A Balanced Filtering Directional Coupler Based on Slotline Using Asymmetric Parallel Loaded Branches

A balanced filtering directional coupler (BFDC) based on slotline is designed in this article. The proposed coupler is composed of balanced microstrip/slot (MS) transition feed structures, three-line cross-finger-coupled resonant structures, and asymmetric parallel loaded branch. The MS transition feed structure is adopted at the input and output ports to make the differential mode (DM) response independent of the common mode (CM) one and achieve a good CM suppression. Two transmission zeros (TZs) are generated and controlled by the coupling of three-line cross-finger-coupled resonant structure; thereby, the out-of-band selectivity can be enhanced. Asymmetrical parallel loaded branch can effectively improve the flatness of the coupler. In order to verify the design strategy, the coupler was designed, fabricated, and measured, and the measurement results are in good agreement with the simulation ones. The DM operating band of the designed coupler covers 4.2–5.3 GHz [fractional bandwidth (FBW) $=24$ %] with 15-dB return loss. Isolation is better than 15 dB and CM suppression is better than 40 dB.