Open type parallel plate waveguide probe and complex dielectric constant measuring method

Abstract

The invention discloses an open type parallel plate waveguide probe and a complex dielectric constant measuring method. According to the probe, a conical transition parallel plate waveguide structure is adopted to be matched with a metal flange, and a single-port reflection coefficient measurement value is converted into a complex dielectric constant by establishing a coaxial admittance model. Compared with the prior art, the device has the advantages of no low-frequency limitation, capability of measuring anisotropic materials, compact structure and the like. Simulation shows that the real part measurement error of wood is less than 3.1% compared with that of a simulation database, and actual measurement shows that the measurement result of polymethyl methacrylate (PMMA) is consistent with the data value reported in the existing literature.

Description

Technical Field

[0001] The present invention belongs to the field of microwave measurement technology, and specifically relates to a probe device based on an open parallel plate waveguide (PPW) structure and a complex dielectric constant measurement method thereof, which can be applied to fields such as materials science, food engineering, and nondestructive testing. Background Art

[0002] The measurement of dielectric constant is crucial in fields such as food engineering, nondestructive testing, and microwave engineering. Currently, there are various methods for measuring the complex dielectric constant of materials, including the waveguide method, free-space method, resonant cavity method, and open-type method. The open-type method is widely used due to its advantages such as non-destructiveness, ability to measure low frequencies, and wide bandwidth.

[0003] Existing open-type coaxial probes cannot extract parameters from anisotropic materials due to the symmetry of the coaxial main mode electric field, which greatly limits their application. While open-type rectangular waveguides solve the problem of measuring anisotropic materials, their cutoff frequency is limited by their size, making them difficult to operate at low frequencies. This is particularly problematic when the material being measured is small. Although substrate-integrated waveguides (SIWs) and half-mode substrate-integrated waveguides (HMSIWs) have reduced the cutoff frequency to a certain extent, the cutoff frequency of HMSIWs for material parameter testing remains around 2.3 GHz, which still limits their application. Therefore, it is essential to design a device without low-frequency limitations and high polarization purity to expand the measurement capabilities of the open-type method. Summary of the Invention

[0004] To address the existing challenges in measuring the complex permittivity of materials at low frequencies and for anisotropic materials, this paper proposes a method for measuring the complex permittivity of materials using an open-ended parallel plate waveguide (PPW) probe. This method leverages the admittance model of an open waveguide to correlate the complex permittivity of the material with the measured single-port reflection coefficient, achieving accurate measurement of the material's complex permittivity, particularly suitable for low-frequency applications.

[0005] The technical solutions of the present invention are as follows:

[0006] A method for measuring the complex dielectric constant of a material based on an open parallel plate waveguide probe. The measurement system consists of an open parallel plate waveguide probe, a vector network analyzer, and a computer. The specific steps are as follows:

[0007] Step 1: Install the measurement system and connect the open parallel plate waveguide probe to the vector network analyzer, with the probe facing the material to be measured. The open parallel plate waveguide probe uses a coaxial feeding and gradient line matching structure, and is equipped with a metal flange to ensure good contact with the material to be measured. Its structural parameters are as follows: Figure 2As shown, the specific dimensions are a=10mm, b=4mm, w=2mm, d=10mm, mm, mm.

[0008] Step 2: Measure the aperture field distribution of the waveguide. The measurement results are as follows: Figure 5 As shown in the figure, it can be seen that most of the electric field energy is concentrated within the waveguide, with minimal edge effects and high electric field polarization purity. The waveguide aperture field is purely polarized, enabling the use of waveguide probes to measure the complex dielectric constant of anisotropic materials. Furthermore, the low leakage wave characteristics of the open waveguide make the model error acceptable.

