Measurement of low-permittivity materials based on a spectral-domain analysis for the open-ended coaxial probe

A new coaxial probe was designed to measure materials with a low dielectric constant at frequencies up to 1.5 GHz. Inconsistencies found while measuring samples of different thicknesses were solved by abandoning the model of Levine and Papas for the probe and developing a new model. Using the spectral-domain technique, a closed-form expression is obtained for the admittance of a hanged open-ended coaxial line radiating into a planar-stratified material backed by a metal plate. This new expression can be considered as a correction to the model of Levine and Papas. The model considers the dominant mode as well as the effect of higher order modes. With this new model measurements of low dielectric constant materials were performed together with a perturbation analysis of the influence of air gaps. Theory and measurements were found to be in good agreement