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

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 Q ₀. 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