Soil Permittivity Extraction by Time-Domain Free-Space Calibration-Free Microwave Measurements With No Thickness Information

An attractive microwave free-space method is proposed for permittivity extraction of soil samples using calibration-free time-domain measurements. Different than other similar methods in the literature which use the layer-stripping approach in forward direction in the extraction process, its formalism is based on the layer-stripping approach in backward direction, whereby allowing extraction of permittivity from interfacial reflection coefficient only without resorting to any thickness information, especially useful for time-domain analysis of multilayer structures having multiple reflection terms. Effects of angle misalignment, loss, and surface roughness on the extraction performance of the proposed method are analyzed by numerical examples. Permittivity measurements of three low-loss flat solid samples (polypropylene, polyethylene, and polyoxymethylene) were first performed by a free-space setup operating over 1−18 GHz frequency range to validate our methodology and compare its accuracy against other similar methods in the literature. Thereafter, permittivity measurements of two soil samples sieved with similar and different sizes (coarse- and fine-grained soil samples) by the same free-space setup were then considered to access the applicability and performance of our method for electromagnetic property analysis of homogeneous and partially inhomogeneous soil samples.