Stable non-iterative electromagnetic parameter characterization method based on transmission reflection method and clamp

Abstract

The invention relates to the technical field of electromagnetic characterization, in particular to a stable non-iterative electromagnetic parameter characterization method based on a transmission reflection method and a clamp, and the method comprises the steps: S1, loading a to-be-measured medium into a measurement clamp, dividing the measurement clamp into a to-be-measured medium region and an air region, and enabling the two regions to be equivalent to a two-port network; s2, measuring a scattering parameter S of two ports in the two-port network, and converting the scattering parameter S into an ABCD matrix form; s3, calculating an initial ABCD matrix of the to-be-measured medium according to the total ABCD matrix of the measurement clamp and the ABCD matrix of the air section; s4, calibrating the initial ABCD matrix of the to-be-detected medium, eliminating the influence of an air section, and obtaining a final ABCD matrix of the to-be-detected medium; and S5, converting the final ABCD matrix of the to-be-measured medium back to the scattering parameter S. The method is suitable for various magnetic and non-magnetic materials, and is still effective for materials with complex dielectric constant and magnetic conductivity dependent on frequency.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic characterization technology, and more particularly to a stable non-iterative electromagnetic parameter characterization method and a fixture based on a transmission reflection method. Background Art

[0002] Electromagnetic parameters play an important role in material characterization, providing a way to gain insight into the structure of the material, the temperature in the environment, and the amount of impurities in it. Measuring the electromagnetic parameters of a material requires accurate characterization of the complex permittivity and magnetic permeability within the required frequency band, and plotting the curves of their real and imaginary parts versus frequency. In many microwave applications, it is often necessary to have an accurate understanding of the complex permittivity and magnetic permeability of the material. However, each material has its own unique electromagnetic spectrum characteristics, which depend on its physical parameters. Moreover, the complex permittivity and magnetic permeability are two frequency-dependent parameters, and the stronger the correlation, the more difficult the characterization.

[0003] Current technologies, such as reflection and transmission-based methods, have been developed to measure the complex dielectric constant and permeability of materials. This method inserts the sample material into the cavity in the transmission line, measures its reflection coefficient and transmission coefficient through the instrument, and solves it based on the correspondence between the two sets of parameters and the electromagnetic parameters of the material. However, they often find it difficult to achieve stable electromagnetic parameter characterization in a wide frequency band. The test results of some methods are only valid in a narrow frequency band, while some methods can only characterize electromagnetic parameters at a few selected frequency points. However, with the development of microwave technology, in order to understand the dielectric dispersion characteristics of materials, it is necessary to accurately characterize the complex dielectric constant and permeability in a wider frequency range. In addition, microwave detection technology, as an emerging detection method with many advantages such as non-destructive and non-ionizing, is also booming. In many fields today, designing an effective material detection method based on electromagnetic parameters will help replace many traditional detection and classification modes. Using new methods to overcome the defects of existing measurement methods, achieve more accurate and convenient electromagnetic characterization, or combine electromagnetic parameter measurement to realize a new material detection application platform will have important research value and significance.

[0004] Generally speaking, the dielectric constant and permeability obtained by the transmission reflection method are more accurate than those by the single reflection method. However, the analytical method of obtaining S parameters through the transmission reflection method and then extracting the intrinsic electromagnetic parameters of the material to be tested has a resonance problem, which affects the actual measurement application.

[0005] Therefore, how to provide an intuitive and simple electromagnetic parameter characterization method applicable to various materials is an urgent problem to be solved by technicians in this field. Summary of the invention

[0006] In view of this, the present invention provides a stable non-iterative electromagnetic parameter characterization method and fixture based on the transmission reflection method, which is applicable to various magnetic and non-magnetic materials, and is still effective for materials whose complex dielectric constant and magnetic permeability are frequency-dependent.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a stable non-iterative electromagnetic parameter characterization method based on a transmission reflection method, comprising the following steps:

[0009] S1. Load the medium to be tested into the measuring fixture, and divide the measuring fixture into a medium area to be tested and an air area. The two areas are equivalent to a two-port network.

