Bandwidth-Enhanced Branch-Line Coupler Using the Center-Loaded Vertical Line and Distributed Capacitors

Abstract

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 degrees coupler demonstrates more than 11% return-loss/isolation bandwidth increase as compared to the conventional quadrature hybrid. Furthermore, the 3dB +/- 1dB magnitude- and 90 degrees +/- 5 degrees phase-difference bandwidths are 3% and 14% increased, respectively.