Design of miniaturised branch-line and rat-race hybrid couplers with harmonics suppression

Shunt open-stubs are used to design reduced size branch-line and rat-race couplers with suppressed harmonics. The band stop response of shunt stubs has been utilised to suppress the harmonics. Design graphs are presented to ease the design process. Even-odd mode analysis is carried out to investigate in-band and out-of-band performances of the couplers. Two branch-line and one rat-race couplers have been fabricated. Experimental results of the designed couplers agree well with the theoretical predictions. For the branch-line coupler, up to fifth harmonic suppression and 37% size reduction are obtained. Up to seventh harmonic suppression and 69% size reduction are obtained for the rat-race coupler.