Suspended Ring Resonator for Dielectric Constant Measurement of Foams

The present study introduces a microstrip-based resonant device that is specifically intended for measurements of polymer foam permittivity at frequencies of 1 to 10 GHz. The device utilizes the standard microstrip ring resonator. However, the ring and the ground plane/feed network are physically located in separate planes, enabling the placement of an arbitrary sample of material to be measured in the substrate of the ring resonator. A transmission line model is developed and tested in order to predict the frequency response of the circuit. Experimental data is reported and compared with theory. The measurements indicate a high potential resolution of the method.