CN-121995622-A - Symmetrical quasi-optical cavity design method for dielectric constant measurement

CN121995622ACN 121995622 ACN121995622 ACN 121995622ACN-121995622-A

Abstract

The invention discloses a symmetrical quasi-optical cavity design method for dielectric constant measurement, which belongs to the technical field of quasi-optical cavity design and comprises a mirror design method and a coupling design method, wherein the mirror design method comprises the following steps that a quasi-optical cavity is composed of a spherical mirror with a transmission type symmetrical structure, parameters required to be designed mainly comprise the curvature radius of the spherical mirror, the mouth surface radius of the spherical mirror, the distance between two spherical mirrors and the spherical mirror material, and factors considered during design comprise intrinsic spectral line purity, stability of an open cavity, Q value of the open cavity and beam waist radius, and the coupling design method comprises the following steps of feeding signals out of the symmetrical quasi-optical cavity in a coupling ring mode. The design method of the symmetrical quasi-optical cavity creatively proposes that a coupling ring is placed between two spherical mirrors to feed in/feed out signals, replaces the traditional fixed aperture coupling mode, flexibly adjusts the coupling strength of the coupling ring through parameters such as ring diameter, line diameter, position and the like, and solves the problems that the coupling strength of the coupling ring cannot be adjusted and the flexibility is poor after aperture coupling processing.

Inventors

FENG LIYING
LIU TING
ZHANG NA
DENG SHUPEI
LI YING

Assignees

北京无线电计量测试研究所

Dates

Publication Date: 20260508
Application Date: 20251226

Claims (10)

1. The design method of the symmetrical quasi-optical cavity for dielectric constant measurement is characterized by comprising a mirror design method and a coupling design method; the design method of the mirror is as follows: the quasi-optical cavity is an open cavity and consists of spherical mirrors with transmission type symmetrical structures, and parameters to be designed mainly comprise the curvature radius of the spherical mirrors, the radius of the mouth surface of the spherical mirrors, the distance between the two spherical mirrors and the material of the spherical mirrors; Factors considered in design include intrinsic line purity, open cavity stability, open cavity Q value, and beam waist radius, Q being a quality factor value; the coupling design method comprises the following steps: The coupling ring is used for feeding signals into and out of the symmetrical quasi-optical cavity, the coupling ring realizes energy exchange through 'magnetic field coupling', the coupling effect is related to ring diameter, ring line diameter, turns and position parameters, and the coupling ring design needs to balance the requirements of coupling efficiency, impedance matching and mode compatibility.
2. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 1, wherein the specific design method for mirror design comprises the steps of: S11, preliminarily obtaining the relation between the cavity length L and the curvature radius R according to the stability of the open cavity; S12, comprehensively pushing out the relation between the cavity length L and the curvature radius R according to the purity of the intrinsic spectral line and the stability requirement of the open cavity; s13, estimating a Q value design target according to the requirement of the material loss tangent measurement precision; S14, designing a target and a target test frequency according to the Q value of the open cavity, and determining the relation between the cavity material of the open cavity and the coating material and the cavity length; S15, determining the radius of the spherical mirror surface according to the beam radius and the cavity length; s16, determining the optimal cavity length according to the Fresnel number; The specific design method of the coupling design comprises the following steps: s21, determining the diameter range of the ring according to the coupling strength; s22, determining a loop line diameter range by impedance matching and conductor loss; s23, designing turns; s24, determining the position of the coupling ring by the main mode of the quasi-optical cavity.
3. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 2, wherein S11 is specifically: The stability of the quasi-optical cavity is described by adopting a curvature factor g i , and when the curvature factor is 0<g i g i <1, the resonant cavity is stable and L/2< R < L is satisfied.
4. A symmetrical quasi-optical cavity design method for dielectric constant measurement according to claim 3, wherein the specific design of S12 is as follows: Δf 1 ＝f 00q -f 11(q-2) =2a-3ab; Δf 2 ＝f 10(q-1) -f 00q =2ab-a; f 00q is the frequency of the TEM 00q mode, f 11(q-2) is the frequency of the higher order mode to the left of the TEM 00q mode, f 10(q-1) is the frequency of the higher order mode to the right of the TEM 00q mode, Δf 1 is the difference in frequency between the TEM 00q mode and the higher order mode to the left, and Δf 2 is the difference in frequency between the TEM 00q mode and the higher order mode to the right; When b is selected, delta f 1 and delta f 2 are both as large as possible, and the relation between the cavity length L and the curvature radius R is that
5. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 4, wherein S13 is specifically: the accuracy of the loss tangent measurement depends on the quality factor, and the formula is: n is the refractive index of the sample to be measured, k is the wave number, t is half of the thickness of the sample, D' =d-t, d=l/2; The loss tangent of a common low loss material is on the order of 10 -4 , and the Q value design goal is 5 ten thousand or more, by the ability to measure the reciprocal of the quality factor.
6. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 5, wherein S14 is specifically: The relationship between the quality factor and the specular loss is: Q=L/2δ wherein delta is skin depth; the minimum cavity length calculation formula is: L>2δQ; The step S15 specifically comprises the following steps: The calculation formula of the beam radius w s at the spherical mirror is as follows: lambda is wavelength, omega 0 is beam waist radius; The spherical mirror aperture radius A is greater than the beam radius at the spherical mirror by a factor of 2.2.
7. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 6, wherein S16 is specifically: The relation between the cavity length of the symmetrical quasi-optical cavity and the Fresnel number and the radius of the spherical mirror surface is as follows: n is fresnel number, g=1-L/R; if the confocal cavity is, the following steps are: the range of the sphere cavity length and the curvature radius is And R=L is taken by using the confocal cavity, wherein A, L and R are both equal to each other, the optimal cavity length L is obtained, and the corresponding curvature radius R and the spherical mirror surface radius A parameter are obtained.
8. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 7, wherein S21 is specifically that the effective action range of the magnetic field in the cavity is required to be matched with the diameter r of the ring, and r is more than or equal to 0.1 lambda and less than or equal to 0.3 lambda; the S22 is specifically that impedance matching is carried out to obtain a line diameter d range, wherein d is more than or equal to 0.05λ and less than or equal to 0.1λ; The S23 specifically adopts a single-turn ring; The S24 specifically comprises the steps that a coupling ring is placed at the strongest part of a magnetic field, a quasi-optical cavity for testing the dielectric constant of a material works in a TEM 00q mode, the maximum value of the magnetic field intensity is located at the axis in the radial direction, and the maximum value of the magnetic field intensity is located at the center of a symmetrical quasi-optical cavity in the axial direction.
9. The method for designing a symmetrical quasi-optical cavity for dielectric constant measurement according to claim 8, wherein the coupling ring in S24 is located in the middle of two spherical mirrors of the symmetrical quasi-optical cavity, at the radial center position.
10. The method for designing symmetrical quasi-optical cavity for dielectric constant measurement according to claim 9, wherein the range of values of the ring diameter and the wire diameter is related to frequency, and parameter scan simulation is performed using a simulation software value range to obtain an optimal value according to the use scenario.

