Dual Band Branch-Line Coupler Using Stub-Loaded Lines

Abstract

This paper describes the design of a center-tapped stub loaded branch line coupler for dual bandoperation. In this design, the quarter wavelength transmission lines of the conventional branch line coupler arereplaced with an equivalent T-shaped transmission lines. Design equations for evaluating the characteristicimpedances and the electrical lengths of the coupler arms are derived based on ABCD-matrix. The proposedcoupler operates at 2 and 4 GHz corresponding to S-band applications like weather radar and satellitecommunications. Simulations are done using Agilent ADS software on Roger RO4003 substrate with dielectricconstant of 3.38. The simulated results of the S-parameters show that the return loss and isolation loss are wellbelow -20 dB at the designed frequencies. The amplitude imbalance between the two output ports S21 and S31does not exceed 0.3 dB. The phase difference between the output ports of the designed coupler is -89.5o at 2GHz and -270o at 4 GHz. Efforts are done on reducing the size of the coupler by folding the stub towards theinner area of the coupler to be ready for fabrication.