Determination of Complex Permittivity by Inverting Coaxial Transmission Line Data Using FDTD

The coaxial transmission line measurement is the most common technique to measure the permittivity of granular materials in wideband. The technique measures S-parameters of a coaxial line filled with a material by vector network analyser. An analysis is required for determining the complex permittivity of the material from the measured S-parameters. The paper proposes a new analysis method, which uses finite difference time domain (FDTD) simulations as the forward modelling. The response of a coaxial line filled with a sample material is calculated by the FDTD method with exciting sine waves at different frequencies, and the frequency dependencies of permittivity and conductivity are determined by iteratively minimising the error between the measurement and modelling at each frequency. The paper demonstrates the method with some dielectric materials and discusses considerations for further developments.