Dual-channel superheterodyne free-space method and apparatus for measuring the dielectric constant of materials

Abstract

This invention provides a dual-channel superheterodyne free-space method and apparatus for measuring the dielectric constant of materials, addressing the challenge of accurately measuring the dielectric constant of low-loss transparent materials. The embodiment employs a dual-channel superheterodyne circuit structure, using two correlated but different frequency waveguide channels to generate mid / low-frequency measurement signals and a synchronization signal, respectively. The measurement signal is input to an inductive voltage divider and then fed into a lock-in amplifier, while the synchronization signal is also input to the lock-in amplifier. The channel transmission coefficient modulus is calculated based on the voltage division ratio of the inductive voltage divider before and after the material is placed in the measurement position, and the voltage division ratio after voltage compensation. The phase shift of the channel transmission coefficient is calculated based on the phase difference between the measurement signal and the synchronization signal before and after placement, as measured by the lock-in amplifier. After phase correction of the channel transmission coefficient, the end-face transmission coefficient of the material is obtained. Finally, the real part of the dielectric constant and the loss tangent of the material are calculated using the free-space method.

Description

Technical Field

[0001] This application relates to the field of dielectric constant measurement technology, and in particular to a dual-channel superheterodyne free space method and apparatus for measuring dielectric constant of materials. Background Technology

[0002] Accurate measurement and characterization of the electromagnetic intrinsic parameters of materials, especially the complex permittivity, are crucial for the design and development of microwave and millimeter-wave devices or radio frequency systems across various fields. For example, in the development of quasi-optical components (such as dielectric lenses and quasi-optical filters), precise knowledge of the material's complex permittivity is essential during the design phase to achieve the desired simulation performance. In cryogenic receiver systems in radio astronomy, accurate characterization of the material's complex permittivity plays a key role in selecting materials with the lowest loss to achieve optimal receiver performance. In millimeter-wave radar for automotive driver assistance systems, accurate measurement of the complex permittivity properties of materials such as radar PCB substrates, radar radomes, and bumpers is extremely important for the design and performance evaluation of automotive radar and its components.

[0003] There are three main methods for measuring the complex dielectric constant of materials in the millimeter-wave band:

[0004] The first method is to use terahertz time-domain spectroscopy (THz-TDS) to measure the dielectric parameters of a material by analyzing the time-domain or frequency-domain response of the material after a pulse wave penetrates it. This method has a very wide measurement frequency range, but its frequency resolution is not high, and its measurement accuracy is limited, especially in that it cannot accurately measure the dielectric loss of the material.

[0005] The second method is the resonant cavity method based on various resonance principles, including closed resonant cavities, split resonant cavities, and quasi-optical open resonant cavities. This type of method has high measurement accuracy and is especially suitable for measuring low-loss materials (loss tangent less than 0.01). However, it can only work at discrete resonant frequencies.

[0006] The third method is the continuous wave measurement method based on a vector network analyzer (referred to as a network analyzer). This type of method can be divided into free space method, coaxial line method, and waveguide method according to different transmission forms. Compared with other methods, the free space method measurement device has more advantages because it is not limited by sample processing and preparation, can realize non-destructive testing of material dielectric parameters over a wider frequency range, and can be more conveniently compatible with various complex high and low temperature testing environments.

[0007] In classic free-space measurement systems, the dielectric parameters are typically calculated by measuring the transmission and reflection signals of a material using a vector network analyzer. Theoretical analysis shows that the accuracy of measuring the real part of the dielectric constant depends primarily on the accuracy of measuring the phase of the transmission coefficient, while the accuracy of measuring the loss tangent depends primarily on the accuracy of measuring the magnitude of the transmission coefficient. For low-loss transparent materials, traditional methods, such as free-space testing devices based on vector network analyzers, result in poor repeatability of the magnitude and phase of the transmission coefficient, leading to significant measurement uncertainty and failing to meet high-precision measurement requirements. In the millimeter-wave band, the measurement uncertainty specifications for the transmission coefficient provided by commercial vector network analyzer manufacturers only guarantee measurement accuracy for high-loss materials (loss tangent not less than 0.01), and are not suitable for the accurate measurement of low-loss transparent materials.

[0008] In summary, the broadband dielectric property measurement and characterization of low-loss dielectric materials in the millimeter-wave band remains challenging. Due to the large measurement uncertainties in the magnitude of the transmission coefficient and the phase angle, the classic free-space method based on commercial vector network analyzers is insufficient to solve the problem of accurate measurement of low-loss transparent materials. Summary of the Invention

[0009] This application provides a dual-channel superheterodyne free-space method and apparatus for measuring the dielectric constant of materials, which improves the accuracy of measuring the dielectric constant of low-loss transparent materials.

[0010] To achieve the above objectives, this application adopts the following technical solution:

[0011] Based on one aspect of the embodiments of the present invention, the present invention provides a dual-channel superheterodyne free-space method for measuring the dielectric constant of materials, the device comprising:

[0012] The first superheterodyne circuit, located in the first waveguide channel, is used to generate mid / low frequency measurement signals, including a first band transmitting antenna and a first band receiving antenna;

[0013] The second superheterodyne circuit, located in the second waveguide channel, is used to generate a mid / low frequency synchronization reference signal;

[0014] The first superheterodyne circuit and the second superheterodyne circuit use the same signal source;

[0015] The second frequency synthesizer is used to convert the synchronization reference signal into a medium / low frequency synchronization signal with the same frequency as the measurement signal;

[0016] An inductive voltage divider is used to measure and distribute the voltage of a measurement signal. One of its output ports is connected to a lock-in amplifier to feed the measurement signal into the lock-in amplifier.

[0017] A lock-in amplifier is used to measure the phase difference between the input measurement signal and the synchronization signal, as well as to measure the voltage of the input measurement signal.

[0018] The compensation module is used to obtain a first voltage division ratio (D1) from an inductive voltage divider and a first voltage (V1) of the measurement signal from a lock-in amplifier before the material under test is placed at the measurement position between the first-band transmitting antenna and the first-band receiving antenna; and to obtain a second voltage (V2) of the measurement signal from the lock-in amplifier after the material under test is placed at the measurement position, and to adjust the voltage division ratio of the inductive voltage divider according to the difference between the first voltage and the second voltage, so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage to compensate for the reduced voltage; after compensation, the voltage division ratio of the inductive voltage divider is the second voltage division ratio (D2).

