Multiobjective Optimization Design of Microstrip Directional Coupler With Generalized Coupling Slots

A novel technique for designing slot-coupling microstrip directional coupler is proposed by optimizing the generalized coupling slot on the printed circuit board (PCB). The generalized coupling slot is characterized by connecting distributed contour nodes with slot line segments and necessary slot branches. The optimal slot can be acquired by implementing multiobjective optimization search for the optimal distribution of the contour nodes and the optimal slot width. For computer optimization search, a general node connection scheme and a binary design matrix encoding the node distribution and slot width are proposed. For demonstration, a 20-dB microstrip directional coupler operating at a center frequency of 2 GHz is designed with uniform slot width of all slots on PCB in a very compact size. The measurement indicates that high directivity over 40 dB in an operation bandwidth of approximately 53% can be acquired. In addition, the tolerance of the fabrication error is analyzed by using Latin hypercube sampling to model the fabrication errors of node distribution and slot width. Several structural factors are investigated, which may affect the coupler performance, including the use of slot branches, the number of nodes, the design space on the PCB, and the respective slot width of each segment.