Design of dual-band rat-race couplers

New designs of dual-band rat-race couplers are proposed. The desired dual-band operations are realised by adding tapped open stubs to the branches of the conventional rat-race coupler. It is found that these stubs can be implemented in two different ways to satisfy the conditions for dual-band applications, which results in two types of designs. The two different designs can be separately used to achieve low and high ratios between two operating bands, therefore providing enhanced design flexibility. The design formulas are developed using ABCD matrix and the even-odd mode analysis. On the basis of the derived design equations, a type I rat-race coupler working at 1 GHz/3.5 GHz and a type II coupler working at 2 GHz/5 GHz are devised and fabricated on Rogers´s RO3006 printed circuit boards. Measurement results prove the design concepts.