Bandwidth-enhanced branch-line coupler using the center-loaded vertical line and distributed capacitors

In this paper, the technique that loads the center of the horizontal transmission lines in a conventional branch-line coupler by a vertical line section as well as the distributed capacitors is presented to enhance the coupler bandwidths. Based on the proposed structure, the numerical solution derived from the even/odd-mode decomposition analysis for optimized bandwidth enhancement is applied for structural parameters. Experimental results show that the developed 90° coupler demonstrates more than 11% return-loss/isolation bandwidth increase as compared to the conventional quadrature hybrid. Furthermore, the 3dB±1dB magnitude-and 90°±5° phase-difference bandwidths are 3% and 14% increased, respectively.