End Plate and Side Wall Currents in Circular Cylinder Cavity Resonator

In waveguides or in cavity resonators, a knowledge of the electromagnetic field distribution is of prime importance to the designer. Representations of these fields for the lower modes in rectangular, circular and elliptical waveguide, as well as coaxial transmission line, have frequently been described. For the most part, however, these representations have been diagrammatic or schematic, intended only to give a general physical picture of the fields. In actual designs, such as high Q cavities for use as echo boxes,1 accurately made plates of the distributions were found necessary to handle adequately problems of excitation of the various modes and of mode suppression. One use of the charts is to determine where an exciting loop or orifice should be located and how the field should be oriented for maximum coupling to a particular mode. Optimum locations for both launchers and absorbers can be found. Naturally, when attention is concentrated on a single mode these will be located at the maximum current density points. If, however, two or more modes can coexist, and only one is desired, compromise locations can sometimes be found which minimize the unwanted phenomena. Also, in a cylindrical cavity resonator of high Q with diameter large compared with the operating wavelength, there are many high order modes of oscillation whose resonances fall within the design frequency band. Some of these are undesired and one of the objectives of a practical design is to reduce their responses to a tolerable amount.