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A tri-band frequency selective surface (FSS) with close band spacing based on split ring slots for y-polarised incident waves is investigated. The close band response is obtained by reducing the strong mutual coupling between the closely packed concentric rings through splitting the ring slots and redistributing the location of the surface currents at resonances. To validate the proposed FSS, a Ka band FSS was designed, fabricated and tested. Experimental results showed closely spaced resonances at 27.3, 31.7 and 36.2 GHz. Frequency ratios of 1.16 for the second to the first resonance, and 1.14 for the third to the second one were obtained. Also, good stability is observed for oblique incidence.

In this paper, a tunable triband frequency selective surface unit cell based on varactor-loaded split ring slots is presented. By using the third resonant mode, a considerable mutual coupling reduction between adjacent ring slots is obtained ensuring both close band response as well as independent band tuning. Simulated results show the tunable properties of the proposed approach by shifting the three passbands resonant frequencies from 8.368, 10.276, and 12.010 GHz (for the unloaded split ring slots), to 7.732, 9.202, and 10.900 GHz, respectively, when each split ring slot is loaded with a capacitance of 140 fF. Furthermore, the metal shorts that split the ring slots as well as the metal rings located in between the slots are used to build the conducting paths to bias each varactor with minimum disruption in the RF response. A demonstrator based on an X-band diaphragm was designed, fabricated, and experimentally tested. Measured results using a varactor with capacitance range of 30-65 fF show resonant frequencies for the unbiased varactors at 8.53, 9.70 and 11.47 GHz with insertion loss of 1.1, 2.3 and 2.4 dB, respectively, while biasing the diodes at maximum reverse voltages (24 V) provides resonant frequencies at 8.74, 10.03 and 11.77 GHz with insertion loss of 0.8, 1.4 and 1.7 dB, respectively. Frequency shifts of 210, 330 and 300 MHz for the first, second and third resonant frequencies were obtained, respectively. The -10 dB reflection coefficient bandwidths were 495 MHz for the first band, 436 MHz for the second band and 418 MHz for the third band. Close band response is verified obtaining frequency band ratios of 1.18 and 1.14 for the third to the second resonance, and for the second to the first resonance, respectively.INDEX TERMS Frequency selective surfaces, split ring resonators, mutual coupling. communication systems or radar applications. Therefore, AFSSs are an attractive solution to enhance flexibility, security, and efficiency of the upcoming generations of wireless communication systems [3][4][5][6][7].Recently, multiband FSSs have been developed by virtue of their versatility in multifrequency/multistandard system implementations [8][9][10][11][12][13]. Often, multiband communication systems use channels or services that are closely spaced in frequency. Nowadays multiple X-band systems are deployed in urban areas. Point-to-point and point-tomultipoint telecommunication systems operate at 10.75-11.25 GHz, weather radars occupy 9.3-9.5 GHz band, and 11.45-11.70 GHz frequency band is destined to ensure

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. Fullwave 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°.