Second-Order Quad-Band Bandpass Frequency Selective Surface with Multiple Transmission Zeros

This paper presents the design of an innovative second-order high-selectivity quad-band bandpass frequency selective surface (FSS) with multiple close-to-passband transmission zeros (TZs). Its constituent unit cell comprises three metal elements, for which two identical metal patterns are printed on the top and bottom layers, and they are in a back-to-back configuration along with a middle-layer metal element in between. Meanwhile, two thin dielectric substrates are employed to separate these three metal layers. Based on the proposed design, the engineered FSS can attain a quad-band bandpass-filter (BPF) response. The four created transmission passbands are centered at 7.8, 12.3, 15.1, and 17.6 GHz, respectively, and their in-band power-reflection levels are higher than 10 dB. Due to the generated multiple stopband TZs located at the close-to-passband regions, sharp-rejection filtering capabilities for the proposed FSS are obtained. In addition, further merits of the conceived quad-band bandpass FSS are demonstrated, such as (i) its compact circuitry size, (ii) simple geometry of its building unit cell, and (iii) polarization-insensitive filtering response with high angular stability up to 30° incident angles for small variations of the bandwidths as confirmed at the electromagnetic-(EM)-simulation level.