A high-directivity microstrip directional coupler with feedback compensation

A new directivity-enhancement method for a microstrip directional coupler is presented. The method utilizes feedback elements between the collinear ports of the parallel-line coupler to generate an isolation zero at the desired frequency. The closed-form equations for designing such compensating elements are developed. Experiments are conducted to verify the proposed method. More than 30 dB directivity improvement is achieved, compared to the conventional microstrip parallel coupler.