Pseudoelliptic Waveguide Filters Using U-Shaped Ridge Resonators

A new structure for the realization of inline waveguide pseudoelliptic filters using U-shaped ridge resonators has been presented. The basic structure is a U-shaped ridge placed inside a rectangular waveguide and can be utilized to realize both a pole and a transmission zero by either adjusting its offset from the waveguide center axis or by making the lengths of its two arms different from each other. The dominant mode of the feeding waveguide acting as the nonresonating mode, bypasses the U-shaped ridge and thus creates an alternate energy path, realizing a transmission zero. The dominant mode of rectangular waveguide also excites the resonating mode in U-shaped ridge because of the asymmetries introduced by either offset or different length arms. A transmission zero below the passband is achieved by offsetting the U-shaped ridge relative to the waveguide center axis. A transmission zero above the passband is realized by using a centered U-shaped ridge with different length arms. Both adjustments can also be made simultaneously to achieve further flexibility in realizing the desired filter response. A fourth-order filter with two transmission zeros, one in the lower stopband while the other in the upper stopband is designed, fabricated and measured. Measurements show good agreement with the simulations thus demonstrating the practicability of the proposed structure.