Broadband Complex Permittivity Determination of Thin Material Using Open-Ended Coaxial Probe and an Integrated GA–HFSS Computational Method

In this article, we present an innovative approach for a simple, nondestructive, and accurate method for broadband determination of complex permittivity of thin materials including those often used in patterning advanced metamaterial absorbers. While the measurement procedure involves using a simple coaxial probe method, the determination of complex permittivity values is based on an iterative procedure involving High Frequency Structure Simulation (HFSS) and a genetic algorithm (GA)-based zero finding technique. The HFSSs include modeling the coaxial probe when in contact with the thin material under test (MUT) and when backed by standard material and ground plane and calculation of de-embedded reflection coefficient values, $S_{11}$ , at the probe and material interface. Through the developed GA-HFSS software interface, a GA fitness function was used to determine the complex permittivity values that best fit the measured S-parameters. Measurements were made on several standard materials and obtained results were validated when compared with measurements using the shunt capacitance at the end of a coaxial line method. Results from the two measurements methods were within 1%–2% difference in complex permittivity values in the frequency band from 500 MHz to 15 GHz, the compact radar band of interest. Measurement errors due to air gaps at the probe sample interface are analyzed to help guide measurements and achieve accurate results.