transceiving antenna 8 is connected with the vector network analyzer 10 through a coaxial cable, the metal elliptical reflecting surface 7 is used for focusing the electromagnetic waves emitted and received by the transceiving antenna 8, and makes the emitted electromagnetic waves on the surface of the microwave material 5 to be measured approximate to uniform plane waves, the focal plane of the metal elliptical reflecting surface 7 is coincided with the plane of the microwave material to be measured; the vector network analyzer 10 is used for measuring the S parameters of the microwave material to be measured in the free space, the computer 9 is used for controlling the network analyzer, realizing automatic measurement of the material, and calculating the electromagnetic parameters of the microwave material to be measured through the S parameters.

[0040] When the dielectric constant is tested by the free space terminal short circuit method, it is necessary to reduce the interference factors between the antenna and the material to be tested as much as possible to ensure the accuracy of the test results. However, since the temperature gradient needs to be constructed for the material to be tested, a heating unit and a cooling unit must be introduced. If the cooling unit is arranged on the upper cover plate, it will have a great influence on the test accuracy of the free space terminal short circuit method; if the heating unit is arranged on the upper cover plate, how to reduce the influence of the heating unit on the test accuracy of the free space terminal short circuit method becomes the key. The heating unit is designed innovatively, the shape, material and position of the heating unit are optimized, the interference of the physical structure on the antenna signal is reduced as much as possible, and the stability of the antenna signal is ensured. The non-contact heating technology is adopted to homogenize the temperature of the surface of the material to be tested, and the temperature of the material to be tested is accurately controlled. The scheme of the application not only improves the test accuracy, but also ensures the accurate control of the temperature gradient, and provides more reliable technical support for the dielectric constant test.

[0041] Embodiment 1

[0042] The quartz material is tested by the dielectric performance test system based on the free space terminal short circuit method under the temperature gradient, including the following steps:

[0043] Step 1. The metal plate is placed on the plane where the focal point of the reflected surface of the transceiving antenna is located, and the placement direction of the transceiving antenna is adjusted so that the E plane direction of the antenna is the same as the direction of the metal grid line, and the spatial position of the antenna is adjusted so that the reflection parameter S11 of the antenna is higher than-15dB;

[0044] Step 2. The test frequency bandwidth, intermediate frequency bandwidth and other parameters are set in the vector network analyzer, and the whole free terminal short circuit system is calibrated at room temperature SOL;

[0045] Step 3. The calibration reflection plate is placed on the focal plane of the transceiving antenna, i.e. the calibration reflection surface is placed on the top layer of the test, the upper surface of the calibration reflection surface is heated to 200℃ by the heating unit, and the lower surface of the calibration reflection surface is cooled to-20℃ by the liquid cooling unit, after the temperature is stable for 10 minutes, the single port short circuit reflection parameter under the required temperature gradient is measured by the vector network analyzer

[0046] Step 4. The calibration reflection plate is replaced by the quartz material and placed on the focal plane of the transceiving antenna, the upper surface of the material to be tested is heated to 200℃ by the heating unit, and the lower surface of the material to be tested is cooled to-20℃ by the alcohol liquid cooling unit, after the temperature is stable, the single port short circuit reflection parameter under the required temperature gradient is measured by the vector network analyzer

[0047]

[0048] Step 5. Using the calibration data obtained in step 3 and the test data obtained in step 4, the actual reflection parameters of the quartz material at the target temperature gradient are calculated as follows:

[0049]

[0050] Step 6. The dielectric constant of the quartz material at the target temperature gradient is obtained by inverting the real reflection coefficient of the quartz material obtained in step 5, and the specific process is as follows:

[0051] The propagation path diagram of electromagnetic wave is shown in Figure 3 , in which the gray area represents the quartz material to be measured, the oblique area represents the metal short circuit plate, H represents the magnetic field, k represents the propagation direction, E represents the electric field, the subscript 1 represents the air area, 2 represents the material to be measured, i represents the incidence, and r represents the reflection. Then for TE wave, we have:

[0052]

[0053]

[0054] T 1,2 = 1 + Γ 1,2 Γ 1,2 = -Γ 2,1 (4)

[0055]

