An optimal circular-waveguide dual-mode filter without tuning screws

A novel circular-waveguide dual-mode (CWDM) filter structure(1) is proposed . The coupling between the degenerate modes in the same cavity is provided by an off-centered circular iris built in at the middle of the resonant cav ities, Considering the facts of: 1) simplicity of mechanical process; 2) a potentially high Q; 3) a wide spurious-free frequency band; and 4) the effe ctiveness of electromagnetic (EM) modeling, the structure can be considered as an optimal coupling structure for CWDM filters when a precise EM design is required. A rigorous mathematical proof is given for explaining its wor king mechanism. The detailed formulations for designing the new coupling st ructure is also discussed. To validate the new structure, a narrow-bandwidt h four-pole Ku-band elliptic filter is designed, manufactured and tested. V ery good agreement is obtained between the calculated and measured response s.