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Abstract
Measurement of dielectric constant and loss factor of a material plays an important role in microwave technology. There are several techniques developed for accurate measurement: free space methods, guided-wave methods, cavity resonator methods, coaxial probes, and impedance methods. Among these methods, the free space measurement system is non-destructive, contactless, and sample preparation requirements are minimal. Therefore, it can be easily adapted for measurement at high frequencies. All other methods have the disadvantages of narrow band or single frequency and the difficulty of accurate sample preparation for different frequency bands.
In this thesis, it is suggested to use the microwave non-destructive testing based on free space measurement system to extract the real and imaginary parts of the complex relative permittivity for a low-loss dielectric material. The bulky horn antennas used in the well known free space setup are replaced by low cost, miniaturized microstrip antennas fed through coaxial probes. This antenna is modeled using commercial software based on the finite element method, High Frequency Structure Simulator (HFSS). Using HFSS, microstrip patch antenna model integrity is guaranteed by comparing the simulation results of the resonant frequency and the magnitude of the reflection coefficient with those previously measured and published using the same antenna.
The measurement system contains two identical microstrip patch antennas, one for transmitting the microwave signal and the other for reception. The material under test is fitted in between these two antennas. The Finite Element Method (FEM) is used to model that part of measurement setup. For the first time, the discontinuity between the antenna and the sample as well as the multiple reflections through the sample in between the transmitting and receiving antennas are included in modeling. The discontinuity problem is considered by extracting the Generalized Scattering Matrices (GSM). These GSMs include higher order modes. Multiple reflections are considered in the model for complex permittivity extraction.
Computation of transmission coefficient and quality factor gives the dielectric constant and loss tangent, respectively.
To guarantee the accuracy of the proposed modeling, a material which complex permittivity is known a priori is placed in between the transmitting and receiving antennas. The model gives accurately the complex permittivity.