[0056] where T 1,2 and Γ 1,2 represent the transmission coefficient and reflection coefficient from region 1 to region 2, T 2,1 represents the transmission coefficient from region 2 to region 1, Γ 2,1 represents the reflection coefficient from region 2 to region 1, is the generalized reflection coefficient from region 1 to region 2, is the generalized reflection coefficient from region 2 to region 3, subscript 1 represents region 1, which is the air area, subscript 2 represents region 2, which is the material to be measured, and subscript 3 represents region 3, which is the metal short circuit plate area, η1 is the wave impedance of air, η2 is the wave impedance of the material to be measured, η is the wave impedance of the material, ε r is the dielectric constant of the material, μ r is the magnetic permeability of the material, k1 is the wave number of the material to be measured, and d1 is the thickness of the material to be measured.

[0057] The surface of the material to be measured is partially reflective, and the surface of the metal reflective plate is fully reflective. Because the surface of the metal reflective plate is fully reflective, we have Thus, the reflection coefficient Γ 1,2 of the surface of the material to be measured can be calculated; because the magnetic permeability μ r of the material to be measured isGiven this information, the dielectric constant ε of the microwave material under test under the temperature gradient can be obtained. r .

[0058] For a two-layer medium, electromagnetic waves undergo total internal reflection at the metal reflector, thus resulting in Γ. 2,3 =-1; thus, the reflectance coefficient Γ of the surface of the material under test can be calculated. 1,2 Because the material to be tested is quartz, its μ r =1, and the dielectric constant ε of the microwave material under test under the temperature gradient can be obtained. r .

[0059] The test results of quartz under temperature gradient in this embodiment are as follows: Figure 4 As shown in the figure, the test results under temperature gradient and the test results at room temperature are completely consistent. This is because the dielectric properties of quartz are stable under the influence of temperature and are not affected by temperature. The figure shows that the device of the present invention can achieve accurate testing of the dielectric constant of the material under temperature gradient.

[0060] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A dielectric property test system based on free space terminal short circuit method under temperature gradient, characterized in that, The application relates to a dielectric performance test system, which comprises a temperature control unit, a liquid cooling unit, a support, a metal short-circuit plate, a heating unit, a metal ellipsoidal reflecting surface, a transceiving antenna, a data processing unit, a vector network analyzer, a first temperature sensor and a second temperature sensor. The metal short-circuit plate is arranged on the support, a microwave material to be measured is arranged on the metal short-circuit plate, the heating unit is arranged on the microwave material to be measured, the heating unit adopts metal heating grid lines, and the arrangement direction of the grid lines is the same as the E-plane direction of the transceiving antenna; the temperature control unit is connected with the heating unit and is used for controlling the heating unit to heat so that the upper surface of the microwave material to be measured is kept at a first temperature; a liquid cooling tank is arranged in the metal short-circuit plate, the liquid cooling tank is connected with the liquid cooling unit, the cooling liquid in the liquid cooling unit circulates in the liquid cooling tank and is used for keeping the lower surface of the microwave material to be measured at a second temperature; the first temperature sensor is used for measuring the temperature of the upper surface of the microwave material to be measured, and the second temperature sensor is used for measuring the temperature of the lower surface of the microwave material to be measured. The metal ellipsoidal reflecting surface is arranged directly above the wave-transparent material plate. The transceiving antenna is connected with the vector network analyzer through a coaxial cable, the metal ellipsoidal reflecting surface is used for focusing the electromagnetic waves transmitted and received by the transceiving antenna and makes the transmitted electromagnetic waves on the surface of the microwave material to be measured approximate to uniform plane waves, and the plane where the focus of the reflected electromagnetic waves is located is coincident with the upper surface of the microwave material to be measured. The vector network analyzer is used for measuring the parameters of the microwave material to be measured in free space and transmitting S parameters to the data processing unit for processing to obtain the electromagnetic parameters of the microwave material to be measured.

2. The dielectric performance test system of claim 1, wherein, The transceiving antenna is a double-mode horn antenna or a corrugated horn antenna.

3. The dielectric performance test system of claim 1, wherein, The metal heating grid lines are obtained by arranging a plurality of heating rods on the microwave material to be measured, the metal heating grid lines are not in direct contact with the surface of the microwave material to be measured, and the interval between the adjacent two heating rods is determined according to the temperature uniformity and the influence on the free space measurement accuracy.

