Design and validation of miniaturize Rat Race Coupler based microstrip Balun

This work presents a compact Balun with enhanced bandwidth and better isolation between output ports using Interdigital Capacitor (IDC). Firstly, a Rat Race Coupler (RRC) is designed, later Balun is proposed by replacing the isolation port with a 50 Ω isolation resistor. The proposed Transmission line (TL) is designed using an IDC in parallel with a high impedance TL. Based on a π-equivalent circuit the proposed TL has been investigated and optimized to achieve 70.7 Ω impedance. The Balun is designed and fabricated. The simulated and fabricated results are in accord. The fractional bandwidth (FBW) of RRC and Balun are 37.5% and 33% respectively at centre frequency 2.4 GHz. The size occupied by the proposed Balun is 0.51λg × 0.22λg. The structure is low cost, uniplanar and compact.