Novel miniaturized branch-line coupler using bond-wire slow-wave structure

In this paper, one compact branch-line coupler is presented. The new structure consists of four lambda/4 transmission line which is shorten by slow-wave effect. Each slow-wave transmission line consists of inductive section using a bondwire as series inductor, and capacitive section using a metal island as capacitor. Because of slow-wave effect, the proposed coupler has 31% physical size of the conventional coupler at 1 GHz. The analyzed result has been achieved phase difference between S21 and S31, which is within 90deg plusmn 1deg. Furthermore, the insertion loss of the proposed branch-line coupler is comparable to that of conventional branch-line coupler. The new coupler can be easily implemented by using the standard printed-circuit-board etching processes and the wire bonding process that has very low tolerance. The proposed structure is very useful for wireless communication system.