Design of High-Directivity Wideband Microstrip Directional Coupler With Fragment-Type Structure

A novel design for a microstrip wideband directional coupler is proposed by using fragment-type structures. The use of a fragment-type structure may provide satisfactory flexibility and excellent performance. For a given design space, a fragment-type wideband coupler can be designed by first gridding the space into fragment cells and then metallizing the fragment cells selected by a multi-objective optimization searching algorithm, such as a multi-objective evolutionary algorithm based on decomposition combined with enhanced genetic operators. For demonstration, a 20-dB wideband microstrip directional coupler is designed and verified by test. A 45% bandwidth centered at 2 GHz has been measured in terms of maximum variation of 0.5 dB in the 20-dB coupling level. In the operation band, the designed coupler has directivity above 37 dB, and a maximum directivity of 48 dB at 2 GHz. In addition, some technique aspects related to multi-objective optimization searching, such as effects of design space, control of coupling level, and efficiency consideration for optimization searching, are further discussed. Fragment-type structures may also be used to design high-performance wideband directional couplers of tight coupling level.