Broadband branch-line coupler with very loose coupling utilizing open/short-circuited coupled-transmission lines

In this paper broadband branch-line couplers with loose couplings (less than -10 dB) utilizing open/short-circuited coupled-transmission lines are proposed. By applying the equivalent admittance approach to the proposed branch-line coupler, broadband characteristics with a relative bandwidth of 47.5% for 20 dB return loss and directivity can be obtained in the case of the branch-line coupler with a coupling factor of -20 dB. The validity of this design procedure has been demonstrated by electromagnetic simulations and experiments.