Plano-Concave Fabry-Perot Open Resonator for Measurements of Conductivity up to 70 GHz

In this paper, we present a setup based on Plano-Concave Fabry-Perot Open Resonator (PC FPOR) suitable for metallic sample measurement of conductivity ranging from 103 S/m (e.g., carbon-fiber-based laminates) up to 107S/m (e.g., good room-temperature conductors like copper). As multiple TEM0,0,q modes are considered within the cavity, the setup delivers the properties of the sample-under-test at multiple frequencies in the band of 30 to 70 GHz with separation of ca. 1.5 GHz between consecutive frequency points. This work is an extension of our recently published method towards higher frequencies. Therefore, additional loss sources must be introduced to the cavity's numerical description to cover this range properly. Firstly, volumetric losses of air can no longer be neglected as below 40GHz, especially due to the oxygen absorption peak located at ca. 61 GHz. Secondly, the influence of both coupling holes has to be accounted for. Such a calibration is necessary to measure without additional systematic errors.