Design and Optimization of a Miniaturized Stub-Based Branch-Line Coupler at 900 MHz for GSM Applications

Abstract

The branch-line coupler (BLC) is a four-port passive component widely used in wireless and microwave systems to divide or combine power equally with a 90° phase difference between its output ports. Because each branch of a conventional BLC is a quarter-wavelength long, the circuit becomes bulky at low frequencies such as the 900 MHz GSM band. This study presents the design and optimization of a miniaturised stub-based BLC operating at 900 MHz. We first designed a conventional coupler with 50 Ω ports on a Rogers RT5880 substrate (εr = 2.2, h = 0.78 mm) and simulated it in CST Studio Suite. Each quarter-wavelength branch was then replaced by an equivalent compact section loaded with stubs, and the stub dimensions were optimized by a parametric sweep to recover bandwidth. The conventional coupler gave a return loss (S11) and isolation (S41) of about -32 dB at 0.92 GHz, through and coupled transmission (S21, S31) of about -3.1 dB at 900 MHz and a -10 dB bandwidth of 379 MHz. The stub-loaded coupler reduced the circuit area by 50.2% (from 9109.9 mm² to 4536.6 mm²), with S11 and S41 reaching about -48.6 dB, S21 and S31 of about -2.9 dB and -3.4 dB at 900 MHz (0.5 dB amplitude imbalance) and a bandwidth of 307 MHz. After optimization, the bandwidth increased to 420 MHz (0.715-1.135 GHz) with S11 and S41 better than −18 dB at 900 MHz, but the amplitude imbalance between S21 and S31 rose to about 3.1 dB. These simulation ...