Complex permittivity measurements of moulding plastic using hybrid time and frequency methods

An improved method of measuring complex permittivity using a coaxial line test cell is described. Only two wideband reflection measurements on the same short circuited cell, with and without the sample present, are required. A hybrid time and frequency domain method, using gating, allows both the adaptor discontinuities and the frequency dependence of the line resistance to be accurately characterised from the empty line measurement. From the second measurement, a transcendental equation for the permittivity is solved conventionally in frequency domain but using the time domain response to provide an ambiguity-free starting estimate. With proper inclusion of the discontinuity and resistance effects, measurements of low loss tangents can be undertaken.