Amendment of cavity perturbation method for permittivity measurement of extremely low-loss dielectrics

Abstract

The quality factor of a resonant cavity may increase after introducing an extremely low-loss dielectric, so the conventional cavity perturbation method, widely used in dielectric permittivity measurement, may be invalid for extremely low-loss dielectric samples. After a brief review of the conventional cavity perturbation theory, this paper discusses the change of quality factor of a resonant cavity due to the introduction of a dielectric sample. A new concept, expected quality factor Q0, is introduced in this paper to denote the quality factor of a resonant cavity loaded with a strictly no-loss sample, and a calibration procedure is proposed to find the frequency dependence of Q0. The conventional resonant perturbation formulas are then amended by substituting the quality factor before the perturbation with the expected quality factor Q0 corresponding to the frequency after the perturbation. Experiments show that the accuracy of resonant perturbation method has been greatly increased after the amendment, especially for extremely low-loss dielectric samples.