A novel multimode bandpass filter with high and wide rejection band is proposed by using an open stub. The open-stub has two functions in the filter: 1) perturbation for the multimode operation and 2) zero point generation at the stopband for the stopband control. With proper input/output coupling, additional two poles, i.e., four poles, can be generated for the proposed filter structure. The design is then verified by experiment. The single structure four-mode filter has low insertion loss, good stopband performance, and compact size and linear phase.

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