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This communication presents the miniaturized low-profile second-order (SO) bandstop frequency selective surfaces (FSSs) for single- and dual-band resonances. The unit cell comprises two identical first-order (FO) elements in a back-to-back configuration with the metal grid in between. Two thin dielectric substrates separate the three metal layers. The proposed FO unit cell consists of intercell-connected spiral resonators (SRs) with capacitively coupled cross loop dipole. The SO filter response of the proposed FSS exhibits a wide stopband ($S_{21} < -10$ dB) bandwidth (BW) of 27.6%. The unit cell has the periodicity of $0.162\lambda _{0}$ , where $\lambda _{0}$ is the free space wavelength at the lower passband. The novelty of the proposed work is a miniaturized simple unit cell geometry, at the millimeter-waveband but not limited to it, with polarization insensitive and high angular stable filter response up to 60° with small variations in the BW. An unconventional four-unit cell element FSS is designed for dual-band SO stopband resonances with the BWs of 8.3% and 13.8%. Prototypes of both FSSs were fabricated and their measured results were found consistent with the simulated filter responses. [ABSTRACT FROM AUTHOR]

This article presents a new method of enhancing the capacitance of a single-layer conformal frequency selective surface (FSS) for multifunctional operation. The unit cell of the FSS consists of gridded square loop-loaded modified Jerusalem cross (JC) element. End capacitance of the conventional JC FSS is enhanced by placing parallel diagonal strips, resulting in a pair of reflection zeros near the sidebands. The modified geometry provides high selective stopband filtering that reflects the signal at 20.16 GHz with S21 < −10 dB stopband bandwidth of 20.52%. The FSS shows stable stopband reflection up to a high incidence angle of 85°. The stability in the filter response is also achieved at various conformal angles. The proposed FSS is optimized for dual-band near-perfect absorption. At two resonances 12.76 and 22.54 GHz, the absorption are 99.88% and 99.1%, respectively. Prototypes of the stopband FSS and the dual-band absorber are fabricated to verify the feasibility of the proposed concept. The measured filter and absorber responses are found consistent with the simulated results. As a conformal multifunctional structure, the proposed FSS has the merits of electromagnetic interference shielding and radar cross section reduction in stealth application. [ABSTRACT FROM AUTHOR]