Dual-band Rat-Race Coupler with Bandwidth Enhancement

This paper presents the novel design of a dual-band rat-race coupler with enhanced bandwidth. This is accomplished by the insertion of two additional shunt stubs for bandwidth optimization. The proposed circuit features planar structure, low insertion loss and no via-hole required. For demonstration, the measured performance of a microstrip rat-race coupler operating at 2.45/5.8 GHz is shown. An improvement in bandwidth of 50-100% is achieved with line impedance ranging from 40-90 Omega