D-Band Material Characterization Using a Balanced-Type Circular Disk Resonator With Waveguide Interfaces and a Modified Full-Wave Modal Analysis

This study presents material characterization in the ${D}$ -band using a balanced-type circular disk resonator (BCDR) with waveguide interfaces. In the developed BCDR, ultrafine and brittle coaxial lines are covered by waveguide interfaces, which alleviates the robustness issues that occur when increasing the measurement frequency of conventional BCDRs with coaxial-line interfaces. The BCDR provides broadband complex permittivity and conductivity measurements of low-loss substrate materials over the entire ${D}$ -band, owing to the mode-selective behavior of the resonator. Moreover, a modified modal analysis is derived to improve the accuracy of determining the permittivity from the measured resonant frequency based on the full-wave circuit analysis. A comparison between the analytical results and those of numerical simulations shows that the proposed analysis provides accurate calculations, even in cases with high-dielectric or thick samples, which typically cause inaccuracies in the conventional analysis. The developed BCDR and modified analysis are experimentally demonstrated by measuring the complex permittivity of three dielectric samples and the conductivity of two copper foil samples in the ${D}$ -band.