A Quasi-Optic Technique for Measuring Dielectric Loss Tangents

A method for measuring dielectric loss tangents is described that combines coherent optical resonator techniques with conventional microwave techniques to yield very high measurement accuracies. A very important feature of the technique is that the equation relating tan ¿ to the experimentally measurable quantities is an algebraic expression instead of a transcendental equation. The method basically consists of perturbing a microwave confocal resonator with a dielectric sheet placed normal to the axis of the resonator. The change in Q and the change in cavitylength necessary to restore resonance can then be related to the loss tangent of the dielectric. For the general case, where the dielectric is placed at any location along the axis of the resonator, the working mathematical equation is a complex transcendental expression. However, if the dielectric sheet is constrained to be at the center of the resonator and each reflector is moved inward an equal amount to restore resonance, then the equation simplifies and is no longer transcendental. These constraints are easily satisfied experimentally. The technique has been used to measure the loss tangents of several common dielectrics at 35 GHz and the results are presented.