Measurement of Axially Inhomogeneous Permittivity Distributions in Resonant Microwave Cavities

The cavity perturbation method is a standard technique for material parameter estimation. It typically assumes that the sample consisting of the material under test (MUT) is small. This and other assumptions are violated in in-process measurement scenarios that involve big and inhomogeneous samples. We analyze the limitations of the classical perturbation method and present a new approach toward the measurement of spatially varying material parameters associated with a large sample in a rectangular or a circular cylindrical microwave cavity. The method is based on the measurement of multiple resonance frequencies and quality factors and is validated by numerical and laboratory experiments. It is demonstrated that relative permittivities and conductivities may be measured with typical accuracies on the order of 1%.