Design Method of Coupled-Microstrip Directional Couplers Based on Equivalent Admittance Approach

摘要

The present paper suggests a simple and effective technique for remarkably improving the directivity of microstrip coupled line coupler. This technique is based upon the equivalent admittance approach, and requires only properly designed matching networks. We treat a 1/4-wave length parallel-coupled microstrip coupler of 15dB, and try to improve the directivity of the couplers by optimizing the parameters of matching circuits. As a result, the improved directivity of 20 or more dB can be obtain over a relative bandwidth of about 19 % using impedance steps. Furthermore, a relative bandwidth can be remarkably widened by the use of impedance steps and 1/4-wave short-circuited stubs, and its value increases to about 50 %. Finally, the design results are confirmed by an em-simulator.