Correction to open-resonator permittivity and loss measurements

A correction to the length of a dielectric sample placed in an open resonator is derived using perturbation theory. The correction takes account of the additional volume of sample extending beyond the curved phase front normally assumed in the beam wave theory. Results are presented for a range of sample lengths of a high-density polyethylene, which illustrate the improvement obtained in both permittivity and loss measurements when this correction is used.