Building a resonant cavity for the measurement of microwave dielectric permittivity of high loss materials

The design of a cavity resonator implies to solve the Maxwell equations inside that cavity, respecting the boundary conditions. As a consequence, the resonance frequencies appear as conditions in the solutions of the differential equation involved The measurement of the complex permittivity, epsilon* = epsilon'-i epsilon can be made using the small perturbation theory. In this method, the resonance frequency and the quality factor of the cavity, with and without a sample, can be used to calculate the complex dielectric permittivity of the material. We measure the shift in the resonant frequency of the cavity, Delta t, caused by the insertion of the sample inside the cavity, which can be related to the real part of the complex permittivity, epsilon', and the change in the inverse of the quality factor of the cavity, Delta (I/Q), which can be related with the imaginary part, epsilon. This is valid for very small perturbations of the electric field inside the cavity by the insertion of a sample. For materials with high losses, the perturbation can be very high, making impracticable the use of this technique. The solution is to use high volume cavities. In this work we report the design and the performance tests of a cavity to be used with high loss materials. (C) 2007 Wiley Periodicals, Inc.