Precise measurement of dielectric properties at radio frequencies

Abstract

A new and improved apparatus is described for the measurement of dielectric loss and permittivity at radio frequencies using a resonance technique. Although designed primarily for use with low-loss polymers, such as polythene (tanδ = 80μrad at 6MHz), the instruments developed are capable of covering the properties of most practical insulating materials. The equipment may be used either with the Hartshorn and Ward air-substitution method or with a liquid-immersion technique. The former is preferred for accurate loss-angle measurements on low-loss materials; repeatability using the same 127 mm-diameter test specimen is about 0.4μrad throughout the 0.1−30 MHz frequency range. By using a 53mm-diameter specimen, together with suitable electrodes, the upper frequency limit may be extended to 100MHz, with only a small sacrifice of resolution. The accuracy of measurements by this methods is advanced by the use of a conductance calibration procedure. Permittivity can be measured by the air method to within about 1% which is adepquate for some purposes; otherwise a liquid-immersion technique with an error of 0.03% is available. A full describtion of calibration procedures required to support these measurements is given, and the accuracy of the equipment is assessed in detail for the specific case of a low-density polythene.