Input impedance of a coaxial line terminated with a complex gap capacitance - numerical and experimental analysis

A full-wave numerical analysis was performed for a coaxial line terminated with a complex gap capacitance using a finite-element high-frequency structure simulator. The scattering parameters, input impedance, and spatial distribution of the electromagnetic field have been obtained in the frequency range of 100 MHz to 19 GHz for specimens 8 to 320 μm thick, with a dielectric constant of up to 80. It was found that the residual inductance of the specimen affects the impedance characteristic of the network. The inductance-capacitance resonance is coupled with the cavity resonance. The specimen inductance is linearly dependent on the specimen thickness. At frequencies near the cavity resonance, the specimen section can be treated as a network of a transmission line with a capacitance, where the fundamental mode propagates along the diameter of the specimen. Results of the numerical analysis were verified experimentally using water as a model material with a high dielectric constant. Our closed-form formula for input impedance of the network is valid in a wider frequency range than the lumped-element method. The results are useful in improving the accuracy of broadband dielectric measurements in the extended frequency range of thin films with high dielectric constant that are of interest to bio- and nanotechnology.