Three-line microstrip directional coupler with high directivity

This article proposes a novel microstrip directional coupler with high directivity. The proposed coupler was configured using three coupled transmission lines and two reflectors to cancel the leakage components in the isolation ports. To demonstrate the validity of the proposed coupler, we designed and fabricated the microstrip directional coupler with 15 dB coupling at 5.8 GHz. The measurement showed the directivity of 50.0 dB at 5.83 GHz, which was improved by 35.9 dB compared with that of conventional one.