Reactance of electrically short radio frequency coaxial probes

An experimental technique is presented to determine the aperture reactance of a short radio frequency (RF) coaxial probe. The method is based on the inversion of the measured reflection coefficient at the probe´s input port. The work was done at 500 MHz using an HP8510C network analyzer and high quality calibration standards. For precise inversion the attenuation and phase constants of the coaxial line must be determined with high accuracy. To a good approximation the measured probe capacitance was found to increase linearly with length from the value for a flush probe. This allows the CW capacitance of a short probe, with length up to 1/10 the free-space wavelength, to be modeled using a simple two-term model. Bare and insulated probes, both shielded and unshielded, were studied.