Wideband Highly-Selective Bandpass Filtering Branch-Line Coupler

This paper presents a novel design of a wideband highly-selective bandpass filtering branch-line coupler (FBLC). By integrating a coupled microstrip line, and an open-ended stub at each port of a single-section BLC, bandpass filtering characteristics with excellent selectivity and broad operating bandwidth have been achieved. The proposed circuit has been verified through EM simulations and physical measurements of the fabricated prototype. Experimental results demonstrate over 71-percent fractional bandwidth (2.25 GHz to 4.74 GHz) with a power imbalance lower than 0.5 dB. At the same time, the measured return loss and isolation are better than 15dB with the phase imbalance of 5 +/- 5 degrees. This level of performance, especially in terms of bandwidth and selectivity, has not been reported in the literature thus far.