Design of tri-band branch-line couplers using novel tri-band transmission lines

A novel design of tri-band branch-line couplers using tri-band transmission lines is presented in this paper. The proposed tri-band transmission line employs a Pi-type line with open- and short-ended stubs. The line with Pi-type structure is applied to realize the dual-band operation at the first two frequencies (f₂/f₁). To achieve the third operating frequency ratio (f₃), the open- and short-ended stubs are tapped at the center of the Pi-type lines. An ABCD-matrix is employed to derive the design formulas for the proposed tri-band transmission line. To verify this design concept, a planar tri-band coupler operating at 1/2.5/4.5 GHz is designed and fabricated. The simulation and measurement results show good agreement with the design theory.