In this paper, a directional coupler incorporating the bandpass frequency response characteristic is proposed and characterized. This structure is modified from a conventional branch-line coupler. Two wide open-ended coupled lines are used instead of microstrip branch lines. As such, good restraint performance outside the passband can be achieved. The even-odd mode technique is employed to analyze and synthesize such a coupler. After that, frequency tuning is achieved by modifying the electrical length of the open-ended coupled lines with varactors. Measured results validate the correctness of our theory and design.
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