Development of circularly polarized microwave cavity filters

The use of circularly polarized microwave cavities leads to simplification in the art of microwave filter design. Only one cavity (with two coupling apertures) is required to produce a four-port waveguide filter that is a reflectionless diplexer (i. e., a filter for mixing or separating signals of different frequency). A rectangular waveguide with one coupling aperture can excite a circularly polarized cavity mode by Hxand Hzor Eyand Hxcoupling. The latter type of coupling can be accomplished through apertures in the side walls of a circular cylinder cavity, freeing the end plates for tuning and temperature compensation. For cornrnercial use of these filters the TE112cavity mode was chosen as the optimum degenerate mode on the basis of minimum cavity wall losses and maximum separation of unwanted cavity modes. Experimental test results at X-band frequencies are included for several of these cavities.