Low loss dielectric measurements in the frequency range 1-70 MHz by using a vector network analyser

The resonance of a coil inductor and a parallel-electrode capacitor can be used to determine the permittivity and loss angle of sheets of material at RF frequencies. By using high-stability instrumentation to measure resonant frequency and Q-factor, the loss angle of very low-loss materials can be determined with a resolution of below 10 microradians. In the 1970s much work went into developing Q-meter-based systems, based on series resonance, which were typically used for measuring the loss angle of polymers such as polyethylene. These systems were also used for measuring ceramics used as RF windows in prototype fusion reactors. This paper describes how one of these systems was updated to replace the now obsolete Q-meter with a vector network analyser (VNA), using parallel resonance. A second aspect to the work is calculation of the fringing (edge) capacitance between the electrodes, which are unguarded, by using MIT FastCap software. This enables measurements obtained by the equivalent-thickness method to be corrected for the effects of fringing capacitance. Measurements using an unmodified Q-meter system, a VNA-based system, and two other techniques are compared for polyethylene, fused silica, macor and alumina specimens. Good agreement is obtained. The measurements are presented with comprehensive evaluations of uncertainty.