Air-gap effect and the importance of the electrodes on the sample in the lumped-capacitance method for dielectric measurements

A theoretical analysis is presented for the effect of an air gap in the lumped-capacitance method of dielectric measurements in the microwave frequency region. It is shown that when the material to be measured has a large permittivity, a very small equivalent air gap arising from poor contact between the sample surface and the end of the inner conductor (or the short-circuit) can cause a large error if neglected. The importance of a metal layer on the sample surface is emphasized, in order to eliminate the equivalent air gap and thus reduce the measuring error. Experimental results are given which confirm the analysis