Application of an Open Coaxial-Line Resonator for the Determination of the Local Permittivity of Low-Loss thin Slabs at Microwave Frequencies

A non-destructive method for the determination of the local dielectric constant of thin slab-like materials is investigated. The dielectric material is brought into the electromagnetic fringing field of a suitable unflanged open coaxial-line resonator with a small aperture area. The change of resonant frequency is related to the real part of the permittivity of the tested material. A so called "open-circuit" measurement arrangement is used. The shunt capacitance Coc of the resonator terminating structure as a function of the sample thickness - which has a decisive influence on the frequency shifting - and of the dielectric constant ¿rd of the tested material is computed in the electrostatic case for a flanged open coaxial-line. Additionally, measured datas from well known calibration materials serve to derive an empirical formula for Coc, to get the dielectric constant from measured resonant frequency and sample thickness.