[0010] S2, measure the scattering parameters S of the two ports in the two-port network, and convert the scattering parameters S into an ABCD matrix form;

[0011] S3, calculating the initial ABCD matrix of the medium to be measured according to the total ABCD matrix of the measuring fixture and the ABCD matrix of the air segment;

[0012] S4, calibrating the initial ABCD matrix of the medium to be measured, eliminating the influence of the air segment, and obtaining the final ABCD matrix of the medium to be measured;

[0013] S5. Convert the final ABCD matrix of the medium to be measured back into the scattering parameter S.

[0014] Furthermore, S3 includes:

[0015] S31. Assuming that the medium to be measured is set close to the first port, the total ABCD matrix of the measuring fixture is expressed as:

[0016]

[0017] S32, the ABCD matrix of the air segment is expressed as:

[0018]

[0019] Wherein, γ0 is the propagation constant when unloaded, d is the length of the measuring fixture at the end of the loading medium, and Z0 represents the characteristic impedance of the measuring fixture when unloaded;

[0020] S33. Calculate the ABCD matrix of the medium to be tested:

[0021]

[0022] Furthermore, S4 includes:

[0023] S41. Measure the measuring fixture without loading the medium to be measured, record the measured scattering parameters as unloaded transmission line parameters, and rewrite them into an ABCD matrix form:

[0024] S42, when a certain volume of the medium to be tested is added, the ABCD matrix of the medium to be tested in S33 is transformed to obtain the final ABCD matrix of the medium to be tested, which is expressed as:

[0025]

[0026] in, Represents the ideal ABCD matrix of a coaxial air segment of length d.

[0027] Further, in S41, when the measuring fixture is not loaded with the medium to be measured, its ABCD matrix is ​​expressed as:

[0028]

[0029] in, and They respectively represent the ABCD ideal matrix with and without the medium to be measured when the object to be measured with a volume of d is added to one side of the measuring fixture with a length of L.

[0030] Furthermore, the transformation relationship between the ABCD matrix and the scattering parameter S is:

[0031]

[0032] Among them, S11 represents the scattering parameter when the first port transmits and the first port receives, S12 represents the scattering parameter when the first port transmits and the second port receives, S21 represents the scattering parameter when the second port transmits and the first port receives, and S22 represents the scattering parameter when the second port transmits and the second port receives.

[0033] In a second aspect, the present invention provides a measurement fixture, which is used in the stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method as described above, comprising: an outer conductor and an inner conductor;

[0034] The outer conductor is a hollow structure, and the inner conductor is inserted into the outer conductor, and the two form a coaxial transmission line; an annular cavity is formed between the inner conductor and the outer conductor, and the medium to be measured is placed in the annular cavity;

[0035] The inner conductor has a first blind hole and a second blind hole at both ends thereof; the first blind hole and the second blind hole are connected to an external SMA connector needle respectively.

[0036] Furthermore, the total length of the coaxial transmission line is 80 mm, and the characteristic impedance is 50Ω in a no-load state.

[0037] Furthermore, the inner diameter of the outer conductor and the outer diameter of the inner conductor satisfy the following relationship:

[0038]

[0039] Where Z0 represents the characteristic impedance, r represents the relative dielectric constant, D represents the inner diameter of the outer conductor, d represents the outer diameter of the inner conductor, and ln represents the natural logarithm.

[0040] Further, when only the TEM mode is allowed to exist in the coaxial transmission line, the following formula is satisfied:

[0041] λ min >λ=π(a+b)

[0042]

[0043] Among them, λ and f represent the wavelength and frequency of the transmission signal when it is filled with air, c represents the speed of light in a vacuum, a and b represent the outer radius of the inner conductor and the inner radius of the outer conductor of the coaxial transmission line, respectively, and ε r and μ r represent the relative permittivity and magnetic permeability of the filling medium respectively.

