Branch-line coupler with four bands applied in LTE-Taiwan and GPS

The present invention relates to a branch-line coupler, particularly applied in a system of 4G LTE-Taiwan and a global positioning system (GPS). The branch-line coupler is developed by double-sideband [pi] equivalence of a conventional branch-line coupler and using a multiband circuit to replace a [pi] -equivalent admittance value jB. The branch-line coupler with four bands includes a main body and four multiband circuits each separately coupled to a first input port, second input port, a third output port and a fourth output port. Each of the multiband circuits further comprises first, second, third and fourth band circuits all connected in series. Thus, the circuit size for the branch-line coupler can be miniaturized for further application in a system of 4G LTE-Taiwan and a global positioning system.