Dual-band filters based on TM dual-mode cavities

This paper presents a novel geometry for the realization of compact dual-band waveguide filters. It is based on TM dual-mode cavities that allow for asymmetric filtering functions with N transmission poles and N transmission zeros. In contrast to previous works, that are limited to structures with a single passband, here the TM cavity geometry is used to generate one pole in each passband and two transmission zeros located below, above or between the two passbands. Thus, a basic building-block for the design of multiple-cavity asymmetric dual-band filters is achieved. The use of TM cavities significantly minimizes the filter length without reducing the RF performances, mainly in terms of insertion loss. A three-cavity TM dual-band filter with six asymmetric transmission zeros has been designed and manufactured to validate the proposed geometry.