Measurement of Complex Permittivity normal to Substrates for Medium-loss Materials Using a PTFE Loaded Balanced-type Circular Disk Resonator at Microwave and Millimeter Wave Frequencies

Abstract

The complex permittivity normal to substrates can be measured by the balanced-type circular disk resonator method at microwave and millimeter wave frequencies. However, in the case of substrates for medium-loss materials with a dielectric loss tangent of about 10^-2 and thickness of 1mm or less, the unloaded Q of the resonator decreases to about several tens, causing distortion in the resonance response. Therefore, it becomes difficult to measure the complex permittivity accurately. In this paper, we propose a polytetrafluoroethylene (PTFE) sheet loaded balanced-type circular disk resonator. By loading PTFE sheet, the distance between the conductors of the resonator is widened, and in addition, the concentration of electric field energy into the dielectric sample is weakened adequately, so that the unloaded Q of the resonator increases. The availability of this method is verified by measuring the frequency dependence of the complex permittivity of the FR4 substrate from 5 to 30 GHz.