Frequency-agile directional coupler with good restraint outside the passband based on the coupled transmission line

A frequency-agile directional coupler incorporating the bandpass frequency response characteristic is proposed and characterized in this paper. This structure is modified from a conventional branch-line coupler and composed of two wide open-ended coupled lines. As such, it has good restraint performance outside the passband. Frequency tuning is achieved by modifying the electrical length of the open-ended coupled lines loaded with varactors. Measured results are in agreement with predictions, which validate the correctness of our theory and design.