Broad-band measurements of dielectric permittivity in coaxial line using partially filled circular waveguide

Abstract

The full-wave analysis was applied for a coaxial line (i.e., transmission line) that has a "short-circuited" discontinuity. The discontinuity has a radius less than or equal to the inner radius of the coaxial line. The "sample region" can be considered as a partially filled circular waveguide. Such a structure is very practical and is of particular interest for the dielectric spectroscopy applications. It takes into account the inhomogeneous field distribution, which is the limiting factor for the determination of high dielectric permittivity values at microwave frequencies. The direct problem was solved by using the mode-matching technique, and the relationship between the complex reflection coefficient and the dielectric permittivity of the cylindrical sample was obtained. By solving the inverse problem, it is possible to obtain the complex dielectric permittivity from the experimental values of the scattering matrix. The results were verified by the finite element modeling of the system and applied for particular materials. The correspondence between these approaches is excellent. This method is very suitable for the determination of permittivity, which exceeds several thousands (it is applicable for any type of material). It extends the frequency range where the permittivity can be determined reliably. There is no necessity to prepare samples with different geometries (i.e., surface area and thickness).