Abstract
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.
Highlights
T HE microwave cavity perturbation technique is a commonly used method of defining electromagnetic properties of materials due to its noninvasive and nondestructive qualities [1]–[5]
The sample will have no effect on the frequency response of this mode, meaning measured changes can be assumed to be from the temperature response of the cavity
These experiments were conducted using an aluminum microwave cavity resonator designed for TM measurements with capacitive antenna coupling into the E-field
Summary
T HE microwave cavity perturbation technique is a commonly used method of defining electromagnetic properties of materials due to its noninvasive and nondestructive qualities [1]–[5] This technique involves perturbing a resonant electromagnetic field with a sample and measuring the change in the frequency response. By inserting a sample away from the central axis of the cavity, it is possible to separate the resonant frequencies of the modes This is often referred to as a mode trap [7], [11]. The main advantage of the proposed method is the much narrower bandwidth compared with using a nondegenerate reference mode A further advantage is a reduction in other systematic errors, such as variations in the coupling coefficients as a function of frequency
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