[0044] Furthermore, the diameter of the first blind hole and the second blind hole is 1.25 mm, and the depth is 9 mm; the inner diameter of the outer conductor is 7.0 mm or 4.6 mm; the outer diameter of the inner conductor is 3.0 mm or 2.0 mm; the outer contour of the outer conductor is a rectangular parallelepiped, and connecting holes are provided on both end faces for connecting to an external SMA connector through bolts.

[0045] It can be seen from the above technical solutions that, compared with the prior art, the present invention has the following beneficial effects:

[0046] Compared with the classic NRW extraction method, the method of the present invention has no requirement on the length of the medium to be measured, which reduces the processing difficulty; on the other hand, it avoids the inaccuracy of extracting parameters at the half-wave resonant frequency. Compared with the iterative method, the method proposed by the present invention does not require the estimation of the initial value, which avoids the wrong solution caused by the wrong initial value selection.

[0047] In addition, the present invention will use machined coaxial transmission lines to perform theoretical simulation and actual measurement verification on the proposed method to form a more intuitive electromagnetic parameter characterization algorithm.

[0048] The present invention can be used to characterize a wide range of materials, including various magnetic and non-magnetic materials, and is still effective for materials whose complex dielectric constant and magnetic permeability are frequency-dependent. Compared with existing methods, the present invention is simpler and more efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0050] Figure 1 A schematic diagram of the structure of the measuring fixture provided by the present invention;

[0051] Figure 2 A cross-sectional view of the measuring fixture provided by the present invention;

[0052] Figure 3 A coaxial transmission line simulation model provided by the present invention;

[0053] Figure 4 A flow chart of a stable non-iterative electromagnetic parameter characterization method based on a transmission reflection method provided by the present invention;

[0054] Figure 5 It is the transmission and reflection situation when the electromagnetic wave is incident on the interface between air and medium in the algorithm principle of the present invention;

[0055] Figure 6 Comparison of complex electromagnetic parameters of 20 mm virtual materials using different characterization methods, including (a) complex permittivity, (b) complex magnetic permeability;

[0056] Figure 7 A comparison diagram of the simulated S parameters calculated by the NRW method and the method of the present invention;

[0057] Figure 8 Comparison of complex electromagnetic parameters of 30 mm virtual material with different characterization methods: (a) uncompensated phase, (b) compensated phase. DETAILED DESCRIPTION

[0058] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0059] like Figure 1-Figure 2 As shown, an embodiment of the present invention discloses a measuring fixture, comprising: an outer conductor 1 and an inner conductor 2;

[0060] The outer conductor 1 is a hollow structure, and the inner conductor 2 is inserted into the outer conductor 1, and the two form a coaxial transmission line; an annular cavity 3 is formed between the inner conductor 2 and the outer conductor 1, and the medium to be measured is placed in the annular cavity 3;

[0061] The inner conductor 2 has a first blind hole 21 and a second blind hole 22 at both ends thereof; the first blind hole 21 and the second blind hole 22 are connected to the external SMA connector needles respectively.

[0062] The inner diameter of the outer conductor and the outer diameter of the inner conductor satisfy the following relationship:

[0063]

[0064] Where Z0 represents the characteristic impedance, r represents the relative dielectric constant, D represents the inner diameter of the outer conductor, d represents the outer diameter of the inner conductor, and ln represents the natural logarithm.

[0065] When only the TEM mode is allowed to exist in the coaxial transmission line, the following formula is satisfied:

[0066] λ min >λ=π(a+b)

[0067]

[0068] Among them, λ and f represent the wavelength and frequency of the transmission signal when it is filled with air, c represents the speed of light in a vacuum, a and b represent the outer radius of the inner conductor and the inner radius of the outer conductor of the coaxial transmission line, respectively, and ε r and μ r represent the relative permittivity and magnetic permeability of the filling medium respectively.

