Null method for measuring small dielectric loss at microwave frequencies

A method based on the use of a 4-port directional coupler provides an alternative to a method previously described. The specimen is a long bar in a waveguide, nearly filling it in the smaller direction; in the larger direction it may either nearly fill the guide or occupy a small central region. Reflections from the ends of the specimen are reduced by tapers, or by curved sections of guide, and their effects are then eliminated by averaging the results for a series of positions of the specimen. Imperfections of the directional coupler can be eliminated (if they are not too large) by the use of two positions of an auxiliary short circuit. For measuring loss in solids, the method is probably better than that described before, because it does not need a liquid of matching permittivity, which has proved difficult to find.