Design of quadrature hybrid with closely separated dual-passband using three-branch line coupler

Design of quadrature hybrids with two closely separated bands using a three-branch line coupler is proposed. Exact design equations are derived by the fourfold even-odd mode decomposition method. For dual-band operation, the conditions of the 90° hybrid coupler are simultaneously satisfied at the two designated frequencies. Thus, the solutions can exactly give the optimum design. The realizable ranges of frequency ratio and bandwidth are also investigated. A microstrip hybrid coupler operating at 2.5/3.5 GHz for WiMAX applications is designed, fabricated and tested. Simulation and experiment results show a good agreement.