[0069] Specifically, the total length of the coaxial transmission line is 80 mm, and the characteristic impedance is 50 Ω in the no-load state. In order to obtain better test results, the length of the medium to be tested varies with the material type of the medium and is not unique. In order to enable the measurement fixture to connect to the coaxial transmission line of the vector network, two ends of the inner conductor of the coaxial transmission line are drilled with a diameter of d h = 1.25mm, 9mm deep small holes are used as the first blind hole and the second blind hole to connect the SMA connector needle of a specific model. In order to achieve impedance matching, two specifications of coaxial transmission lines are designed. The inner diameter b of the outer conductor of the coaxial transmission line is designed to be 7.0mm and 4.6mm respectively, and the outer diameter a of the inner conductor is set to 3.0mm and 2.0mm respectively. In addition, the overall rectangular shell of the outer conductor is a cuboid with a square cross section and a size d s The diameter of the connector is 12.7 mm, and connection holes are provided on both end faces for connecting to external SMA connectors via bolts.

[0070] In another embodiment, Figure 4 As shown, an embodiment of the present invention also provides a stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method, comprising the following steps:

[0071] S1. Load the medium to be tested into the measuring fixture, and divide the measuring fixture into a medium area to be tested and an air area. The two areas are equivalent to a two-port network.

[0072] S2, measure the scattering parameters S of the two ports in the two-port network, and convert the scattering parameters S into an ABCD matrix form;

[0073] S3. Calculate the initial ABCD matrix of the medium to be measured according to the total ABCD matrix of the measuring fixture and the ABCD matrix of the air segment, specifically including:

[0074] S31. Assuming that the medium to be measured is set close to the first port, the total ABCD matrix of the measuring fixture is expressed as:

[0075]

[0076] S32, the ABCD matrix of the air segment is expressed as:

[0077]

[0078] Wherein, γ0 is the propagation constant when unloaded, d is the length of the measuring fixture at the end of the loading medium, and Z0 represents the characteristic impedance of the measuring fixture when unloaded;

[0079] S33. Calculate the ABCD matrix of the medium to be tested:

[0080]

[0081] S4. Calibrate the initial ABCD matrix of the medium to be measured, eliminate the influence of the air segment, and obtain the final ABCD matrix of the medium to be measured, which specifically includes:

[0082] S41. Measure the measuring fixture without loading the medium to be measured, record the measured scattering parameters as unloaded transmission line parameters, and rewrite them into an ABCD matrix form: It is expressed as:

[0083]

[0084] in, and They respectively represent the ABCD ideal matrix with and without the medium to be measured when the object to be measured with a volume of d is added to one side of the measuring fixture with a length of L.

[0085] S42, when a certain volume of the medium to be tested is added, the ABCD matrix of the medium to be tested in S33 is transformed to obtain the final ABCD matrix of the medium to be tested, which is expressed as:

[0086]

[0087] in, Represents the ideal ABCD matrix of a coaxial air segment of length d.

[0088] S5. Convert the final ABCD matrix of the medium to be measured back to the scattering parameter S, wherein the conversion relationship between the ABCD matrix and the scattering parameter S is:

[0089]

[0090] Among them, S11 represents the scattering parameter when the first port transmits and the first port receives, S12 represents the scattering parameter when the first port transmits and the second port receives, S21 represents the scattering parameter when the second port transmits and the first port receives, and S22 represents the scattering parameter when the second port transmits and the second port receives.

[0091] Since the premise of transmission reflection method measurement is that the reference surface is at both ends of the medium to be measured, it is necessary to perform S4 de-embedding operation, that is, to move the reference surface to the interface between the medium to be measured and the air. Therefore, it is necessary to use a certain method to transform the measured scattering parameters and then use them in the subsequent inversion steps to eliminate the influence of the air segment and obtain more accurate test results.

