A variety of dual-mode ridge cavity filters are presented. The resonances that exist in the dual-mode cavities with all the perturbations present are used to represent and design the filter. Different arrangements which produce similar field distributions are used. Since the degenerate modes of a lossless dual-mode cavity can not be coupled but only rotated, or have their field distributions changed in a complex way, different mechanisms are used to affect such rotations. The first consists in adding standard perturbations or "coupling" elements. The second version consists in physically rotating an almost square ridge. Good results are obtained.
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