Novel design of multiband branch-line coupler using multiband transmission lines

In this article, a novel multiband branch-line coupler employing multiband transmission lines is designed to operate at three different frequencies. To realize multiband operations, a Pi-type-based multiband transmission line network with open- and short-ended stubs is employed. Specifically, the Pi-type line is applied to realize the dual-band operation at the first two working frequencies. For the third operating frequency, the open- and short-ended stubs are tapped at the center of the Pi-type line. ABCD-matrix is employed to derive the design formulas for the proposed triband transmission line. To verify the design concept, a planar coupler operating at 1/2.5/4.5GHz is designed and fabricated. The simulation and measurement results show good agreement with the design theory.