Abstract
This paper discusses the physics behind the operation of dual-mode bandpassfilters from afield theoretical point of view. It is argued that the two degenerate modes of the empty dual-mode cavity, commonly taken as the vertical and horizontal polariza- tions, become nonphysical when and elements are inserted. Instead, the original degenerate modes are rotated, or modified ina complex way, to generate two new modes whose char- acteristics depend on the and elements. It is shown that the elements, as placed in existing dual-mode filter de- signs, act as both and elements. A working dual- mode filter can be designed with only tuning elements present. A physical representation of dual-mode filters in terms of the eigen- resonances of the dual-mode cavities, with the and elements present, is introduced. Two fourth-order dual-mode rect- angular cavity filters with the same response in the passband and itsvicinityarealsopresentedtodemonstratethesimilarroleplayed by tuning and elements in dual-mode cavities. The first filter uses only tuning elements, while the second is based only on coupling elements.
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