Abstract
The need for accurate material property measurements using microwave cavities requires a form of compensation to correct for changes in temperature and other external influences. This paper details a method for temperature correcting microwave cavity perturbation measurements by monitoring two modes; one which is perturbed by the sample and one which is not (referred to as a nodal mode). The nodal modes used (TM310 and TE311 for an axial sample in a cylindrical cavity) are subject only to sample-independent influences. To demonstrate this technique, the bulk permittivity of a PTFE rod has been measured under varying temperature conditions. The results show that without correction, the measured temperature-dependent dielectric constant has large variations associated with the stepped and linear temperature ramping procedures. The corrected response mitigates systematic errors in the real part. However, the correction of the imaginary part requires careful consideration of the mode coupling strength. This paper demonstrates the importance of temperature correction in dynamic cavity perturbation experiments.
Highlights
T HE CAVITY perturbation method is very common for characterizing materials at microwave frequencies because of its precision, simplicity, and noncontact nature
A cylindrical cavity becomes advantageous in measuring temperature-dependent properties of axial samples because with increasing mode number, the field maxima are pushed toward the perimeter of the cavity
Given that the cavity perturbation method already offers such high sensitivity and minimal sample requirements, the aim of this paper is to demonstrate the measurement of temperature-dependent properties in situ, in a temperature varying environment
Summary
T HE CAVITY perturbation method is very common for characterizing materials at microwave frequencies because of its precision, simplicity, and noncontact nature This method involves perturbing a resonant electromagnetic field within a cavity by inserting a sample into the required field region and examining the change in the frequency response of the cavity. With the increasing number of applications for cavity perturbation as a noncontact material probe in chemistry, one must read carefully when dealing with exothermic and endothermic reactions to keep the system stable [17] Since both the unperturbed resonant frequency and bandwidth are functions of the cavity geometry, the cavity response will change due to the natural thermal expansion and temperature-dependent conductivity of the metal cavity. A cylindrical cavity becomes advantageous in measuring temperature-dependent properties of axial samples because with increasing mode number (and Bessel function order), the field maxima are pushed toward the perimeter of the cavity. The chosen nodal modes are the TMm10 and TEm11 modes (where m > 1)
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