[0009] Step 3: Measure the single-port reflection coefficient using a vector network analyzer. Place the metal flange close to the material being measured and measure the three materials of no-load, ideal conductor, and standard part, and record the three conditions. Parameters, representing , and ; Place the metal flange close to the test piece and record the , expressed as ;

[0010] Step 4: Data processing and solution: The measured reflection coefficient data is transmitted to a computer, processed and solved based on the coaxial admittance model to obtain the complex dielectric constant of the material to be measured.

[0011] Furthermore, in step 4, the equivalent admittance model of the open waveguide termination to the material under test is as follows: Figure 1 As shown in the circuit model of the open parallel plate waveguide, the inside of the waveguide is equivalent to a transmission line with a characteristic impedance of Z0 and an equivalent admittance of Y0. The material at the end is equivalent to a complex capacitor with lumped parameters, whose complex permittivity is related to the material parameters. The electric field change in the waveguide interface is equivalent to a lumped edge stray capacitance with a value of The relationship between the reflection coefficient and material parameters is established using this equivalent model.

[0012] Furthermore, in step 4, the terminal admittance expression is , represents the fringe capacitance inside the waveguide, It represents the capacitive component generated by the open waveguide in the medium to be measured. Its real part is the capacitance component, which represents the electric field energy storage, and its imaginary part is the equivalent conductance, which represents the energy consumption. It can be seen that the value of the equivalent complex capacitance is linearly related to the complex dielectric constant. Represents the capacitive component when the medium is air; the terminal reflection coefficient can be expressed as , is the characteristic admittance of the waveguide.

[0013] Furthermore, in step 3, after obtaining the reflection coefficient, the open waveguide can be regarded as a two-port network with a scattering matrix. When the metal flange is connected to three materials: no load, ideal conductor and standard part, the reflection coefficient at the coaxial connector end is , and Reflection coefficient at the metal flange , and The relationship is

[0014]

[0015] in is the relative dielectric constant of the standard part.

[0016]

[0017] ;

[0018] The relative dielectric constant of the device under test can be calculated from the measurement results:

[0019] .

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] The open parallel plate waveguide probe of the present invention has no low-frequency limitations and can measure the complex dielectric constant of materials at frequencies below 2 GHz. The probe's electric field has high polarization purity, making it applicable to the measurement of anisotropic materials and expanding the measurement range. Simulation and experimental verification show that the probe's measurement results are in good agreement with reference values ​​and existing literature results, demonstrating high measurement accuracy and reliability. The probe's compact structure makes it easy to manufacture and use, reducing measurement costs and operational difficulty. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 : Probe equivalent circuit diagram

[0023] Figure 2 : Schematic diagram of the three-dimensional structure of the probe

[0024] Figure 3 : Comparison of simulation and reference values ​​of wood

[0025] Figure 4 :Comparison of PMMA measured data with literature

[0026] Figure 5 : Field distribution at the probe aperture DETAILED DESCRIPTION

[0027] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0028] Example 1:

[0029] The feasibility of this method was verified by performing a full-wave simulation in the simulation software CST Studio Suite 2021. The specific simulation steps are as follows:

[0030] Step 1: Create a Figure 2 The open waveguide model shown in the figure is used, and the length and width of the test sample are set to 60 mm, the thickness of the bottom is 30 mm, and it is close to the end of the waveguide.

[0031] Step 2: Create four project files and set the material of the test sample to air; metal plate; non-destructive material with a dielectric constant of 2.65 as a standard part; and wood as the test board.

[0032] Step 3: Set the excitation and calculate in the CST time domain solver, establish a wave port feed at the SMA coaxial connector port, and correctly set the simulation frequency range and wave port settings.

[0033] Step 4: After obtaining the corresponding reflection coefficient, calculate the relative dielectric constant of wood and compare the obtained result with the data in the CST material library, such as Figure 3 As shown in the figure, for wood, the error in the real part is limited to 5.7%, and the error in the imaginary part does not exceed 27.3%. In the simulation verification, a lossless material with a dielectric constant of 2.65 was used as a reference. In the figure, "Ref." represents the reference value from the CST material library, and "Mea." represents the value obtained by measurement using the recommended PPW probe.

