Millimeter-Wave Resonant Cavity for Complex Permittivity Measurements of Materials

A system to quickly and accurately measure the complex permittivity of materials is presented. The system is based on a rectangular waveguide resonant cavity and its associated resonant modes. The resonant cavity is modified to introduce a fixture of high-density polyethylene to support a sample of interest at the center of the resonant cavity. The sample of interest can be a solid, a liquid or a powder. The resonant frequency and associated quality factor for various resonant modes in the millimeter wavelength, 15 GHz – 34 GHz, are measured using a programmable network analyzer. The resonant frequency and quality factors are used as inputs to an electromagnetic model of the system. The electromagnetic model is solved for the complex permittivity values that produce the resonant frequency and associated quality factor. The system is verified by comparing results to the analytical solutions and also comparing results to perturbation theory.