[0019] The calculation module is used to calculate the channel transmission coefficient magnitude based on the first voltage division ratio and the second voltage division ratio, and to calculate the channel transmission coefficient phase shift based on the phase difference between the measurement signal and the synchronization signal before and after the material under test is placed in the test. After phase correction of the channel transmission coefficient based on the channel transmission coefficient magnitude and the channel transmission coefficient phase shift, the transmission coefficient of the end face of the material under test is obtained. The module also uses the free space method to invert and calculate the real part of the dielectric constant and the loss tangent of the material under test.

[0020] Furthermore, the first superheterodyne circuit and the second superheterodyne circuit use frequency multipliers of different factors to mix the two waveguide channels at different frequencies respectively;

[0021] The first superheterodyne circuit and the second superheterodyne circuit share the same local oscillator source, which generates a local oscillator signal based on the reference signal provided by the signal source.

[0022] Furthermore, the first superheterodyne circuit includes a first frequency multiplier and a third frequency multiplier, both with a multiplication factor of N; the second superheterodyne circuit includes a second frequency multiplier and a fourth frequency multiplier, both with a multiplication factor of M; wherein N is not equal to M, and both N and M are positive integers greater than 0;

[0023] The first band transmitting antenna and the first band receiving antenna in the first superheterodyne circuit are connected between the first frequency multiplier and the first mixer; the local oscillator signal output by the local oscillator source is multiplied by the third frequency multiplier and mixed with the output signal of the first band receiving antenna to generate the measurement signal;

[0024] The other local oscillator signal output from the local oscillator source is multiplied by the fourth frequency multiplier and then mixed with the output signal of the second frequency multiplier to generate the synchronization reference signal.

[0025] Furthermore, when one of the first waveguide channel and the second waveguide channel is used as the measurement channel, the other channel is used as the synchronization channel;

[0026] The device further includes:

[0027] The switch selection circuit consists of a first switch (K1), a second switch (K2), a third switch (K3), and a fourth switch (K4). The first and second switches are used to select the measurement channel and send the medium / low frequency measurement signal of the measurement channel to the inductive voltage divider. The third and fourth switches are used to select the synchronization channel and send the medium / low frequency synchronization signal of the synchronization channel to the lock-in amplifier.

[0028] The first frequency synthesizer is used to convert the synchronization reference signal output by the first mixer into a medium / low frequency synchronization signal with the same frequency as the measurement signal when the first waveguide channel is used as a synchronization channel; the first frequency synthesizer and the second frequency synthesizer are only connected to the circuit and function when their respective channels are synchronization channels.

[0029] When the first waveguide channel is the measurement channel, the first band transmitting antenna and the first band receiving antenna are actually connected between the first frequency multiplier and the first frequency mixer in the first superheterodyne circuit, and the second frequency multiplier and the second frequency mixer are directly connected at the same time.

[0030] When the second waveguide channel is the measurement channel, the second band transmitting antenna and the second band receiving antenna are actually connected between the second frequency multiplier and the second frequency mixer in the second superheterodyne circuit, while the first frequency multiplier and the first frequency mixer are directly connected.

[0031] Furthermore, the first band transmitting antenna and the first band receiving antenna, the second band transmitting antenna and the second band receiving antenna are standard gain horn antennas or point-focusing lens horn antennas, and the antenna gain is greater than 20dB.

[0032] Furthermore, the first waveguide channel is a W-band (75GHz~110GHz) waveguide channel, and the second waveguide channel is a V-band (50GHz~75GHz) waveguide channel.

[0033] Furthermore, the signal output from the signal source can be input to the first frequency multiplier and the second frequency multiplier respectively through the first directional coupler; the signal output from the local oscillator source can be input to the third frequency multiplier and the fourth frequency multiplier respectively through the second directional coupler.

[0034] Based on another aspect of the embodiments of the present invention, the present invention also provides a dual-channel superheterodyne free-space method for measuring the dielectric constant of materials. This method is applied in the aforementioned apparatus and includes the following steps:

[0035] Before the material under test is placed in the measurement channel at the measurement position between the corresponding band transmitting antenna and receiving antenna, the first voltage of the measurement signal measured by the lock-in amplifier, the first phase difference between the measurement signal and the synchronization signal, and the first voltage division ratio measured by the inductive voltage divider are obtained.

[0036] After the material to be tested is placed in, the second voltage of the measurement signal is obtained from the lock-in amplifier. The voltage division ratio of the inductive voltage divider is adjusted according to the difference between the first voltage and the second voltage so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage to compensate for the reduced voltage.

[0037] After compensation, the second phase difference between the measurement signal measured by the lock-in amplifier and the synchronization signal, and the second voltage division ratio measured by the inductive voltage divider are obtained. The channel transmission coefficient magnitude is calculated based on the first voltage division ratio and the second voltage division ratio, and the phase shift of the channel transmission coefficient is calculated based on the first phase difference and the second phase difference before and after the material under test is placed in the test. The channel transmission coefficient is then phase-corrected based on the channel transmission coefficient magnitude and the channel transmission coefficient phase shift to obtain the transmission coefficient of the end face of the material under test.

[0038] Set the signal source to other frequency points, measure the end face transmission coefficient of the material under test at other frequency points again, and thus establish the frequency response function of the phase of the end face transmission coefficient of the material under test. Calculate the iterative initial value of the real part of the dielectric constant of the material under test based on the slope of the frequency response function.

[0039] Using the free space method and a nonlinear iterative algorithm, the real part of the dielectric constant and the loss tangent of the material under test are calculated by inversion based on the initial iterative value of the real part of the dielectric constant of the material under test.

[0040] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments of the present invention or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained from these drawings of the embodiments of the present invention.

[0042] Figure 1 A schematic diagram of the structure of the dual-channel superheterodyne free-space method for measuring the dielectric constant of materials provided in an embodiment of this application;

[0043] Figure 2 A schematic diagram of a superheterodyne free-space method material dielectric constant measurement device provided in an embodiment of the present invention, in which both channels can be used as measurement channels;

[0044] Figure 3 This is a schematic diagram of the dielectric constant measuring device in one embodiment of the present invention, where the second waveguide channel of the V-band is used as the measuring channel;

[0045] Figure 4 This is a flowchart illustrating the steps of a dual-channel superheterodyne free-space method for measuring the dielectric constant of materials according to an embodiment of the present invention. Detailed Implementation

[0046] The exemplary embodiments will now be described in detail. When the description refers to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this specification; they are merely exemplary embodiments of apparatuses and methods consistent with some aspects of this specification.