[0092] The principle of the method of the present invention is further explained below, mainly the non-iterative algorithm theory. The transmission reflection method originally proposed by Nicolson, Ross and Weir is derived from the boundary conditions of electromagnetic waves. This method is applicable to coaxial line structures that only propagate TEM mode electromagnetic waves and rectangular waveguide structures that only propagate TE01 mode electromagnetic waves. When electromagnetic waves are incident on the interface between air and medium, the transmission and reflection situations are as follows Figure 5 As shown. For a transmission line that has been calibrated at both ends of the reference plane, it can be regarded as a two-port microwave network. When the medium to be tested is loaded into it, the scattering parameters S11 and S21 can be expressed by the following formula:

[0093]

[0094] Among them, Γ is the first reflection coefficient at the interface between air and filling medium at the incident port, and T is the transmission coefficient of the filling medium in the transmission line. According to the transmission line theory, the scattering parameters can be expressed by the electromagnetic parameters of the medium:

[0095]

[0096] T=e -γd

[0097]

[0098] Where Z and Z0 represent the characteristic impedance of the air section and the loaded medium section of the coaxial transmission line, respectively; γ and γ0 represent the propagation constants of electromagnetic waves in the coaxial transmission lines filled with air and medium, respectively; λ represents the wavelength of the electromagnetic wave in free space; λc represents the cutoff wavelength of the coaxial line or rectangular waveguide; d represents the thickness of the loaded medium; μ r Represents the relative magnetic permeability of the medium, describing the influence of the medium on the magnetic field, ε r represents the relative dielectric constant of the medium, and j represents the imaginary unit.

[0099] In order to facilitate the solution, the variable X is introduced: Combining the above formulas, we can get the relationship between the reflection coefficient Γ and the variable X:

[0100] The sign "±" in the above formula needs to satisfy |Γ|<1. ​​Further deduction can be made to obtain the transmission coefficient T:

[0101]

[0102] Finally, combining the above formulas, the relative complex permittivity and magnetic permeability of the sample can be derived:

[0103]

[0104] in, Λ represents another wavelength parameter that is related to the medium.

[0105] When the medium to be measured is a low-loss material, the electromagnetic wave attenuates less when passing through, so the electromagnetic wave will bounce back and forth inside the sample. At the frequency where the medium thickness corresponds to an integer multiple of half the wavelength, the half-wave loss of reflection from the sparse medium to the dense medium causes the reflected electromagnetic waves at different interfaces to have opposite phases and thus cancel each other out. Therefore, the amplitude of the measured scattering parameter S11 is particularly small, and the phase uncertainty becomes very large. At these specific frequencies, the complex dielectric constant and magnetic permeability inverted by the NRW method show abnormal divergence. A literature review found that although the complex dielectric constant and magnetic permeability diverge at the resonant frequency, the product of these two items is stable. Therefore, when the sample is a non-magnetic material, the introduction of the equivalent magnetic permeability μ eff =μ r =1, and then introduce the equivalent dielectric constant ε eff , the divergence phenomenon can be avoided and a stable inversion value can be obtained:

[0106]

[0107] For the characterization of magnetic materials, most methods are difficult to avoid the half-wave resonance problem. The present invention utilizes the inherent dielectric relaxation characteristics of the material, and can stably extract the complex dielectric constant and magnetic permeability by introducing the knowledge of the prior relaxation model. For various materials in nature, the change of the complex dielectric constant usually obeys some specific models, such as the Debye model. The Debye model is one of the most practical and widely used material dispersion models in microwave environments. In the Debye model, the complex dielectric constant obeys the following dispersion relationship:

[0108]

[0109] Where n represents the order of the model. Usually, a first-order model is sufficient to represent the relaxation characteristics of most materials; ε r' (ω) represents the real part of the dielectric constant at frequency ω, ε r” (ω) represents the imaginary part of the dielectric constant at frequency ω; ε ∞ Represents the dielectric constant in the high-frequency limit, that is, the value when ω→∞; Δε j represents the change in dielectric constant of the jth relaxation process; ω represents the frequency; τ j In the coaxial transmission line, in order to intuitively see the relationship between the scattering parameters and the electromagnetic parameters, the complex dielectric constant and magnetic permeability can be expressed as:

[0110]

[0111] Since the product of the complex permittivity and the magnetic permeability is still stable at the half-wave frequency, a variable can be introduced to represent the product value:

[0112] ε r (unstable) r (unstable)=ε r (stable)μ r (stable)=A

[0113] By establishing the relationship between A and scattering parameters through the Debye model, we can obtain:

[0114]

[0115] By measuring N frequency points in the measurement area, N nonlinear equations can be obtained. By solving the equations, the unknown parameters ε of the Debye relaxation model can be obtained. ∞ , Δε, τ, and thus obtain the complex dielectric constant spectrum within the frequency band. At the same time, once the complex dielectric constant is obtained, the complex magnetic permeability can be simply calculated by A / ε r Find out.

