materials Article Design of a Tunable Absorber Based on Active Frequency-Selective Surface for UHF Applications Kainan Qi 1,2,*, Liangsheng Li 2, Jianxun Su 1, Yongqiang Liu 2 and Junwen Chen 1 1 College of Information Engineering, Communication University of China, Beijing 100854, China; [email protected] (J.S.); [email protected] (J.C.) 2 Science and Technology on Electromagnetic Scattering Laboratory, Beijing 100854, China; [email protected] (L.L.); [email protected] (Y.L.) * Correspondence: [email protected] Received: 11 October 2019; Accepted: 26 November 2019; Published: 2 December 2019 Abstract: An ultrathin tunable absorber for the ultrahigh frequency (UHF) band is presented in this paper. The absorber is a single-layer structure based on the topology of a Salisbury screen, in which the conventional resistive layer is replaced by an active frequency-selective surface (AFSS) loaded with resistors and varactors. The reflectivity response of the absorber can be controlled by adjusting the reverse bias voltage for the varactors, which is verified by both simulated and measured results. The experimental results show that the reflectivity response of the absorber can be modulated below 10 dB over a frequency band ranging from 415 to 822 MHz. The total thickness of the absorber, 10 mm, − is equivalent to only λ/72 of the lower limit frequency. The absorbing mechanism for the designed absorber is illustrated by simulating the volume loss density distributions. A detailed analysis is also carried out on the basis of these parameters, such as the AFSS shape, resistor, thickness of the foam, thickness and permittivity of the dielectric substrate, and incident angles, which contribute to the reflectivity of the AFSS absorber. Keywords: tunable absorber; active frequency-selective surface; ultrahigh frequency; varactors 1. Introduction Stealth technology is one of the most important military technologies and is of concern to all nations. Radar-absorbing materials (RAM) can effectively reduce the radar cross section (RCS) [1] of aircrafts and are commonly used in stealth missions. Traditional absorbers [2–4] are well used at high frequencies above 2 GHz. However, low-frequency absorbers are also in great demand. As radar detection equipment extends to the near-meter wavelength regime, high-performance absorbers are required at lower frequencies, especially in the ultrahigh frequency (UHF) band. In addition, low-frequency absorbing materials can also be used for electromagnetic compatibility (EMC) [5], radio frequency identification (RFID) [6], and sub-GHz wireless systems [7]. Metamaterial absorbers (MMA) have attracted much attention in recent years [8–14], after a perfect MMA with near unity absorption in microwave regime was first reported by Landy et al. [15]. MMA are also used for low frequency applications [16–19]; for example, Khuyen et al. [20] proposed an ultrathin polarization-insensitive metamaterial absorber, which exhibits a peak absorption of 97% at 250 MHz. Zuo et al. [21] presented a wideband metamaterial absorber using a metallic incurved structure, which has an absorptivity of more than 90% at 0.8–2.7 GHz. Rozanov [22] discussed the problems of the ultimate thickness-to-bandwidth ratio of a radar absorber, observing that passive absorbers are usually very thick or have a narrow absorption bandwidth below 2 GHz. This problem could be solved by using a tunable absorber, the electromagnetic characteristics of which can be dynamically modulated. There are several methods to make a tunable Materials 2019, 12, 3989; doi:10.3390/ma12233989 www.mdpi.com/journal/materials Materials 2019, 12, x FOR PEER REVIEW 11 of 13 Materials 2019, 12, 3989 2 of 12 superconductors [25], conducting polymers [26], an active frequency-selective surface (AFSS) [27], andabsorber, so on including. In terms thoseof cost based and response on the use time, of graphene AFSS was [23 adopted], liquid in crystals this study [24], superconductorsto design the tunable [25], absorberconducting. The polymers tunable [ 26absorber], an active based frequency-selective on AFSS [28–32] surfacehas an ultrat (AFSS)hin [27 layered], and so structure, on. 