Design of Multipassband Microstrip Branch-Line Couplers With Open Stubs

The microstrip branch-line coupler with multiple passbands is proposed; moreover, the design equations are provided in this paper. In order to obtain multiple passbands, open stubs are added to the conventional branch-line coupler's ports; in addition, these open stubs' impedance and electrical length need to be properly chosen. The proposed couplers can be easily fabricated on the printed circuit board without any lumped element. The agreement between measured results and the theoretical prediction validates the feasible configuration of the proposed coupler.