A novel circuit structure of dual-band bandstop filters is proposed in this paper. This structure comprises two shunt-connected tri-section stepped impedance resonators with a transmission line in between. Theoretical analysis and design procedures are described. The derived synthesis equations have two degrees of freedom which provide more design flexibility in filter synthesis. Notably, three advantages of the proposed filter structure lie in the fact of its increased nonuniform impedances, resulting in a compact size, wide range of realizable frequency ratio, and more realizable impedances. Three experimental dual-band bandstop filters with various frequency ratios were fabricated to demonstrate the feasibility of the new filter structure.
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