Fixed Frequency Permittivity Measurement of Thin Films Using a Tunable FSS Resonator Within a Waveguide

This letter presents a compact method for the precise permittivity extraction of thin films, offering both high sensitivity and a fixed measurement frequency. The proposed system integrates a varactor-tunable frequency-selective surface resonator within a waveguide and employs active bias control to adjust the resonant frequency. Unlike conventional resonant methods, where higher sensitivity leads to undesirable frequency shifts between samples, the proposed approach uses active bias control to compensate for these shifts, keeping the observation frequency constant. The method is validated through full-wave simulations and experiments, achieving permittivity extraction with errors below 3% for the real part of the permittivity and 10% for the loss tangent. The system enables high-sensitivity permittivity measurements at a consistent frequency.