A model for a measurement circuit consisting of a telescopic coaxial transmission line with the sample placed in the gap of the central conductor has been developed and a measurement method, based on this model, was tested. To determine the sample's electrical properties, the method requires the measurement of the two-port scattering matrices of the short-circuited line, the line with one calibration sample, and the line with the sample to be measured inserted. The model takes into account the thermal elongation of the measurement line and can be used for measuring electrical properties of materials at high temperatures in the megahertz and gigahertz ranges. This model also reduces the influence of parasitic reflections in the measuring coaxial line on the measurement results and takes into account a complicated distribution of the electromagnetic field in the measuring capacitor without solving a complicated electrodynamics problem.

An impedance spectrometer capable of accurately measuring solid ion conducting sample impedance spectra by two- or four-electrode methods in either time or frequency domain has been built. The four-electrode measurement mode is implemented by constructing a differential amplifier with a very high input impedance and common mode rejection ratio over a wide frequency range. All of the measurements can be performed in frequencies ranging from 10 Hz to 2 MHz and sample temperatures up to 800 K. The working principle of the spectrometer as well as its technical parameters and accuracy estimation are presented in this paper. The advantage of four-electrode over two-electrode measurement mode is shown by an example of Ce0.9Gd0.1O1.95 solid electrolyte ceramic impedance measurements.