Dual-band branch-line balun for millimeter-wave applications

This paper presents designs of branch-line type dual-band balun for millimeter-wave applications. The proposed balun is based on different types of dual-band impedance transformers. To understand the working principle of the baluns, closed-form equations are derived, where ABCD-matrix method is applied. It is found that the proposed baluns feature flexible realizable frequency ratio, making it suitable for millimeter-wave systems. To verify the design concept, an experimental prototype working at 20 / 44 GHz is designed, fabricated and characterized. The measurement results match well with the simulation results.