Compact broadband microstrip rat-race couplers using microstrip/slotline phase inverters for arbitrary power-dividing ratios

Compact broadband microstrip rat-race couplers using microstrip/slotline phase inverters are proposed. A wide range of power-dividing ratio (1-9) is achieved by varying the impedance ratio of the transmission lines, while the dual-band impedance matching characteristic with frequency ratio (1-2:9) is obtained by varying the impedances of the transmission lines (20-158 Ω) only. For verification, a rat-race coupler with power-dividing ratio of 9: 1 and operating at 1.0/2.5 GHz is designed, fabricated, and measured. The measured results are in good agreement with the simulated ones. The measured results show a bandwidth of 120% with better than 10-dB return loss and 31-dB isolation from 0.69 to 2.74 GHz.