Accurate Free-Space Measurement of Complex Permittivity With the Angular Spectrum Method

This study presents an analytical method based on the angular spectrum method to extract accurate intrinsic material properties from the S-parameter data obtained with a free-space measurement system. Instead of the widely used plane-wave model, the proposed method calculates the beam propagation and the interaction with a dielectric slab for any incident electric field using the angular spectrum method and obtains the exact field coupling of the propagated beam. This method improves the estimation errors of permittivity in a plane-wave analytical model and errors due to higher order modes in a Gaussian beam model. We validated the effectiveness of the proposed analysis method by applying it to the test data calculated with physical optics simulations. We also applied this analysis method to the measurement data obtained by the 67–116 GHz optical system, which was developed for this work. Comparing the results with those measured by the resonator method showed that the complex permittivity of a low-loss material, ultrahigh-molecular-weight polyethylene, can be measured accurately.