Dielectric loss measurement at millimetre wave frequencies using an untuned cavity

The theoretical basis of the untuned cavity method, especially for high permittivity samples, is outlined, and experimental hardware is described. The lid of the cavity opens to introduce a sample or to replace one of the solid sealing disks by one with a circular hole for calibration. The input signal is switched at 11 Hz with a pin diode in a waveguide, to serve as a reference for the two-channel lock-in voltameter which measures output signals from a power detector. An attenuator enables linearity checks to be performed. For very small changes in power at the output of the cavity, an offset facility on the lock-in together with a chart recorder to ease interpretation of the measured changes, enables values of r as low as 0.002 to be detected. These values occur for small volumes of very low loss materials, or when absorption by semiconductor surfaces is measured, if an estimate of reflection coefficient (and hence conductivity) is required.