Compact harmonic suppression branch-line coupler using signal-interference technique

A novel application of signal-interference technique to branch-line coupler design is proposed for miniaturization and wideband harmonic passband suppression. The coupler's second, third and fourth harmonic passband responses are attenuated due to a pole at each of these harmonics. Design equations and graphs are provided using lossless transmission line model. A prototype coupler, having a compact (λ_g/6)² area, that is 44.4% of the (λ_g/4)² conventional coupler area, is fabricated in microstrip form for validation purposes. The input matching, isolation, amplitude and phase balance bandwidths of the proposed coupler are comparable to those of the conventional coupler.