New compact Ka-band CPW rat-race coupler using GaAs MMIC technology

A new compact Ka-band rat-race coupler (RRC) is proposed and fabricated in the 0.15-μm GaAs pHEMTs process. Since the developed RRC is implemented by asymmetrical coplanar-waveguide T-structures, it has a compact circuit footprint and a return-loss bandwidth similar to that of the conventional coupler. Besides, a short-circuited quarter-wave stub is attached into the ring of the RRC to flatten the output phase responses. The developed coupler has a 47.28% 10-dB return-loss bandwidth and only occupies a 20.89% circuit size of the conventional one. By considering the 180° ± 5° phase difference, the frequency range covers from 28.73 to 39.53 GHz with a 31.6% relative bandwidth. Over this frequency range, the amplitude imbalance varies from - 0.269 to 0.8 dB.