Dual-band branch-line coupler with shorted stepped-impedance stubs arranged in a -shaped topology

This article presents a transmission-line-based 3-dB dual-band branch-line coupler. The coupler is easily realizable for ratios between the upper and lower operating frequencies ranging from 1.270 to 2.628. This means that it can be designed for two operation frequencies that are closer to each other in comparison with existing simple circuit topologies based on transmission lines, when considering a compact design. The complete set of equations for all possible responses of this dual-band 90 degrees hybrid coupler is derived. To evaluate its practical feasibility, the proposed circuit is analyzed with respect to the characteristic impedances of its transmission lines. A step-by-step procedure is presented to help the circuit designer deal with the use of the equations and all their degrees of freedom. For experimental validation, a microstrip dual-band branch-line coupler has been designed for operation at 3.835 and 5.285GHz. A good agreement is observed between simulated and measured results.