[0047] The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of this specification. The singular forms “a,” “described,” and “the” as used herein are also intended to include the plural forms unless the context clearly indicates otherwise.

[0048] It should be understood that the terms "first," "second," "third," etc., may be used in this specification to describe various information or structural modules for the purpose of more clearly describing the solution. These terms should not be construed as indicating or implying relative importance or implicitly specifying the number, order, or position of the indicated technical features. Therefore, a feature defined with "first," "second," "third," etc., may explicitly or implicitly include one or more of that feature. In the description of this specification, unless otherwise stated, "a plurality of" means two or more; "if" can be interpreted as "when," "when," or "in response to determination."

[0049] In this specification, directional terms such as "up," "down," "left," "right," "north," "south," "west," and "east" may be defined relative to the orientation of the components shown in the accompanying drawings. It should be understood that these directional terms can be relative concepts, used for relative description and clarification, and may change accordingly depending on the orientation of the components in the accompanying drawings.

[0050] In this specification, unless otherwise expressly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral part; it can be a direct connection or an indirect connection through an intermediate medium. Furthermore, the term "coupled connection" can be a direct electrical connection or an indirect electrical connection through an intermediate medium. The term "contact" can be direct contact or indirect contact through an intermediate medium.

[0051] In this specification, "and / or" describes the relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following associated objects are in an "or" relationship.

[0052] In the traditional method of measuring the reflection and transmission characteristics of electromagnetic waves by materials using a vector network analyzer (VNA) in free space, the VNA is used as the core instrument. The VNA is connected to a transmitting antenna and a receiving antenna (such as a horn antenna). The material under test is placed flat between the transmitting antennas to ensure that the electromagnetic waves are incident perpendicularly. The VNA transmits a swept-frequency microwave signal. After the microwave signal penetrates the material, it is captured by the receiving end. After obtaining the S-parameters of the material under test (including the S11 reflection coefficient and the S21 transmission coefficient), the dielectric constant of the material under test is inverted using the Nicolson-Ross-Weir (NRW) algorithm.

[0053] However, the aforementioned traditional method for measuring the reflection and transmission characteristics of materials to electromagnetic waves using the VNA self-space method has at least the following technical problems when applied to the measurement of the dielectric constant of low-loss transparent materials:

[0054] (1) When the microwave signal of a VNA penetrates a low-loss transparent material, the electromagnetic wave attenuation is minimal, and the transmission coefficient amplitude is close to 1 (|S21|=1 for an ideal lossless material). In actual measurement, the minute changes in |S21| need to be accurately captured in order to calculate the loss tangent. However, the amplitude measurement accuracy of commercial VNAs is usually ±0.1dB. For low-loss transparent materials, this error will be amplified, resulting in drastic fluctuations in the calculated tanδ value.

[0055] (2) The accuracy of measuring the dielectric constant of low-loss transparent materials depends on the measurement accuracy of the transmission coefficient S21 phase. The phase jitter of the local oscillator of the VNA will be directly transmitted to the phase measurement result, resulting in more significant phase noise in the millimeter wave band (such as above 30 GHz).

[0056] (3) Solve the dielectric constant by directly solving the complex equations of reflection coefficient S11 and transmission coefficient S21 using the NRW formula. ε r When there is a small error in the S-parameter measurement, the inversion result will amplify the error exponentially (especially when the transmission coefficient S21 is close to 1).

[0057] (4) Millimeter waves have extremely short wavelengths. Commercial VNAs suffer from deterioration in dynamic range and phase noise in the millimeter wave band (e.g., phase noise > -90 dB / Hz), making it difficult to distinguish weak signal changes in low-loss materials. VNA specifications are usually calibrated based on medium- to high-loss materials (tanδ ≥ 0.01), and there is a lack of error compensation mechanisms for low-loss materials.

[0058] Based on extensive research and analysis of the technical problems and causes of traditional methods for measuring the dielectric constant of materials using commercial vector network analyzers and free-space methods, the inventors propose a dual-channel superheterodyne free-space method for measuring the dielectric constant of materials. This method differs from traditional methods using commercial vector network analyzers. It employs a dual-channel superheterodyne circuit structure, using two correlated but different frequency waveguide channels to generate mid / low-frequency measurement signals and synchronization signals respectively. The low-loss transparent material under test is placed between the transmitting and receiving antennas of the corresponding channel band. The measurement signal is input to an inductive voltage divider and then fed into a lock-in amplifier, while the synchronization signal is input to the synchronization terminal of the lock-in amplifier. The channel transmission coefficient magnitude is calculated based on the voltage division ratio of the inductive voltage divider before and after the material is placed in the measurement position, and the phase shift of the channel transmission coefficient is calculated based on the phase difference between the measurement signal and the synchronization signal measured by the lock-in amplifier before and after placement. This achieves more accurate measurement of both the channel transmission coefficient magnitude and phase shift. Then, after phase correction of the channel transmission coefficient, the end-face transmission coefficient of the material under test is obtained. The real part of the dielectric constant and the loss tangent of the material under test are then calculated using the free-space method. This scheme achieves a dynamic measurable attenuation range of approximately 90 dB, and its attenuation and phase shift measurement uncertainty indicators are far superior to traditional measurement schemes using commercial network analyzers, significantly improving the measurement accuracy of the dielectric constant of low-loss transparent materials.

[0059] Figure 1 This is a schematic diagram of the structure of a dual-channel superheterodyne free-space method dielectric constant measurement device provided in an embodiment of this application. The measurement device in this embodiment includes a first waveguide channel and a second waveguide channel. The channel used for measurement can also be called the measurement channel, and the channel used for synchronization can also be called the synchronization channel. The measurement device includes:

[0060] The first superheterodyne circuit, located in the first waveguide channel, is used to generate mid / low frequency measurement signals. f test This includes a first-band transmitting antenna and a first-band receiving antenna.

[0061] The second superheterodyne circuit, located in the second waveguide channel, is used to generate a mid / low-frequency synchronization reference signal.

[0062] The first and second superheterodyne circuits use the same signal source, which makes the measurement signal and the synchronization signal mutually correlated.

[0063] A second frequency synthesizer is used to convert the synchronization reference signal into a medium / low frequency synchronization signal with the same frequency as the measurement signal. f sync ).