[0116] In order to verify the feasibility of the above formula derivation, the material simulation and inversion verification are carried out through the finite element software AnsysHFSS. Figure 3 As shown in the figure, in the simulation software, the de-embedding operation of the measuring fixture is performed, that is, the measured Convert to To obtain accurate scattering parameters.

[0117] Set up a virtual dispersive magnetic material, assume that its complex dielectric constant satisfies the first-order Debye model, and its parameters are the same, and there is ε ∞ =2, Δε=2, τ=2. The magnetic permeability is 1.5-0.015i. The 0-6 GHz frequency band coaxial transmission line measurement fixture of the present invention is used, and the material to be tested is set as a ring-shaped sample with a thickness of 20 mm, an outer diameter of 7 mm, and an inner diameter of 3 mm.

[0118] Figure 6 The simulated S parameter results are shown, and it can be seen that there is a resonance phenomenon near 3GHz and 6GHz. The amplitude of S11 is very low near the resonance frequency, and the phase of S11 changes rapidly. In addition, it can be seen that since the thickness of the medium does not exceed one wavelength, the phase change of S21 does not exceed 360 degrees, that is, there is no phase ambiguity of S21 in the characterization of electromagnetic parameters at this time, and only the influence of thickness resonance is observed.

[0119] In order to analyze the feasibility of the method of the present invention, the simulated S parameters were calculated using the NRW method and the method of the present invention, and the complex electromagnetic parameters were inverted. The real and imaginary parts of the complex dielectric constant and magnetic permeability of the virtual material were obtained in the 0.1-6 GHz frequency band. The comparison results are shown in Figure 2. Figure 7 (a). For easy observation, the set reference value is also shown in Figure 7 (a). It can be seen that at the resonant frequency, i.e., near 3 GHz and 6 GHz, the real and imaginary parts of the dielectric constant extracted by the NRW method are significantly deviated from the set values, and the same is true for the complex magnetic permeability. At the same time, in the entire frequency range, the error of the NRW method is much greater than that of the method of the present invention. Whether it is the real and imaginary parts of the complex dielectric constant or the magnetic permeability, the absolute value of the error of the method of the present invention is less than 0.01 at most frequency points. This fully demonstrates that the method of the present invention can effectively eliminate the divergence phenomenon during the inversion of electromagnetic parameters, while having a high degree of accuracy.

[0120] In addition, in order to observe the phase blur phenomenon, the length of the medium was increased to 30 mm and the simulation was performed again. Figure 8The S parameter results of the simulation are shown. Due to the increase in thickness, the half-wave resonant frequency moves to the left, and three resonant frequency points appear near 2, 4, and 6 GHz. The S11 amplitudes of these three resonant points gradually increase. This is because the existence of material loss reduces the amplitude of the electromagnetic wave reflected from the second interface, thereby weakening the cancellation effect of the electromagnetic wave reflected from the first interface. Figure 7 (b) It can be seen that the S11 phase mutation phenomenon still occurs at the half-wave resonant frequency.

[0121] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0122] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A stable non-iterative electromagnetic parameter characterization method based on transmission reflection method, characterized in that: The following steps are involved: S1. Load the medium to be tested into the measuring fixture, and divide the measuring fixture into a medium area to be tested and an air area. The two areas are equivalent to a two-port network. S2, measure the scattering parameters S of the two ports in the two-port network, and convert the scattering parameters S into an ABCD matrix form; S3, calculating the initial ABCD matrix of the medium to be measured according to the total ABCD matrix of the measuring fixture and the ABCD matrix of the air segment; S4, calibrating the initial ABCD matrix of the medium to be measured, eliminating the influence of the air segment, and obtaining the final ABCD matrix of the medium to be measured; S5. Convert the final ABCD matrix of the medium to be measured back into the scattering parameter S.

