Coaxial to circular waveguide transition as high-precision easy-to-handle measuring cell for the broad band dielectric spectrometry of liquids

For an abrupt transition from a coaxial to circular waveguide with a dielectric interface in the plane of discontinuity, the input admittance has been calculated as a function of frequency and of relevant geometrical and electrical quantities. The calculations have been performed applying a mode matching technique. A simple lumped-element representation has been derived from the theoretical considerations which was found to provide an excellent description of the input admittance of the discontinuity over a wide range of values of the parameters of interest. Network parameters are discussed for transitions that are most suited as sample cells for dielectric measurements in both the time domain and the frequency domain. Results from test measurements are reported to show the agreement of our numerical predictions with precise capacitance values of other authors and also to demonstrate the sensitivity and accuracy attainable with such cells.