[0064] An inductive voltage divider is used to measure and distribute the voltage of a measurement signal. One of its output ports is connected to a lock-in amplifier, feeding the measurement signal into the lock-in amplifier. The voltage division ratio between the measurement signal voltage input to the input port and the measurement signal voltage distributed to the lock-in amplifier can be obtained through the inductive voltage divider.

[0065] A lock-in amplifier is used to measure the input measurement signal. f test ) and synchronization signal ( f sync The phase difference between the two phases, and the voltage of the measurement signal fed in; the phase difference before the material under test is placed in is the first phase difference ( The phase difference after the tested material is placed is the second phase difference ( ).

[0066] The compensation module is used to obtain a first voltage division ratio (D1) from an inductive voltage divider and a first voltage (V1) of the input measurement signal from a lock-in amplifier before the material under test is placed at the measurement position between the first-band transmitting antenna and the first-band receiving antenna; and to obtain a second voltage (V2) of the input measurement signal from the lock-in amplifier after the material under test is placed at the measurement position. The module adjusts the voltage division ratio of the inductive voltage divider based on the difference between the first and second voltages, so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage, thus compensating for the reduced voltage. After compensation, the voltage division ratio measured by the inductive voltage divider is the second voltage division ratio (D2). Preferably, the compensation module mainly consists of a control circuit, a data acquisition circuit, and an embedded computing unit, enabling automatic voltage acquisition, voltage balance indication, and automatic compensation.

[0067] The calculation module is used to obtain the first voltage division ratio and the second voltage division ratio from the inductive voltage divider (IVD) and calculate the channel transmission coefficient modulus. The first and second phase differences are obtained from the lock-in amplifier, and the phase shift of the channel transmission coefficient is calculated. Based on the channel transmission coefficient modulus and the channel transmission coefficient phase shift, the channel transmission coefficient ( After phase correction, the transmission coefficient of the end face of the measured material is obtained. Preferably, the calculation module can automatically read the voltage division ratio signal from the inductive voltage divider and the phase difference signal from the lock-in amplifier through computer programming, and obtain the end-face transmission coefficient of the material under test through data processing and calculation. The calculation module can be implemented using an embedded computing unit or a general-purpose computer, and the dielectric constant calculation process can be formed into an algorithm library, which can be called and executed by the calculation module.

[0068] The calculation module is also used to invert and calculate the real part of the dielectric constant and the loss tangent of the material under test using the free-space method. During the inversion calculation, the end-face transmission coefficient and single-reflection coefficient of the material under test are used as the basis for the calculation. ) and single transmission coefficient ( T The relationship between the real and imaginary parts of the dielectric constant of the material under test is calculated using a nonlinear iterative algorithm, and then the loss tangent is calculated.

[0069] Preferably, the first superheterodyne circuit and the second superheterodyne circuit use frequency multipliers of different factors to mix the two waveguide channels at different frequencies, thereby reducing the mutual interference between the two superheterodyne circuits.

[0070] Preferably, the first superheterodyne circuit and the second superheterodyne circuit share the same local oscillator source, which generates its local oscillator signal based on a reference signal provided by the same signal source. In other embodiments of the present invention, each superheterodyne circuit may use its own local oscillator source, using the same reference signal provided by the same signal source to generate its own local oscillator signal, but this increases the cost.

[0071] Preferably, the first superheterodyne circuit includes a first frequency multiplier and a third frequency multiplier, both with a multiplication factor of N, and the second superheterodyne circuit includes a second frequency multiplier and a fourth frequency multiplier, both with a multiplication factor of M; wherein N is not equal to M, and both N and M are positive integers greater than 0.

[0072] The first band transmitting antenna and the first band receiving antenna in the first superheterodyne circuit are connected between the first frequency multiplier and the first frequency mixer.

[0073] The first frequency multiplier in the first superheterodyne circuit and the second frequency multiplier in the second superheterodyne circuit are based on the same signal source output radio frequency signal. f 1 ) respectively generate first radio frequency signals of different frequencies ( Nf 1 ) and second radio frequency signal ( Mf 1 );

[0074] The local oscillator outputs a local oscillator signal based on the reference signal provided by the signal source. The local oscillator signal is then multiplied by a third frequency multiplier and a fourth frequency multiplier to output a third radio frequency signal. Nf 1 + fm1 ,in f m1 (for intermediate frequency / low frequency signals) and fourth radio frequency signals ( Mf 1 + f m2 ,in f m2 (For intermediate / low frequency signals);

[0075] The first superheterodyne circuit includes a first-band transmitting antenna and a first-band receiving antenna. These antennas are connected between a first frequency multiplier and a first frequency mixer. A space is reserved between the first transmitting antenna and the first receiving antenna to hold the material under test, serving as the measurement position. The first frequency multiplier outputs a first radio frequency signal (…). Nf 1 After passing through the first band transmitting antenna and the first band receiving antenna, the third radio frequency signal output by the third frequency multiplier ( Nf 1 + f m1 The output of the intermediate / low frequency measurement signal after mixing in the first mixer ( f test );

[0076] In the second superheterodyne circuit, the second radio frequency signal ( Mf 1 ) and the fourth radio frequency signal ( Mf 1 + f m2 The output synchronization reference signal is after mixing in the second mixer. f m2 The synchronization reference signal is output as a medium / low frequency synchronization signal after passing through the second frequency synthesizer. f sync The function of the second frequency synthesizer is to synthesize the synchronization reference signal ( f m2 ) converted to mid / low frequency measurement signals ( f test ) Mid / low frequency synchronization signals with the same frequency and related frequency ( f sync );

[0077] Medium / low frequency measurement signals ( f test The input is fed into an inductive voltage divider (IVD) and then into the input of a lock-in amplifier. The mid / low frequency synchronization signal ( f sync The input is sent to the synchronization port of the lock-in amplifier;

[0078] Based on the above device, the voltage division ratio obtained from the inductive voltage divider and the phase change measured from the lock-in amplifier before and after the material under test is placed in the measurement position can be used to accurately measure the magnitude of the transmission coefficient and the phase shift of the material under test in the measurement channel. Then, the dielectric constant of the material under test can be obtained by inversion calculation based on the free space method.

