Measuring Complex Permittivity of Soils By Coaxial Transmission Line Method and 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. Determining the complex permittivity of the material requires an analysis on the measured S-parameters. The paper proposes a new method to calculate the complex permittivity from the S-parameters, which uses FDTD simulations as the forward modelling. The response of a coaxial line filled with sample material is calculated by the FDTD method with inputting sine waves of different frequencies, and the frequency dependencies of permittivity and conductivity are determined iteratively minimising the error between measurement and modelling. We show measurements of a soil at various water contents and the proposed method works well for frequencies below approx. 5 GHz where attenuation within the measuring cell is not too high.