Inline Waveguide Pseudo-Elliptic Filters Using Non-Resonating Modes With Folded-Waveguide Resonators

In this paper, a new structure to implement inline waveguide pseudo-elliptic filters using folded-waveguide (FWG) resonators along with non-resonating modes has been proposed. The basic structure is an FWG resonator fed by a rectangular waveguide through inductive irises. The $TE_{10}$ mode of the feeding rectangular waveguide excites both dominant and first higher order modes in the FWG structure. The dominant mode of the FWG acting as a non-resonating mode creates an alternate energy path from source to load thus producing a transmission zero (TZ), while the first higher order mode acts as the resonant mode resulting in a pole. The structure therefore acts as a singlet capable of realizing a pole and a TZ either below or above the passband depending on the offset of the feeding irises from the center of the FWG resonator and relative to each other. The structure also provides flexibility in shifting the FWG center axis relative to the feeding rectangular waveguides, thus giving more coupling control and realizing wider range of singlet responses. The proposed singlet structure does not require any increase in the cross-sectional size relative to the standard rectangular waveguide. Two prototype filters using the proposed singlet structures have been designed, manufactured and tested. Measurements show good agreement with the simulations, verifying the feasibility of the proposed structure.