Analysis of transmission/reflection methods for complex permittivity determination using waveguides

Abstract

The parameters determining interaction of electromagnetic fields with matter are the dielectric properties. The complex permittivity of materials is the most important parameter at microwave frequencies, and its accurate determination allows precise field distribution simulation and calculation of absorption characteristics. This study compares different rectangular waveguide techniques used to determine complex permittivity of materials at X-band (8.2 to 12.4 GHz). Both reflection and transmission measurements were used in laboratory measurements in order to obtain the scattering parameters of low loss materials. Several methods were developed to obtain complex permittivity from the measured scattering parameters using Matlab, each having their own limitations such as techniques which allowed the use of arbitrary sample lengths, derivation of non-iterative methods and of iterative methods. Computer simulations using CST Microwave Studio were carried out to obtain the scattering parameters of low loss materials. Validation of the CST models was performed by comparison with experimental measurements. After validation, the model was used to calculate the scattering parameters of a high loss material, NaCl solution. A method to determine the frequency-dependent complex permittivity of NaCl solution using the transmission coefficient was derived. Lastly, measurements of the scattering parameters of the high loss material were carried out using transmission methods.