An ultra-thin broadband active frequency selective surface absorber for ultrahigh-frequency applications

At frequencies below 2 GHz, conventional microwave absorbers are limited in application by their thickness or narrow absorption bandwidth. In this paper, we propose and fabricate an ultra-thin broadband active frequency selective surface (AFSS) absorber with a stretching transformation (ST) pattern for use in the ultrahigh-frequency (UHF) band. This absorber is loaded with resistors and varactors to produce its tunability. To expand the tunable bandwidth, we applied the ST with various coefficients x and y to the unit cell pattern. With ST coefficients of x = y = 1, the tunability and strong absorption are concisely demonstrated, based on a discussion of impedance matching. On analyzing the patterns with various ST coefficients, we found that a small x/y effectively expands the tunable bandwidth. After this analysis, we fabricated an AFSS absorber with ST coefficients of x = 0.7 and y = 1. Its measured reflectivity covered a broad band of 0.7-1.9GHz below -10 dB at bias voltages of 10-48V. The total thickness of this absorber, 7.8mm, was only similar to lambda/54 of the lower limit frequency, similar to lambda/29 of the center frequency, and similar to lambda/20 of the higher limit frequency. Our measurements and simulated results indicate that this AFSS absorber can be thin and achieve a broad bandwidth simultaneously. (C) 2015 AIP Publishing LLC.