Directivity improvement of microstrip coupled line couplers based on equivalent admittance approach

The present paper suggests a simple and effective technique for remarkably improving the directivity of inhomogeneous coupled line couplers such as a microstrip coupler. The technique is based upon the equivalent admittance approach and requires only properly designed matching networks. In this paper, we treat a 1/4-wave parallel-coupled microstrip coupler of 15.8 dB, and obtain the directivity improvement of 20 or more dB over a bandwidth of about 1.2 GHz at a center frequency of 3.5 GHz. Finally the design results are confirmed by an em-simulator.