[0079] In one embodiment of the present invention, the first waveguide channel and the second waveguide channel correspond to two different electromagnetic wave frequency bands, for example, the first waveguide channel is a W-band (75GHz~110GHz) waveguide channel and the second waveguide channel is a V-band (50GHz~75GHz) waveguide channel. The present invention does not specifically limit the electromagnetic wave frequency band range of the two waveguide channels; preferably, the two frequency band ranges do not overlap, and the two frequency bands can be millimeter wave bands, microwave bands, etc.

[0080] In one embodiment of the present invention, in order to achieve multiple functions such as signal monitoring, power distribution, and isolation protection, the signal output from the signal source can be input to the first frequency multiplier and the second frequency multiplier respectively through the first directional coupler; the signal output from the local oscillator source can be input to the third frequency multiplier and the fourth frequency multiplier respectively through the second directional coupler. The present invention does not specifically limit the way the output signals of the signal source and the local oscillator source are connected to the frequency multipliers; methods such as directional couplers, direct connection, waveguides, and coaxial cables can be selected according to the waveguide frequency band and the requirements of the measurement scenario.

[0081] Figure 2 This is a schematic diagram of a superheterodyne free-space method material dielectric constant measurement device provided in one embodiment of the present invention, where both channels can be used as measurement channels. To meet the need to measure the dielectric constant of the material under test at different frequency bands and to improve the flexibility and practicality of the measurement device, this embodiment adds a switch selection circuit and a first frequency synthesizer, a second-band transmitting antenna, and a second-band receiving antenna. Through the cooperation of the switch selection circuit and the two frequency synthesizers, flexible switching of channel roles is achieved, allowing either channel to be used as a measurement channel while the other channel serves as a synchronization channel.

[0082] In this embodiment, the switch selection circuit includes a first switch (K1), a second switch (K2), a third switch (K3), and a fourth switch (K4). K1 and K2 can be bidirectional switches, while K3 and K4 can be unidirectional switches. K1 and K2 are used to select the measurement channel, directly sending the mid / low-frequency measurement signal of the measurement channel to the inductive voltage divider. K3 and K4 are used to select the synchronization channel, sending the mid / low-frequency synchronization signal processed by the frequency synthesizer in the synchronization channel to the lock-in amplifier.

[0083] In this embodiment, when one waveguide channel is used as a measurement channel, the other waveguide channel is used as a synchronization channel; the first frequency synthesizer and the second frequency synthesizer are only connected to the circuit and function when their respective channels are synchronization channels, and are used to convert the synchronization reference signal into a medium / low frequency synchronization signal with the same frequency as the measurement signal;

[0084] When the first waveguide channel is used as the measurement channel, the first-band transmitting antenna and the first-band receiving antenna are actually connected between the first frequency multiplier and the first mixer in the first superheterodyne circuit. Simultaneously, in the second waveguide channel, the second frequency multiplier and the second mixer need to be directly connected, and the second-band transmitting antenna and the second-band receiving antenna are bypassed. At the same time, four switches need to be controlled to directly input the measurement signal output from the first mixer to the inductive voltage divider, and to input the signal output from the second mixer to the second frequency synthesizer, so that the synchronization signal output from the second frequency synthesizer is input to the lock-in amplifier. Specifically, K1 is placed on one side of the inductive voltage divider, disconnecting it from the first frequency synthesizer; K2 is placed on one side of the second frequency synthesizer, disconnecting it from the IVD; K3 disconnects it from the first frequency synthesizer, while K4 closes it from the second frequency synthesizer.

[0085] When the second waveguide channel is used as the measurement channel, the second-band transmitting antenna and the second-band receiving antenna are actually connected between the second frequency multiplier and the second mixer in the second superheterodyne circuit. Simultaneously, in the first waveguide channel, the first frequency multiplier and the first mixer need to be directly connected, and the first-band transmitting antenna and the first-band receiving antenna are bypassed. At the same time, four switches need to be controlled to directly input the measurement signal output from the second mixer to the inductive voltage divider, and to input the signal output from the first mixer to the first frequency synthesizer, so that the synchronization signal output from the first frequency synthesizer is input to the lock-in amplifier. Specifically, K1 is placed on the first frequency synthesizer side and disconnected from the inductive voltage divider side; K2 is placed on the inductive voltage divider side and disconnected from the second frequency synthesizer side; K3 is closed to the first frequency synthesizer, and K4 is disconnected from the second frequency synthesizer.

[0086] In this embodiment of the invention, the transmitting antenna and receiving antenna in the measurement channel can be a standard gain horn antenna or a point-focusing lens horn antenna. Preferably, the antenna gain should be greater than 20dB.

[0087] Figure 3 This is a schematic diagram of the dielectric constant measuring device in one embodiment of the present invention, where a second waveguide channel in the V-band is used as the measurement channel. In this embodiment, the second waveguide channel is a V-band waveguide channel, which is used as the measurement channel to measure the dielectric constant of the material under test. The first waveguide channel is a W-band waveguide channel, which serves as the synchronization channel.

[0088] Signal source output signal (frequency) f 1 The signal is fed into the V-band waveguide channel (measurement channel) via the first directional coupler, and then multiplied by a 2x multiplier (second frequency multiplier) to generate a frequency of 2 in the range of 50GHz to 75GHz. f 1 The signal is fed into the W-band waveguide channel (synchronization channel) and then multiplied by a 3x3 frequency multiplier (first frequency multiplier) to generate a frequency of 3 in the range of 75GHz to 110GHz. f 1 The signal.

[0089] This oscillator generates a frequency based on a reference signal provided by a signal source. f 1 A stable +5kHz local oscillator signal is split into two paths by the second directional coupler. One path is multiplied by a 2x multiplier (fourth multiplier) to generate 2... f 1 A +10kHz millimeter-wave signal is used, and this signal is mixed with the signal from the V-band waveguide channel in the second mixer to output a 10kHz intermediate frequency measurement signal; another local oscillator signal is generated by a ×3 frequency multiplier (third frequency multiplier). f 1 The +15kHz millimeter-wave signal, after being mixed with the W-band waveguide channel in the first mixer, outputs a 15kHz intermediate frequency synchronization reference signal.

[0090] By controlling switch K1 to the first frequency synthesizer, K2 to the inductive voltage divider side, K3 closed, and K4 open, the 15kHz intermediate frequency synchronization reference signal is processed by the first frequency synthesizer (i.e., the ×2÷3 frequency synthesizer) to obtain a 10kHz intermediate frequency synchronization signal with the same frequency as the intermediate frequency measurement signal. The synchronization signal is then input to the lock-in amplifier. The 10kHz intermediate frequency measurement signal output from the second mixer is input to the inductive voltage divider.

