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.
A novel combined-element frequency selective surface (FSS) is proposed for multi-band applications. In this design, complementary square loops and meandering slots are combined to construct the composite FSS. The proposed FSS provides three transmission poles and two transmission zeros, transmitting the desired signals at 8.5, 11 and 16 GHz while reflecting the unnecessary signals at 9.6 and 13.1 GHz. This FSS has the advantages of flexible and simple structure, easy processing and low profile compared with traditional multi-pole FSSs. The design strategy of the proposed FSS and the associated equivalent circuit model are provided to analyse the frequency characteristics. A prototype is fabricated and measured, showing a superior and stable frequency response in case of oblique incidence for both the transverse-electric (TE) and transverse-magnetic (TM) polarisations. Good agreement between the simulated and measured results is observed. Finally, the higher-order FSS with good frequency selectivity is investigated.
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