Temperature Correction Using Degenerate Modes for Cylindrical Cavity Perturbation Measurements

Microwave cavity perturbation measurements are a useful way to analyze material properties. Temperature changes can be introduced during these measurements either intentionally or as a result of some other process. The microwave cavity itself also has a temperature-dependent response, which can affect the results. A common method to correct is to use another resonant mode separately to the measurement mode, which is not affected by the sample. Instead of using independent modes, this paper describes a method to use split degenerate TMm10 modes of cylindrical cavities. TMm10 consists of two modes with identical field patterns with a relative rotation between them and identical resonant frequencies. A strategically placed perturbation reduces the frequency of one of the TMm10 modes and affects the coupling of both modes by reconfiguring the fields. This can be used for temperature correction by placing a sample such that both modes are equally coupled. The lower frequency, the perturbed mode is used as a measurement mode. The higher mode is used as a reference for temperature correction as it is unaffected by the sample. This technique was verified by measuring the permittivity of pure water using an aluminum microwave cavity resonator at 3.96 GHz. The temperature was swept between 20 °C and 60 °C, and the results was verified against the literature.