Systematic Design Technique for Dual-Band Branch-Line Coupler Using T- and Pi-Networks and Their Application in Novel Wideband-Ratio Crossover

In this paper, a generalized analysis and design methodology of a dual-band branch-line coupler (BLC) is presented. The proposed design, although based on the prevalent method of replacement of various arms of a single-band BLC with an equivalent T-/Pi-network, simplifies the design approach by only replacing either pair of arms and therefore provides four distinct topologies. Closed-form design equations along with insightful comments are reported for all the four topologies of the BLC. A number of cases are investigated to demonstrate the effectiveness of the proposed design for equal and unequal power division ratio and band-ratio (= f(2)/f(1)). As an application, a novel dual-band crossover as a cascade of BLCs is also presented. While investigating the limitations of previous designs, a strategy to obtain the crossover with so far the widest band-ratio and simple layout is also presented. An unequal power division dual-band BLC and a dual-band crossover are designed which operate at 1/2 and 1/4 GHz, respectively. These designs are validated from the electromagnetic simulations and prototypes implemented on RT/Duroid 5880 substrate.