[0091] Similarly, when measuring the dielectric constant of the material under test in the W-band, the W-band waveguide channel is used as the measurement channel, and the V-band waveguide channel is used as the synchronization channel. The V-band transmitting and receiving antennas in the V-band waveguide channel need to be bypassed. The ×2 frequency multiplier is directly connected to the second mixer, and the W-band transmitting and receiving antennas are connected between the ×3 frequency multiplier and the first mixer in the W-band waveguide channel. By controlling switch K1 to the inductive voltage divider side, K2 to the second frequency synthesizer side, K3 open, and K4 closed, a 15kHz intermediate frequency measurement signal is input to the inductive voltage divider. The 10kHz synchronization reference signal output from the second mixer is processed by the second frequency synthesizer (i.e., the ×3÷2 frequency synthesizer) to obtain a 15kHz intermediate frequency synchronization signal with the same frequency as the intermediate frequency measurement signal. This synchronization signal is then input to the lock-in amplifier.

[0092] The dual-channel superheterodyne free-space method dielectric constant measurement device provided in this embodiment includes both V-band and W-band waveguide channels. It can simultaneously perform wideband measurement of the dielectric constant of the material under test in the V-band and W-band frequency bands covering 50GHz-110GHz in one device, and has better wideband measurement characteristics, better practicality and flexibility.

[0093] The dual-channel superheterodyne free-space method material dielectric constant measurement device provided in this embodiment includes two waveguide channels, each of which is a complete single-channel superheterodyne circuit system. The two channels serve as a reference to each other. Because the radio frequency signals in the two channels have different frequencies, crosstalk between the channels can be effectively eliminated, resulting in better anti-crosstalk performance.

[0094] In the measurement device provided in this embodiment of the invention, since the radio frequency signals of the two waveguide channels come from the same signal source, their radio frequency signal frequencies are correlated, and the mid / low frequency signals obtained after mixing are also completely correlated. Therefore, the stability of the lock-in amplifier measurement data can be guaranteed, thereby obtaining more accurate measurement results.

[0095] Although the two channels in the above embodiments use the V-band and W-band respectively, those skilled in the art will understand that the measurement device and method provided in the embodiments of the present invention can also be extended to other waveguide frequency bands. If other electromagnetic wave frequency bands are used, it is only necessary to replace the waveguide devices and transceiver antennas used in the two band channels accordingly to adapt to the measurement requirements of the corresponding bands.

[0096] Figure 4 This is a schematic flowchart illustrating the steps of a dual-channel superheterodyne free-space method for measuring the dielectric constant of materials according to an embodiment of the present invention. The method is applied to the measuring device provided in this embodiment. Based on the voltage division ratio measured by the inductive voltage divider and the lock-in amplifier, and the phase difference between the measurement signal and the synchronization signal, the attenuation and phase shift of the channel transmission coefficient are calculated. After phase correction, the complex form end-face transmission coefficient of the material under test is obtained. Then, based on the free-space transmission and reflection theory, the real part of the dielectric constant and the loss tangent of the material under test are calculated. Since the measurement uncertainty of attenuation and phase shift obtained by this method is far superior to that of commercial network analyzers, this method can solve the problem of accurate measurement of low-loss materials (loss tangent less than 0.01), and can achieve high-precision measurement of the dielectric constant of low-loss transparent materials in the millimeter-wave band.

[0097] Figure 4 The example method is based on a scenario where the measurement channel (e.g., V-band channel) and synchronization channel (e.g., W-band channel) have been determined, and the switching circuit has completed the routing operation. Before starting the measurement steps, the output frequency of the signal source is first set ( f1 The output frequency and power level of the local oscillator are adjusted accordingly to ensure that the lock-in amplifier can be synchronized normally and have a stable output reading.

[0098] The method includes the following steps:

[0099] Step 401: Before placing the material to be tested into the measurement position between the corresponding band transmitting antenna and receiving antenna in the measurement channel, (by the compensation module) acquire the first voltage (V1) of the measurement signal measured by the lock-in amplifier, and (by the calculation module) acquire the first phase difference between the measurement signal measured by the lock-in amplifier and the synchronization signal. And obtain the first voltage division ratio (D1) measured by the inductive voltage divider.

[0100] Step 402: After the material to be tested is placed in the measurement position, the second voltage (V2) of the measurement signal is obtained from the lock-in amplifier (by the compensation module). The voltage division ratio of the inductive voltage divider is adjusted according to the difference between the first voltage and the second voltage so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage (V1) to compensate for the reduced voltage.

[0101] The measuring device and method provided in this invention are applicable to non-destructive measurement of the test material of a flat plate medium. The measurement position of the test material is located at the center between the transmitting antenna and the receiving antenna. After the test material is placed in the plate, both the transmitting antenna and the receiving antenna need to be aligned with the test material. Preferably, the size of the test material should be 3 to 4 times larger than the beam diameter of the antenna at the location of the test material.

[0102] Step 403: After compensation, (by the calculation module) obtain the second phase difference between the measurement signal measured by the lock-in amplifier and the synchronization signal. The first voltage divider and the second voltage divider ratio (D2) are used to calculate the channel transmission coefficient magnitude of the measurement channel based on the first voltage divider ratio and the second voltage divider ratio. The phase shift of the channel transmission coefficient is calculated based on the phase difference between the measurement signal and the synchronization signal before and after the material is placed in the test. After phase correction of the channel transmission coefficient based on the channel transmission coefficient magnitude and the channel transmission coefficient phase shift, the end-face transmission coefficient of the material being measured is obtained. );

[0103] The channel transmission coefficient modulus is calculated based on the first voltage division ratio (D1) and the second voltage division ratio (D2) measured by the inductive voltage divider. The method is as follows:

[0104] (Equation 1)

[0105] Based on the first phase difference between the measurement signal measured by the lock-in amplifier and the synchronization signal ( ) and second phase difference ( ) Calculate the phase shift of the channel transmission coefficient ( The method is as follows:

[0106] (Equation 2)

[0107] Based on the modulus of the channel transmission coefficient ( ) and channel transmission coefficient phase shift ( After phase correction of the channel transmission coefficient, the end face transmission coefficient of the measured material is obtained. The method is as follows:

[0108] (Equation 3)

[0109] In the formula, k0 is the free space propagation wavenumber. , f The frequency of electromagnetic waves, and These are the vacuum permittivity and permeability, respectively. d The thickness of the sample. To improve measurement accuracy, the thickness of the material being measured is... d The average value can be obtained after multiple measurements.