4. The dielectric performance test system of claim 1, wherein, The dielectric performance test system further comprises a first temperature display instrument and a second temperature display instrument, the first temperature display instrument is connected with the first temperature sensor, the second temperature display instrument is connected with the second temperature sensor, and the temperature display instrument is used for displaying the temperature value measured by the temperature sensor.

5. The dielectric performance test system of claim 1, wherein, The surface of the microwave material to be measured should be smooth and flat to meet the requirement that the uniform plane wave propagation direction is perpendicular to the surface of the microwave material to be measured.

6. The dielectric performance test system of claim 1, wherein, The metal short-circuit plate and the support are made of high-temperature metal, and the material of the metal ellipsoidal reflecting surface is brass.

7. The dielectric performance test system of claim 1, wherein, The cooling liquid is alcohol or liquid nitrogen.

8. The dielectric performance test system of claim 1, wherein, The first temperature is above 150 DEG C, and the second temperature is below -20 DEG C.

9. A method for testing dielectric constant based on the dielectric property testing system according to any one of claims 1-8, characterized in that, The application further discloses a dielectric performance test method. Step 1: the metal calibration plate is placed on the plane where the focus of the reflected electromagnetic waves is located, the placement direction of the transceiving antenna is adjusted so that the E-plane direction of the antenna is the same as the arrangement direction of the metal grid lines, and the spatial position of the antenna is adjusted so that the reflection parameter S11 of the antenna is higher than a set threshold value; Step 2: the parameters in the vector network analyzer are set, and the whole free terminal short-circuit system is calibrated at normal temperature. Step 3. Place the calibration reflector plate at the focal plane of the transceiver antenna, heat the upper surface of the calibration reflector plate to a first temperature using the heating unit, and at the same time, cool the lower surface of the calibration reflector plate to a second temperature using the liquid cooling unit, and after the temperature is stable, measure the one-port short-circuit reflection parameter under the required temperature gradient using the vector network analyzer Step 4. Replace the calibration reflector plate with the microwave material to be measured, the thickness of the microwave material to be measured is the same as that of the calibration reflector plate, heat the upper surface of the microwave material to be measured to the first temperature by using the heating unit, at the same time, cool the lower surface of the microwave material to be measured to the second temperature by using the liquid cooling unit, after the temperature is stable, the single-port short-circuit reflection parameter under the required temperature gradient is measured by using the vector network analyzer Step 5. Using the calibration data obtained in step 3 and the test data obtained in step 4, the actual reflection parameters of the microwave material to be tested under the target temperature gradient are calculated, and the calculation method is as follows: Step 6. The dielectric constant of the microwave material to be tested under the target temperature gradient is obtained by inverting the real reflection coefficient of the microwave material to be tested obtained in step 5, and the specific process is as follows: T 1,2 = 1 + Γ 1,2 Γ 1,2 = -Γ 2,1 (4) wherein T 1,2 and Γ 1,2 represent the transmission and reflection coefficients of region 1 to region 2, T 2,1 represents the transmission coefficient of region 2 to region 1, Γ 2,1 represents the reflection coefficient of region 2 to region 1, is the generalized reflection coefficient of region 1 to region 2, represents the generalized reflection coefficient of region 2 to region 3, the subscript 1 represents region 1, which is an air region, the subscript 2 represents region 2, which is a material region to be measured, the subscript 3 represents region 3, which is a metal short-circuit plate region, η1 is the wave impedance of air, η2 is the wave impedance of the material to be measured, η is the wave impedance of the material, ε r is the dielectric constant of the material, μ r is the magnetic permeability of the material, k1 is the wave number of the material to be measured, and d1 is the thickness of the material to be measured. The surface of the material to be measured is partially reflective, and the surface of the metal reflecting plate is totally reflective. Because the surface of the metal reflecting plate is totally reflective, there is Therefore, the reflection coefficient Γ of the surface of the material to be measured can be calculated 1,2 ; because the magnetic permeability μ of the material to be measured r is known, the dielectric constant ε of the microwave material to be measured under a temperature gradient can be obtained r .

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