Simple Planar Microstrip Crossover Coupler with Independent Control over Bandwidth and Selectivity

This paper presents the design, optimization, and measurement validation of a highly compact and frequency-selective Microstrip Crossover architecture for microwave applications, specifically targeting the 2.4 GHz band. The proposed design features a modified hexagonal structure enhanced with Sierpinski carpets, four loading stubs on the access lines, and Defected Ground Structures (DGS), along with two via holes to excite a new operating mode. This integration achieves an outstanding miniaturization ratio, occupying 62% less surface area compared to conventional crossover designs. Through simulation, the optimized structure demonstrates superior performance with low insertion loss of 1 dB and Isolation/Matching better than 20 dB at 2.4 GHz. A key contribution is the demonstrated response controllability, where the loading stubs and DGS serve as independent tuning elements: the stub length controls the frequency of the lower band transmission zeros for selectivity control, while the DGS dimensions govern the passband width and Fractional Bandwidth (FBW).