Broadband Perpendicular Permittivity Measurements Up to 220 GHz Using a Balanced-Type Circular Disk Resonator Without Air Gap Errors

This paper presents broadband perpendicular permittivity measurements at frequencies up to 220 GHz by using a balanced-type circular disk resonator (BCDR). To achieve this, we develop a BCDR excited by 0.6-mm coaxial lines. The developed resonator is designed to be connectable to a commercially available ultra-broadband vector network analyzer system with 0.6-mm coaxial interfaces, and provides broadband characterization of low-loss substrate materials owing to its mode-selective behavior. Moreover, we propose a new sample configuration for the BCDR method involving dielectric sheets metallized on both sides to eliminate air gaps between the sample and the resonator, which lead to measurement inaccuracies. The developed BCDR and proposed sample configuration are experimentally demonstrated up to 220 GHz by measuring three ceramic samples: fused silica, aluminum nitride, and alumina. The measured perpendicular permittivity results are in good agreement with those obtained using the split-cylinder resonator method and previously reported literature values. We also observe that conventional BCDR measurements using bare samples, without metallization on either side, underestimate the relative permittivity by 1.5 %, 5.5 %, and 4.9 % for fused silica, aluminum nitride, and alumina samples, respectively, owing to the air gap errors. Accurate perpendicular permittivity measurements using the improved BCDR method are expected to contribute to the development of advanced materials for 6G applications.