Modelling of the reflection of an open-ended coaxial line and its use for accurate complex permittivity measurements at frequencies up to 20 GHz

An open-ended coaxial line can be used as a sensor in nondestructive dielectric measurements. The technique is based on the study of the fringing field of an open-ended coaxial line and its perturbation by a lossy dielectric. The open end of the coaxial line is terminated by the sample material and the input reflection coefficient of the sensor is measured using a network analyzer. A number of models have been developed in order to relate these data to the complex permittivity of the material. The equivalent circuit model was shown to be suitable only up to a few gigahertz, particularly for lossy dielectric measurements. In this paper, a comparison is made between two different formulations which allow the complex permittivity of the terminating material to be derived from the reflection coefficient of a coaxial sensor. It is found that there is a systematic difference between the values of aperture admittance predicted by the two models. When a calibration procedure was performed at the measuring end of the probe, the two models were found suitable for accurate dielectric measurements up to 20 GHz