Accurate Microwave Cavity Sensing Technique for Dielectric Testing of Arbitrary Length Samples

In this article, an attractive resonant technique is devised for calculating the dielectric properties of arbitrary length material under test (ALMUT), which is partially filled inside a waveguide cavity sensor. The proposed technique exempts two major assumptions viz. the depolarization effect and the theory of images, which were neglected in earlier cavity perturbation methods. A numerical model of the depolarizing factor, which precisely considers the impact of the relative permittivity and the sinusoidal variation of the electric field, is also formulated. First, the technical modification and the numerical analysis are provided and after that, a new set of formulas for characterizing the complex permittivity of ALMUT is developed. The numerical results suggest the accuracy and validity of the scheme for a broad range of dielectric samples, including the typical case, i.e., the full-length dielectric sample. The novelty of the presented technique can be ascertained from the fact that it provides higher accuracy irrespective of the actual sample length when compared with standard cavity techniques. A few standard dielectric samples with the partially filled arrangement are measured using the WR90 cavity, and the results are compared with their corresponding values obtained using the earlier reported conventional and modified techniques. It is found that the results evaluated using the devised scheme are consistent and independent of the actual dimensions of ALMUT. On the other hand, error in estimated complex permittivity using the earlier reported approaches shows substantial dependence on the sample size.