Independently Tunable Closely Spaced Triband Frequency Selective Surface Unit Cell Using the Third Resonant Mode of Split Ring Slots

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.