[0110] For channel transmission coefficient ( )use The correction is made to express the phase delay caused by the electromagnetic wave passing through air of the same thickness as the material being measured.

[0111] Step 404: Set the signal source to other frequencies ( f i ), and then measure the material under test at other frequencies ( f i The end-face transmission coefficient under the condition is used to establish the end-face transmission coefficient of the measured material. The frequency response function of the phase is used to calculate the initial iterative value of the real part of the dielectric constant of the material under test based on the slope of the frequency response function.

[0112] Let the transmission coefficient of the end face of the material being measured be ( The expression for the frequency response function of the phase is:

[0113] y=a f +b (Equation 4)

[0114] in, f y represents the measurement frequency (the frequency of the output signal from the first or second harmonic source), and y represents the phase of the transmission coefficient of the end face of the measured material. a is the slope, b is the offset, and both a and b are constants. The values ​​of a and b can be determined based on the successively measured (f1, y1) and (f... i ,y iThe frequency response function is determined by the straight line defined by the two points.

[0115] The initial value of the real part of the dielectric constant of the material under test is obtained by the following equation 5:

[0116] (Equation 5)

[0117] Where 'a' is the slope obtained from the phase frequency response function of the transmission coefficient at the end face of the material under test. The vacuum permittivity, The permeability of free space, d The thickness of the material being measured.

[0118] Step 405: Using the free space method and a nonlinear iterative algorithm, based on the initial iterative value of the real part of the dielectric constant of the material under test, the real part of the dielectric constant and the loss tangent of the material under test are calculated by inversion.

[0119] The single reflection coefficients of air and the end face of the tested material are The single-pass transmission coefficient between the two end faces of the tested material is T Based on the analysis of multiple transmission and reflection of electromagnetic waves by the tested material, the transmission coefficient of the tested material end face measured in equation (3) can be obtained. )and and T It has the following relationship:

[0120] (Equation 6)

[0121] Among them, the reflection coefficient ( The calculation method for ) is as follows:

[0122] , (Equation 7)

[0123] in, Let be the dielectric constant of the material being measured in complex form. and These are the real and imaginary parts of the dielectric constant of the material being tested, respectively. Equation 6 describes the transmission coefficient of the end face of the material being tested obtained from the measurement. (Including the combined results of multiple reflections and transmissions) and the single transmission coefficient of the tested material ( T ) and reflection coefficient ( The quantitative relationship between them.

[0124] The single-pass transmission coefficient of the tested material ( T The dielectric constant of the measured material has the following relationship with the real and imaginary parts:

[0125] (Equation 8)

[0126] (Equation 9)

[0127] in, The speed of light in a vacuum. d The thickness of the material being measured. f It is the frequency of the electromagnetic wave.

[0128] By combining equations (6), (8), and (9), and employing a nonlinear equation iterative algorithm, based on the initial iterative value of the real part of the dielectric constant of the material under test obtained in step 404, the real and imaginary parts of the dielectric constant of the material under test are calculated, thereby obtaining the loss tangent value. .

[0129] The foregoing has described exemplary embodiments of this specification. It should be understood that in some cases, the modules described in this specification may be divided in a manner different from that in the embodiments, and the described actions or steps may be performed in a different order than that in the embodiments, while still achieving the desired result. Furthermore, the processes depicted in the accompanying drawings do not necessarily require a specific or sequential order to achieve the desired result. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0130] Other embodiments of this specification will readily occur to those skilled in the art upon consideration of the specification and practice of the invention claimed herein. This specification is intended to cover any variations, uses, or adaptations that follow the general principles of this specification and include common knowledge or customary techniques in the art not illustrated herein.

[0131] The above description is merely a preferred embodiment of this specification and is not intended to limit this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this specification should be included within the scope of protection of this specification.

Claims

1. A dual-channel superheterodyne free-space method for measuring the dielectric constant of materials, characterized in that, The device includes: The first superheterodyne circuit, located in the first waveguide channel, is used to generate mid / low frequency measurement signals, including a first band transmitting antenna and a first band receiving antenna; The second superheterodyne circuit, located in the second waveguide channel, is used to generate a mid / low frequency synchronization reference signal; The first superheterodyne circuit and the second superheterodyne circuit use the same signal source; The second frequency synthesizer is used to convert the synchronization reference signal into a medium / low frequency synchronization signal with the same frequency as the measurement signal; An inductive voltage divider is used to measure and distribute the voltage of a measurement signal. One of its output ports is connected to a lock-in amplifier to feed the measurement signal into the lock-in amplifier. A lock-in amplifier is used to measure the phase difference between the input measurement signal and the synchronization signal, as well as to measure the voltage of the input measurement signal. The compensation module is used to obtain a first voltage division ratio from an inductive voltage divider and a first voltage of the measurement signal from a lock-in amplifier before the material under test is placed at the measurement position between the first-band transmitting antenna and the first-band receiving antenna; and after the material under test is placed at the measurement position, to obtain a second voltage of the measurement signal from the lock-in amplifier, and to adjust the voltage division ratio of the inductive voltage divider according to the difference between the first voltage and the second voltage, so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage to compensate for the reduced voltage; after compensation, the voltage division ratio of the inductive voltage divider is the second voltage division ratio. The calculation module is used to calculate the channel transmission coefficient magnitude based on the first voltage division ratio and the second voltage division ratio, and to calculate the channel transmission coefficient phase shift based on the phase difference between the measurement signal and the synchronization signal before and after the material under test is placed in the test; after performing phase correction on the channel transmission coefficient based on the channel transmission coefficient magnitude and the channel transmission coefficient phase shift, the transmission coefficient of the end face of the material under test is obtained. The calculation module calculates the initial iterative value of the real part of the dielectric constant of the material under test based on the slope of the frequency response function; the calculation module uses the free space method and a nonlinear iterative algorithm to inversely calculate the real part of the dielectric constant and the loss tangent of the material under test based on the initial iterative value of the real part of the dielectric constant of the material under test; wherein, the frequency response function is obtained by measuring the material under test at the first frequency point (f1) and other frequency points (f2, f3, f4, f5) using the device. f i The end-face transmission coefficients are established under these conditions; The first superheterodyne circuit and the second superheterodyne circuit use frequency multipliers of different factors to mix the two waveguide channels at different frequencies respectively; The first superheterodyne circuit and the second superheterodyne circuit share the same local oscillator source, which generates a local oscillator signal based on the reference signal provided by the signal source. The first superheterodyne circuit includes a first frequency multiplier and a third frequency multiplier, both with a multiplication factor of N; the second superheterodyne circuit includes a second frequency multiplier and a fourth frequency multiplier, both with a multiplication factor of M; wherein N is not equal to M, and N and M are both positive integers greater than 0; The first band transmitting antenna and the first band receiving antenna in the first superheterodyne circuit are connected between the first frequency multiplier and the first mixer; the local oscillator signal output by the local oscillator source is multiplied by the third frequency multiplier and mixed with the output signal of the first band receiving antenna to generate the measurement signal; The other local oscillator signal output from the local oscillator source is multiplied by the fourth frequency multiplier and then mixed with the output signal of the second frequency multiplier to generate the synchronization reference signal.

