High-performance tight coupling microstrip directional coupler with fragment-type compensated structure

In this study, a design scheme for high-performance tight coupling microstrip coupler is proposed by adding a fragment-type compensated structure between two loosely coupled lines. Owing to its flexibility and adaptability, the fragment-type compensated structure can not only provide a quarter-wavelength phase shift for coupling enhancement, but also compensate the difference in the phase velocities of the even- and odd-modes for high directivity. Design of the fragment-type compensated structure can be implemented by multi-objective optimisation searching with several design objectives characterising tight coupling couplers. A high-efficiency optimisation searching strategy by using two-dimensional median filtering operator is employed to improve the efficiency of multi-objective optimisation. For demonstration, a high-performance 3-dB tight coupling directional coupler operating at 2 GHz is designed. The measurement data demonstrates a maximum directivity of 47 dB, above 25 dB directivity in a 21.5% bandwidth, a maximum variation of 0.3 dB in the coupling level and a maximum power division ratio of 0.6 dB, which indicate the overall performance better than the previously reported 3-dB couplers. In addition, a simplified theoretical analysis of the proposed coupler and full-wave simulated results are provided for better understanding of the fragment-type compensated structure.