A bandwidth-enhanced 3-dB lumped-element branch-line coupler based on asymmetrical E-equivalent sections

In this paper, a new 3-dB lumped-element branch-line coupler using asymmetrical E-equivalent sections is proposed, designed, and experimentally verified. The proposed asymmetrical E-equivalent section is evolved from the high- and low-impedance transmission line structure. As the transmission lines of the conventional branch-line coupler are replaced by the asymmetrical lumped-element E-equivalent sections, it can enhance the return loss bandwidth. The measured |S₁₁| of the developed coupler is better than -10 dB from 0.751 GHz to 1.158 GHz with a 42.6 % relative bandwidth. It is broader than that of the conventional distributed branch-line coupler. Considering 90° ± 5° phase difference, the frequency range covers from 0.785 GHz to 1.107 GHz with a 34 % relative bandwidth.