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The adjacent tunability resonant ranges units, from and 415 theto 8 absorbed22 MHz, withfrequency a reflectivity can be tuned below continuously −10 dB and bya thickness adjusting of the 10 bias mm voltage that corresponds of varactors. to Theonly experimental λ/72 of the resonanceresults validate frequency. the tunability This work of theis important designed absorber.for stealth The and tunability other microwave ranges from applications 415 to 822 in MHz, the withfuture. a reflectivity below 10 dB and a thickness of 10 mm that corresponds to only λ/72 of the resonance − frequency. This work is important for stealth and other microwave applications in the future. 2. Design, Simulations, and Experiments 2. Design, Simulations, and Experiments 2.1. Structure of Proposed Absorber 2.1. Structure of Proposed Absorber Figure 1 shows the structure of the proposed AFSS absorber. The top layer is a FR4 dielectric substrate,Figure which1 shows has thea relative structure permittivity of the proposed of 4.4(1 AFSS-j0.02) absorber. and a thickness The top of layer 1 mm. is The a FR4 next dielectric layer is thesubstrate, AFSS, whichwhich has is aloaded relative with permittivity resistors of and 4.4(1-j0.02) varactors and and a thickness printed ofonto 1 mm. the TheFR4 next substrate; layer is the AFSS,thickness which of the is loaded copper with is 0.018 resistors mm. and The varactors third layer, and use printedd as an onto independent the FR4 substrate; layer, is the a thickness9 mm-thick of the copper is 0.018 mm. The third layer, used as an independent layer, is a 9 mm-thick foam with very foam with very low dielectric loss ( ε r = 1.05, tanδ = 0.002). The bottom layer is a metal ground. low dielectric loss (" = 1.05, tan δ = 0.002). The bottom layer is a metal ground. Figure1b shows the Figure 1b shows the rgeometric structure of the unit cell, which is based on a dipole. It is composed of geometric structure of the unit cell, which is based on a dipole. It is composed of two bias lines along two bias lines along the x-axis and two strips with six circles along the y-axis. The final geometric the x-axis and two strips with six circles along the y-axis. The final geometric dimensions are given as: dimensions are given as: x = y = 50 mm, h = 3 mm, r = 3 mm, g = 2 mm, w = 1 mm, b = 1 mm, and d = x = y = 50 mm, h = 3 mm, r = 3 mm, g = 2 mm, w = 1 mm, b = 1 mm, and d = 9 mm. A varactor and a 9 mm. A varactor and a resistor are loaded onto the gap between the two strips. resistor are loaded onto the gap between the two strips. h w r FR4 y E resistor g b varactor AFSS foam d conducting plate x (a) (b) Figure 1. Cont. Materials 2019, 12, 3989 3 of 12 Materials 2019, 12, x FOR PEER REVIEW 11 of 13 Materials 2019, 12, x FOR PEER REVIEW 11 of 13 L L C0 C 0 R Cr R Cr (c) (c) Figure 1.1. TheThe designed designed absorber: absorber (a:) ( three-dimensionala) three-dimensional (3D) ( structure,3D) structure, (b) an ( activeb) anfrequency-selective active frequency- Figure 1. The designed absorber: (a) three-dimensional (3D) structure, (b) an active frequency- surfaceselective (AFSS) surface unit (AFSS cell,) andunit (cell,c) the and equivalent (c) the equivalent circuit model circuit of themodel AFSS. of the AFSS. selective surface (AFSS) unit cell, and (c) the equivalent circuit model of the AFSS. 2.2. E Equivalentquivalent C Circuitircuit ModelModel of the Proposed Absorber 2.2. Equivalent Circuit Model of the Proposed Absorber The equivalent circuit modelmodel forfor thethe AFSSAFSS layerlayer isis shown inin FigureFigure1 1c.c. AA varactorvaractor withwith aa variablevariable capacitanceThe equivalent C is connected circuit in model parallel for to the a resistorAFSS layer R.