Research On The Technology Of Complex Permittivity Measurement With Open-ended Coaxial Resonator

Because the measurement technology of complex permittivity with the open-ended coaxial resonator(or transmission line sensor) has the advantages of relatively wide bandwidth, flexible sample shapes and sizes, nondestructive measurement and so on, it has great effect on areas such as aerospace, bio-chemical, medicine, etc. According to requirements of the project, this paper studied the measurement technology of complex permittivity of open-ended coaxial resonator, designed the hardware test system, combined two different algorithms, and measured the complex permittivity of a variety of microwave samples.First, the open-ended coaxial resonator, the core part of the hardware measurement system, was designed. On the basis of some design criteria of coaxial waveguide, the sizes of the inner and outer conductors of the resonator were obtained. In consideration of the factors associated with the quality factor of the resonator, the open end applied the gradient structure. The effect of the coupling mode and the location of the coupling device on the resonator characteristic parameter were studied, and the mode and location of the coupling was determined. The model of the resonator was built and simulated by using HFSS software. Then the resonator was fabricated and debugged. Finally, the resonator with good performance was finished.Secondly, two methods for the measurement algorithm were analyzed, the numerical analysis method based on the perturbation and the equivalent lumped parameter method. After the sample is loaded on the open-ended resonator, the characteristic parameters of the resonator will change because of the perturbation caused by the sample. 3D full-wave analysis and calculation software was employed to model and simulate the resonator, and the basic reference database was obtained. Then the functional relationship between the complex permittivity of the sample and the relative changes of the cavity characteristic parameters was solved from the aspect of numerical analysis. According to the equivalent RLC series resonant circuit and the stratification measurement theory of the microwave material, the objective function equation was obtained.At last, the sample test fixture, the pressure device and the calibration unit weredeveloped. The test system was assembled. Samples made of – quartz, Teflon, Be O, FR-4(glass-epoxy), water and alcohol were measured using the test system. The two different algorithms were applied to analyze the test data, and the test results were compared and analyzed. The error of the test system was discussed. It can be drawn from the analysis of the test results that the test system achieved the following test specifications.Test frequency: 300 MHz ~ 5GHz(five frequency point) Material under test: solid material, liquid material Test error:(Low loss material) +(High loss material)...