A Low-Profile Miniaturized Second-Order Bandpass Frequency Selective Surface

High-order bandpass frequency selective surfaces (FSSs) (N ≥ 1) can achieve high performance with a flat in-band frequency response and fast roll-off. One particular practical issue of designing bandpass FSSs using resonant surfaces is that the thickness of the substrate would be around a quarter of a wavelength. On the other hand, the size of a nonresonant FSS array element is usually large. A new miniaturized FSS capable of exhibiting a second-order bandpass response is proposed in this letter. Two miniaturized resonant surfaces coupled by a nonresonant inductive layer are used to build the proposed FSSs. An FSS operating at around 3.8 GHz is designed to verify the method. The element size is smaller than $0.076\lambda\,\times \,0.076\lambda $ for the proposed structure. This is significantly smaller than the element size of second-order FSSs designed using conventional approaches. The overall thickness is less than λ/24, where λ is the free-space wavelength at the resonant frequency. The method could be particularly useful for the design of FSSs at lower frequencies with longer wavelengths