Mode Coupling in a Fabry-Perot Open Resonator

A Fabry-Perot open resonator (FPOR) is a tool dedicated to broadband accurate microwave and millimeter-wave characterization of dielectric sheets. Despite the various advantages of that method, one of the major challenges limiting its applicability is unintentional coupling of Gaussian modes of interest with a particular family of parasitic higher order modes. The coupling affects the resonance frequency of the resonator loaded with the material under test, thus disturbing the extracted permittivity. It has already been shown that spectral properties of these parasitic modes can be predicted using a recently developed electromagnetic model of a FPOR based on the scattering matrix method. It is shown in this article that the systematic analysis of experimental results supplemented with accurate numerical computations allows us to suppress the impact of mode coupling on the measured permittivity.