A novel design of micostrip bandstop filter (BSF) based on branch-line coupler structure is presented. The present BSF is obtained by modifying the branch-line coupler with open-circuits at the coupled and isolated ports. The usefulness of the present topology is demonstrated by comparing the filter performances with a standard two transmission line (TTL) signal interference BSF of same total electrical length. The proposed structure does not increase the physical dimension of the BSF, but improves the performance significantly. For given impedance ratio, the transmission zero separation bandwidth of the proposed BSF is always larger than the TTL-BSF. Measured prototype at 1.0 GHz validates the design approach.
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