Permittivity Measurement of the Dielectric Material at the Off-Axis Position in a Cylindrical Cavity

In most measurement models of the cylindrical cavity, concentricity between the rod-like sample and cylindrical cavity is a basic premise of an accurate calculation model. Once a sample is randomly placed in the off-axis position in the sample hole of the cavity, the real will not conform to the established calculation model and the off-axis position should be taken into account to the measurement error. In this article, a measurement method to calculate permittivity of the sample placed at the off-axis position in the sample hole is presented. The actual location of the sample in a cylindrical cavity is first obtained in the application of vision measurement. Field distribution in the cavity is calculated by using the mode-matching method. The measured sample in the cavity is divided into finite small elements with arc shapes, and permittivity calculation expression is established by using the perturbation theory. A TM-mode cylindrical cavity that installed two miniature camera probes around the cavity wall is designed and used to measure permittivity of the material. Teflon and fused silica with different off-axis positions are measured and compared with the published results at several resonance frequencies. The maximum permittivity errors of Teflon and fused silica are 1.6% (at around 2 GHz) and 0.68% (at around 10.8 GHz), respectively. The maximum loss tanδ errors are 5.8e ^-5 (at around 12.5 GHz) and 1.44e ^-4 (at around 9.9 GHz), respectively.