An Experimental Technique to Evaluate the Complex Permittivity of Small Samples of Microwave Substrates

Perturbation theory is applied to a rectangular resonant cavity to measure the complex permittivity of small rectangular samples of microwave circuit substrates at X-band frequencies. The technique employed is non-contacting and requires no metallization of the sample. An automatic network analyser is used to determine the resonant frequency and Q-factor of the cavity both with and without the sample and this enables fast and highly accurate measurements to be performed; the real and imaginary parts of the permittivity may be measured to less than 1% and 10% respectively. The accuracy of the technique, however, depends greatly on the sensitivity of the cavity response to errors in the location of the sample and the importance of choosing a suitable position for the sample is illustrated.