Permittivity Measurement From 40 to 70 GHz Using a DC FPOR With a Fixed Specimen Holder and an Off-Axial Feed

This article proposes a double-concave (DC) Fabry–Perot open resonator (FPOR) to measure the complex permittivity of a material under test (MUT) in the band from 40 to 70 GHz. The resonator is fed by waveguides through two feed holes. One of these holes is off-axial and placed at the zero position of the higher order modes fields to suppress the mode coupling from these modes. During the measurement, the MUT is placed on the specimen holder with a fixed position. The fixed sample holder position makes the center of MUT in the resonator change according to its thickness. A method based on the complex-source-point method to calculate the permittivity of the MUT is presented and verified, regardless of whether the MUT is in the center of the resonator. With this method, the motor unit to adjust the position of the MUT to the center of the resonator is no longer needed. A method to measure the exact sample holder position is also given. The validity and accuracy of the proposed DC FPOR and the complex permittivity extraction algorithm are verified by simulation results and by measuring the well-known materials with low, medium, and high permittivity, namely, polytetrafluoroethylene (PTFE), fused silica, and alumina.