2. The stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method according to claim 1 is characterized in that S3 include: S31. Assuming that the medium to be measured is set close to the first port, the total ABCD matrix of the measuring fixture is expressed as: S32, the ABCD matrix of the air segment is expressed as: Wherein, γ0 is the propagation constant when unloaded, d is the length of the measuring fixture at the end of the loading medium, and Z0 represents the characteristic impedance of the measuring fixture when unloaded; S33. Calculate the ABCD matrix of the medium to be tested:

3. The stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method according to claim 2 is characterized in that S4 include: S41. Measure the measuring fixture without loading the medium to be measured, record the measured scattering parameters as unloaded transmission line parameters, and rewrite them into an ABCD matrix form: S42, when a certain volume of the medium to be tested is added, the ABCD matrix of the medium to be tested in S33 is transformed to obtain the final ABCD matrix of the medium to be tested, which is expressed as: in, Represents the ideal ABCD matrix of a coaxial air segment of length d.

4. The stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method according to claim 3 is characterized in that: In S41, when the measuring fixture is not loaded with the medium to be measured, its ABCD matrix is ​​expressed as: in, and They respectively represent the ABCD ideal matrix with and without the medium to be measured when the object to be measured with a volume of d is added to one side of the measuring fixture with a length of L.

5. The stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method according to claim 1 is characterized in that: The conversion relationship between the ABCD matrix and the scattering parameter S is: Among them, S11 represents the scattering parameter when the first port transmits and the first port receives, S12 represents the scattering parameter when the first port transmits and the second port receives, S21 represents the scattering parameter when the second port transmits and the first port receives, and S22 represents the scattering parameter when the second port transmits and the second port receives.

6. A measuring fixture, characterized in that: It is applied in the stable non-iterative electromagnetic parameter characterization method based on the transmission reflection method as described in any one of claims 1 to 5, comprising: an outer conductor and an inner conductor; The outer conductor is a hollow structure, and the inner conductor is inserted into the outer conductor, and the two form a coaxial transmission line; an annular cavity is formed between the inner conductor and the outer conductor, and the medium to be measured is placed in the annular cavity; The inner conductor has a first blind hole and a second blind hole at both ends thereof; the first blind hole and the second blind hole are connected to an external SMA connector needle respectively.

7. The measuring fixture according to claim 6, characterized in that: The total length of the coaxial transmission line is 80 mm, and the characteristic impedance is 50Ω in a no-load state.

8. The measuring fixture according to claim 6, characterized in that: The inner diameter of the outer conductor and the outer diameter of the inner conductor satisfy the following relationship: Where Z0 represents the characteristic impedance, represents the relative dielectric constant, D represents the inner diameter of the outer conductor, d represents the outer diameter of the inner conductor, and ln represents the natural logarithm.

9. The measuring fixture according to claim 6, characterized in that: When only the TEM mode is allowed to exist in the coaxial transmission line, the following formula is satisfied: l min >λ=π(a+b) Among them, λ and f represent the wavelength and frequency of the transmission signal when it is filled with air, c represents the speed of light in a vacuum, a and b represent the outer radius of the inner conductor and the inner radius of the outer conductor of the coaxial transmission line, respectively, and ε r and μ r represent the relative permittivity and magnetic permeability of the filling medium respectively.

10. The measuring fixture according to claim 6, characterized in that: The diameter of the first blind hole and the second blind hole is 1.25 mm and the depth is 9 mm; the inner diameter of the outer conductor is 7.0 mm or 4.6 mm; the outer diameter of the inner conductor is 3.0 mm or 2.0 mm; the outer contour of the outer conductor is a rectangular parallelepiped, and connecting holes are provided on both end faces for connecting to an external SMA connector through bolts.

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