Tri-section stepped-impedance resonators for design of dual-band bandstop filter

In this paper, we present the dual-band bandstop filter design using tri-section stepped-impedance resonators. The proposed tri-section stepped-impedance resonator has the features of dual-band and size reduction due to its nonuniform impedance characteristic. There are two degrees of freedom in the proposed synthesis equations, which can be used for circuit miniaturization and changing impractical impedances into more realizable ones. An experimental bandstop filter was verified by circuit implementation. It showed good agreement between the simulated and measured results. Compared with the conventional parallel-connected λ/4 stubs, the proposed tri-section stepped-impedance structures can achieve more than 23.6% size reduction.