TM dual-mode filters with asymmetric filtering functions

This paper presents a novel geometry for the realization of TM dual-mode cavities and filters having asymmetric filtering characteristics. In contrast to previous works, that are limited to structures with symmetric responses, the proposed cavity does not employ any intra-coupling elements, while the source and load of the cavity are directly coupled to both resonant modes. The latter coupling mechanism is obtained by rotating the input and output waveguides with respect to the cavity. In this way a cavity can generate a single transmission zero located either below or above the two poles, so realizing a basic building block for the design of multiple-cavity asymmetric filters. A further advantage arises from the fact that by avoiding the intra-coupling elements, the whole filter structure can be very efficiently analyzed by simple in-line mode-matching techniques. A two-cavity TM dual-mode filter with a pair of transmission zeros below the pass-band has been designed and manufactured to prove the feasibility of the proposed geometry.