A Novel Frequency-Variant Coupling Structure for Compact Inline Filter with Transmission Zeros

This paper introduces novel frequency-variant coupling (FVC) structure in rectangular waveguide. The key feature of this structure is its ability to generate two transmission zeros (TZs) and one pole, reducing by one the number of physical resonators in waveguide filters with frequency-variant coupling (FVC) presenting TZs in the response. Moreover, the novel coupling structure (here called "Double FVC") allows for large slope parameters of the equivalent rejection resonators modeling the transmission zeros generation. As a consequence, the TZs can be placed very close to the passband, where the classical structures implementing FVC cannot arrive. The paper introduces an equivalent circuit for the double FVC and shows how it can be used in the filters’ implementation. To validate this new proposed structure, a 5-pole filter with a double FVC was fabricated and measured.