Description

Symmetrical quasi-optical cavity design method for dielectric constant measurement Technical Field The invention belongs to the technical field of quasi-optical cavity design, and particularly relates to a symmetrical quasi-optical cavity design method for dielectric constant measurement. Background The closed resonant cavity is widely adopted for measuring the dielectric constant of the small-loss material in the centimeter wave frequency band, but the method has strict requirements on material processing, is not easy to realize abnormal environment simulation, and has a quality factor which is difficult to exceed 1 ten thousand and determines the small-loss measurement capability. The quasi-optical cavity method is widely used in millimeter wave frequency bands with the advantages of non-destructiveness, suitability for high-precision measurement of small-loss materials and the like. The basic principle of the quasi-optical cavity method is that the dielectric constant and the loss tangent are deduced by comparing the changes of the resonant frequency or the cavity length and the quality factor of the measured material before and after the measured material is put into the cavity and combining the parameters of the measured material thickness, the initial cavity length and the like. The dielectric constant of the current test material mostly adopts a semi-symmetrical quasi-optical cavity, and the test material consists of a spherical reflector and a plane reflector. The method has simple structure and is easy to align and use, but is not suitable for high-precision measurement due to factors such as the beam waist is positioned on the surface of the plane mirror and the field distribution is non-ideal. The symmetrical quasi-optical cavity is formed by two spherical reflectors with the same shape, curvature radius and mirror surface width, and the two reflectors are placed to form an open resonant cavity. The structure is complex in alignment and high in cost, but the beam waist of the fundamental mode (TEM 00q mode) is precisely positioned at the center of the cavity, and the field distribution is symmetrical and very uniform about the center, so that the structure is more suitable for high-precision material testing. When the quasi-optical cavity is used for testing the dielectric constant of the material, a small hole coupling mode is mainly adopted to feed signals into the cavity, so that resonance is generated. The small hole coupling is realized by directly processing a through hole on the mirror surface, and the method has the characteristics of simple processing, high system repeatability, high stability and the like. However, because the size and the position of the small hole are fixed, the small hole cannot be adjusted after processing, the flexibility is poor, and the small hole coupling cannot be satisfied when the system is used for expanding multi-frequency band test, high-power environment and the like in the subsequent process. Therefore, the invention provides a novel coupling mode, a coupling ring is placed between two spherical mirrors of the symmetrical quasi-optical cavity for signal feed-in, so that the subsequent integration with other coupling modes is facilitated for cross-frequency band dielectric constant test, broadband tuning, high-power material test environment generation and the like, and convenience is provided for the expansion of material test scenes. The foregoing is not necessarily a prior art, and falls within the technical scope of the inventors. Disclosure of Invention In order to solve the problems, the invention aims to provide a symmetrical quasi-optical cavity design method for dielectric constant measurement, which creatively proposes to place a coupling ring between two spherical mirrors for signal feed-in/feed-out, replaces the traditional fixed aperture coupling mode, ensures that the coupling ring coupling belongs to 'magnetic field coupling', can flexibly adjust the coupling strength through parameters such as ring diameter, line diameter, position and the like, and solves the problems that the aperture coupling cannot be adjusted and has poor flexibility after processing. In order to achieve the above object, the present invention provides a symmetrical quasi-optical cavity design method for dielectric constant measurement, wherein the symmetrical quasi-optical cavity design method comprises a mirror design method and a coupling design method; the design method of the mirror is as follows: The quasi-optical cavity is an open cavity and is composed of spherical mirrors with transmission type symmetrical structures, and parameters to be designed mainly comprise the curvature radius of the spherical mirrors, the radius of the mouth surface of the spherical mirrors, the distance between the two spherical mirrors and the material of the spherical mirrors. Factors considered in design include intrinsic line purity, open cavity stability, open cavity Q value, an