An improved coaxial probe technique for measuring microwave permittivity of thin dielectric materials

A practical coaxial probe technique for convenient, fast and accurate measurements of thin dielectric materials is presented in this paper. The thin dielectric samples, backed by a short circuit, are placed flush with the probe. The complex permittivities of the samples are determined from the coaxial aperture reflection coefficient, which is measured by a vector network analyser. In order to realize fast measurements, only the fundamental coaxial mode is considered in the determination of the permittivity of a sample from the measured reflection coefficient. However, in calibration computations, both the fundamental mode and higher-order modes are considered in order to ensure that the measurement system is accurately calibrated. Two practical measurement set-ups are specifically developed, for thin solid samples and thin liquid samples, respectively, in order to minimize various measurement errors and to make the measurements convenient. To validate this technique, experimental measurements are carried out on several different dielectric materials, including low-, medium- and high-permittivity materials. The measured data are found to be in good agreement with the known data. The limitations of this technique are also discussed.