A novel tri-band frequency selective surface (FSS) is proposed by using multi-layer printed circuit boards with a number of via holes. Three separate propagation paths with two stepped-impedance resonators (SIRs) and a uniform-impedance resonator (UIR) are constructed in each unit cell of the proposed FSS, thus leading to tri-band bandpass performance. The lowest and the middle frequency bands are achieved by two SIRs, respectively, and the highest frequency band is obtained by the resonance of the UIR. Therefore, these three passbands can be realised independently, which can increase the design freedom. The operating principle is explained by analysing the electric field distributions. To verify the design concept, a prototype of the proposed FSS is simulated and tested. Measured results show that the FSS features stable tri-band performance under a large variation of incident angle.

A novel 3D frequency selective structure (FSS), exhibiting bandpass filtering response with high frequency selectivity, is presented. In each unit cell, multiple resonators are constructed to provide multiple transmission zeros/poles. By establishing an equivalent circuit model, it is obtained that the transmission poles in the passband are produced by a short-circuited resonator and an open-circuited resonator, and the transmission zeros close to the passband are attributed to the fundamental and spurious resonant frequencies of another short-circuited resonator. To implement it in high frequencies, multilayer printed circuit board technology is introduced, where several replacements are carried out. The measured results show good agreement with simulations. Furthermore, the proposed FSS exhibits stable performance with incident angles increased from 0 to 50°.

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