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This paper presents a polarization insensitive frequency selective surface (FSS) based on concentric ring slots with five closely spaced passbands. The proposed FSS unit cell contains five ring slots coupled with a supplementary innermost ring slot. The close band response within the first five transmission bands is achieved by the influence of the modified geometry obtained with the inclusion of the sixth ring slot. This added ring slot causes the appearance of a stopband, due to the conductive ring between the fifth and the sixth ring apertures, and a passband produced inherently by the sixth ring aperture. The effect of the additional stopband and passband in the FSS response has been investigated in the paper. Full-wave simulations reveal that a modified geometry with proper sizes effectively enables the close band response of the pentaband FSS. To validate the proposed approach, a 729-element penta-bandpass FSS prototype operating at millimeter-wave frequencies has been designed, fabricated, and tested. Experimental results in a free-space measurement setup indicate resonances at f1 = 26.7, f2 = 29.8, f3 = 33.6, f4 = 38.1, and f5 = 44.2 GHz, with resonant frequency ratios of f2 / f1 = 1.12, f3 / f2 = 1.13, f4 / f3 = 1.13, and f5 / f4 = 1.16. This FSS demonstrates an extremely low ratio of the highest and the lowest resonant frequencies ( f5 / f1 = 1.66) for a penta-band operation. This polarization insensitive FSS demonstrates good performance at oblique angular incidence for elevation angles up to 45&#x00B0;.

Close Band Spacing Pentaband Frequency Selective Surfaces Based on Concentric Ring Slots