[0034] Example 2:

[0035] The feasibility and correctness of the measurement method were verified through experiments. The specific experimental steps are as follows:

[0036] Step 1: Connect the SMA connector of the open parallel plate waveguide probe to the vector network analyzer.

[0037] Step 2: Place the metal flange close to the material to be tested, measure the three materials of no-load, ideal conductor and standard part respectively, and record the corresponding , where F4B material with a dielectric constant of 2.65 is used as the standard part.

[0038] Step 3: Select polymethyl methacrylate (PMMA) as the test material and measure the corresponding .

[0039] Step 4: Input the results of steps 2 and 3 into the calculation to calculate the relative dielectric constant of PMMA.

[0040] The effects of the present invention are as follows Figure 4 As shown in the figure, the measurement range is set to 0.2-2 GHz. The measurement results show that the real part of the PMMA dielectric constant is between 2.589 and 2.856, and the imaginary part is between 0.016 and 0.076, both of which are within the parameter value range reported in existing literature, further proving the accuracy and reliability of the measurement method.

[0041] The above descriptions are only two specific examples of the present invention. Obviously, for professionals in this field, after understanding the content and principles of the present invention, it is possible to make various modifications and changes in form and details without departing from the principles and structure of the present invention. However, these modifications and changes based on the ideas of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. An open parallel plate waveguide probe, characterized in that include: A parallel plate waveguide body (1), a tapered impedance transformation section (2), a rectangular impedance transformation section (3), a metal flange (4) and an SMA coaxial connector (5); the tapered impedance transformation section (2) and the rectangular impedance transformation section (3) together constitute an impedance transformation structure located on the upper surface of the parallel plate waveguide body (1); the metal flange (4) and the SMA coaxial connector (5) are connected on both sides of the parallel plate waveguide body (1); and the central conductors of the rectangular impedance transformation section (3) and the SMA coaxial connector (5) are connected on both sides of the tapered impedance transformation section (2).

2. The probe according to claim 1, characterized in that The tapered section (2) adopts a linear gradient structure, with a width gradually changing from 2 mm at the SMA coaxial connector (5) to 10 mm at the rectangular impedance transformation section (3).

3. The probe according to claim 1, characterized in that The rectangular impedance transformation section (3) structure is 10 mm long, starts from the tapered linear gradient section (2) structure, and extends to the metal flange (4), which is 4 mm wide.

4. The probe according to claim 1, characterized in that The SMA coaxial connector (5) is welded to the narrow side of the tapered linear gradient section (2) structure.

5. The probe according to claim 1, characterized in that The parallel plate waveguide body (1) is 10 mm long, 4 mm wide and 2 mm high.

6. A method for measuring complex dielectric constant, implemented using the probe according to any one of claims 1 to 4, comprising: Step 1: Connect the coaxial connector of the open parallel plate waveguide probe to the spectrum analyzer; Step 2: Measure the metal flange in three situations: no load, ideal conductor and standard parts, and record the three situations. Parameters, representing , and ; Step 3: The open waveguide can be viewed as a two-port network with a scattering matrix. The reflection coefficient at the coaxial connector end is , and Reflection coefficient at the metal flange , and The relationship is , is the relative dielectric constant of the standard ； ； Step 4: Measure the metal flange when receiving the test piece , and recorded as ; Step 5: The quantitative relationship between the relative dielectric constant of the test piece and the measured reflection coefficient is 。 7. A method for measuring complex dielectric constant according to claim 6, characterized in that: The terminal reflection coefficient in step 3 can be expressed as , is the characteristic admittance of the waveguide, is the terminal admittance, , represents the fringe capacitance inside the waveguide, represents the equivalent complex capacitance of the open waveguide in the medium to be measured, Indicates the capacitance when the medium is air.

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