2. The apparatus according to claim 1, characterized in that, When one of the first waveguide channel and the second waveguide channel is used as the measurement channel, the other channel is used as the synchronization channel; The device further includes: The switch selection circuit consists of a first switch, a second switch, a third switch, and a fourth switch. The first and second switches are used to select the measurement channel and send the medium / low frequency measurement signal of the measurement channel to the inductive voltage divider. The third and fourth switches are used to select the synchronization channel and send the medium / low frequency synchronization signal of the synchronization channel to the lock-in amplifier. The first frequency synthesizer is used to convert the synchronization reference signal output by the first mixer into a medium / low frequency synchronization signal with the same frequency as the measurement signal when the first waveguide channel is used as a synchronization channel; the first frequency synthesizer and the second frequency synthesizer are only connected to the circuit and function when their respective channels are synchronization channels. When the first waveguide channel is the measurement channel, the first band transmitting antenna and the first band receiving antenna are actually connected between the first frequency multiplier and the first frequency mixer in the first superheterodyne circuit, and the second frequency multiplier and the second frequency mixer are directly connected at the same time. When the second waveguide channel is the measurement channel, the second band transmitting antenna and the second band receiving antenna are actually connected between the second frequency multiplier and the second frequency mixer in the second superheterodyne circuit, while the first frequency multiplier and the first frequency mixer are directly connected.

3. The apparatus according to claim 2, characterized in that, The first band transmitting antenna and the first band receiving antenna, the second band transmitting antenna and the second band receiving antenna are standard gain horn antennas or point-focusing lens horn antennas, and the antenna gain is greater than 20dB.

4. The apparatus according to claim 2, characterized in that, The first waveguide channel is a waveguide channel from 75GHz to 110GHz, and the second waveguide channel is a waveguide channel from 50GHz to 75GHz.

5. The apparatus according to claim 2, characterized in that, The signal output from the signal source can be input to the first frequency multiplier and the second frequency multiplier through the first directional coupler; the signal output from the local oscillator source can be input to the third frequency multiplier and the fourth frequency multiplier through the second directional coupler.

6. A dual-channel superheterodyne free-space method for measuring the dielectric constant of materials, characterized in that, The method is applied to the apparatus of any one of claims 1 to 5, and the method includes the steps of: Before the material under test is placed in the measurement channel at the measurement position between the corresponding band transmitting antenna and receiving antenna, the first voltage of the measurement signal measured by the lock-in amplifier, the first phase difference between the measurement signal and the synchronization signal, and the first voltage division ratio measured by the inductive voltage divider are obtained. After the material to be tested is placed in, the second voltage of the measurement signal is obtained from the lock-in amplifier. The voltage division ratio of the inductive voltage divider is adjusted according to the difference between the first voltage and the second voltage so that the measurement signal voltage measured by the lock-in amplifier returns to the first voltage to compensate for the reduced voltage. After compensation, the second phase difference between the measurement signal measured by the lock-in amplifier and the synchronization signal and the second voltage division ratio measured by the inductive voltage divider are obtained. The channel transmission coefficient modulus is calculated based on the first voltage division ratio and the second voltage division ratio, and the phase shift of the channel transmission coefficient is calculated based on the first phase difference and the second phase difference before and after the material under test is placed in the test. The end face transmission coefficient of the measured material is obtained by performing phase correction on the channel transmission coefficient based on the channel transmission coefficient magnitude and the channel transmission coefficient phase shift. Set the signal source to other frequency points, measure the end face transmission coefficient of the material under test at other frequency points again, and thus establish the frequency response function of the phase of the end face transmission coefficient of the material under test. Calculate the iterative initial value of the real part of the dielectric constant of the material under test based on the slope of the frequency response function. Using the free space method and a nonlinear iterative algorithm, the real part of the dielectric constant and the loss tangent of the material under test are calculated by inversion based on the initial iterative value of the real part of the dielectric constant of the material under test.

7. The method according to claim 6, characterized in that, The channel transmission coefficient modulus is calculated based on the first voltage division ratio (D1) and the second voltage division ratio (D2). The method is as follows: The first phase difference before and after the test material is placed in the test ( ) and second phase difference ( ) Calculate the phase shift of the channel transmission coefficient ( The method is as follows: The modulus of the channel transmission coefficient ( ) and channel transmission coefficient phase shift ( After phase correction of the channel transmission coefficient, the end face transmission coefficient of the measured material is obtained. The method is as follows: Where k0 is the free-space propagation wavenumber, d The thickness is the sample thickness.

8. The method according to claim 7, characterized in that, The method employing the free-space method and using a nonlinear iterative algorithm, based on the initial iterative value of the real part of the dielectric constant of the material under test, to inversely calculate the real part of the dielectric constant and the loss tangent of the material under test, is as follows: The transmission coefficient of the end face of the tested material ( ) and the single reflection coefficient of air and the end face of the tested material ( ) and the single transmission coefficient between the two ends of the tested material ( T The relationship is: The single-pass transmission coefficient of the tested material ( T ) and the real part of the dielectric constant of the material being measured ( ) and imaginary part ( The relationship is: in, The speed of light in a vacuum. d The thickness of the material being measured. f The frequency of electromagnetic waves; Based on the above relationship, a nonlinear equation iterative algorithm is used. Based on the initial value of the real part of the dielectric constant of the material under test, the real and imaginary parts of the dielectric constant of the material under test are calculated by inversion, and then the loss tangent is further calculated.

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