A Dual-Band Balun Using Partially Coupled Stepped-Impedance Coupled-Line Resonators

In this paper, a recently proposed dual-band resonator structure consisting of a pair of partially coupled stepped-impedance lines is theoretically investigated. The new resonator configuration contains a number of attractive features for dual-band applications. A new type of dual-band balun that is constructed using a pair of such coupled stepped-impedance resonators is fully studied both theoretically and experimentally. Design equations for the balun are derived by an even- and odd-mode approach. A prototype of such dual-band balun operating at 900 MHz and 1.8 GHz is designed, fabricated, and measured. Good agreement between the designed and measured responses proves the concept of the dual-band balun and validates the design formulas.