Open resonators: improvement of confidence in measurement of loss

Open resonators are commonly used for the measurement of complex permittivity of low-loss dielectric materials in the millimetre-wave region of the electromagnetic spectrum. Values for the loss tangent of the materials are derived from measurement of the Q-factor of the resonances obtained. Resonance mode coincidence (partial degeneracy) can give rise to spurious values of the Q-factor and, hence, of the specimen loss tangent. It is usual to check for such spurious results, but their incidence increases with the loss of the specimen because the corresponding resonance widths increase, making it more likely that they overlap. With nonideal specimens, or those having a loss angle greater than 1 mrad, it is difficult to avoid such overlap with conventional open-resonator measurement techniques. The paper shows how additional data can be gathered by measuring total resonator lengths and Q-factors as a function of the spacing between the specimen and one of the resonator reflectors. The technique can be used for obtaining both the real part of the permittivity and the loss angle of specimens. It has the additional advantage that the Q-factor of the empty resonator need not be known. Examples of the measurement technique are provided both for permittivity and loss.