A Tapped Stepped-Impedance Balun With Dual-Band Operations

This letter presents the design of a dual-band balun using tapped stubs and stepped-impedance structures. By adjusting the impedances of the tapped stubs properly, the two output signals of the balun are out-of-phase at the two operating frequencies which is essential for the balun operation. Meanwhile, the stepped-impedance structure is designed to satisfy the dual-band matching condition at the input port. Closed form design equations are derived using the even-odd mode method. Based on these equations, a microstrip balun on Rogers' duroid/RT 5880 substrate operating at 2.45 GHz/5.25 GHz is designed, fabricated, and measured. The measurement results verify the dual-band operations of the balun. Finally, the impedance transformation property of the proposed dual-band balun is discussed.