Dual-band balun with flexible frequency ratios

A novel dual-band balun is presented. The designed 3-port balun is based on a 4-port structure with the fourth port terminated as an open end. Being a 4-port symmetrical network, even-odd-mode analysis is used to analyse the balun´s performance. Based on the derived explicit design equations, a balun is designed to operate at 1.1 and 2 GHz. Numerical simulations show a low reflection coefficient S₁₁, equal magnitude of -3 dB as well as 180° phase difference at the output ports. To validate the theoretical and numerical simulations, the proposed balun is fabricated on the FR4 Isola printed circuit board and measurement results match well with the design theory.