Compact dual-band branch-line and rat-race couplers with stepped coupled-line

This study constructs stepped-impedance coupled-lines for a dual-band coupler design over a frequency ratio range of 1.8-3.0. The proposed structure demonstrates good dual-band performance and compact size due to the centric arrangement of the excitation ports, which allows a more space-efficient circuit configuration. The developed synthesis method utilizes stepped impedance coupled-line to achieve the required quarter-wave impedance transformation at two designated frequencies. The odd-even phase difference in micro-strip implemented coupled-line is investigated in detail for practical circuit implementation. The current work fabricates two experimental circuits, including a dual-band branch-line coupler and a dual-band rat-race coupler. The measured results validate good dual-band performance at 0.9/2.0 GHz, with good agreement with EM simulation in Ansoft HFSS.