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 band operation. In this design, the quarter wavelength transmission lines of the conventional branch line coupler are replaced with an equivalent T-shaped transmission lines. Design equations for evaluating the characteristic impedances and the electrical lengths of the coupler arms are derived based on ABCD-matrix. The proposed coupler operates at 2 and 4 GHz corresponding to S-band applications like weather radar and satellite communications. Simulations are done using Agilent ADS software on Roger RO4003 substrate with dielectric constant of 3.38. The simulated results of the S-parameters show that the return loss and isolation loss are well below -20 dB at the designed frequencies. The amplitude imbalance between the two output ports S21 and S31 does not exceed 0.3 dB. The phase difference between the output ports of the designed coupler is -89.5o at 2 GHz and -270o at 4 GHz. Efforts are done on reducing the size of the coupler by folding the stub towards the inner area of the coupler to be ready for fabrication.

Keywords

• Branch line coupler
• Dual-band
• Stub loaded transmission lines