materials

Article Design of a Tunable Absorber Based on Active -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 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 of the 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 above 2 GHz. However, low-frequency absorbers are also in great demand. As radar detection equipment extends to the near-meter 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], identification (RFID) [6], and sub-GHz systems [7]. absorbers (MMA) have attracted much attention in recent years [8–14], after a perfect MMA with near unity absorption in 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 -insensitive , 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. In terms which of costcan beand used response to simultaneously time, AFSS was control adopted its inreflection this study response to design tothe obtain tunable a wide absorber. absorbing The tunablebandwidth. absorber For instance,based on AFSSMias et [28 al.–32 [33] has] designed an ultrathin a tunable layered microwave structure, absorber which can based be used on a to high simultaneously-impedance surface control andits reflection presented response data showing to obtain that a widethe reflectivity absorbing response bandwidth. of the For absorber instance, can Mias be et controlled al. [33] designed over the a frequencytunable microwave band from absorber 1.72 to based 1.93 GHz. on a high-impedance Zhao et al. [34] surfacedesigned and a presentedtunable metamaterial data showing absorber that the usingreflectivity varactor response diodes, of thewhich absorber had a cantunable be controlled bandwidth over of the1.5 frequencyGHz and bandan absorption from 1.72 rate to 1.93 of more GHz. thanZhao et90% al. [when34] designed the bias a tunable voltage metamaterial changed from absorber 0 to using −19 varactorV. However, diodes, these which works had a tunablecannot simultaneouslybandwidth of 1.5 guarantee GHz and both an absorption tunable absorption rate of more bandwidth than 90% and when material the bias thickness. voltage changed from 0 to 19In V.this However, study, we these design worksed cannota thin absorber simultaneously with a guaranteesufficiently both large tunable tunable absorption bandwidth. bandwidth In this − design,and material the resistors thickness. and varactors are embedded between adjacent resonant units, and the absorbed frequencyIn this can study, be tuned we designed continuously a thin absorberby adjusting with the a subiasfficiently voltage large of varactors. tunable bandwidth. The experimental In this resultsdesign, validate the resistors the tunability and varactors of the are designed embedded absorber between. 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 − 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 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 ground. low dielectric loss (ε = 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 . 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.

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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. L is and shown C are in Figur the distributede 1c. A varactor parameters with a generatedvariable r r 0 0 capacitancecapacitance C Cisr isconnected connected in in parallel parallel toto aa resistorresistor R.R. L and CC0 areare thethe distributeddistributed parameters parameters by the topological structure of the unit cell pattern. C is relative to the gap between the two bias lines 0 0 generatedgenerated by by the the topological topological structure structure of of the the unitunit cellcell pattern. C0 isis relativerelative to to the the gap gap between between the the and can be ignored. The whole impedance of the AFSS is given by twotwo bias bias lines lines and and can can be be ignored ignored. The. The whole whole impedance impedance of the AFSSAFSS isis given given by by 11R = 111 ≈+ωω1 =+R R ZZAFSSZAFSS == ≈+jLjjLωωωL + =+jLjL= jωL + . (1) AFSS 11111 1 1++ ωjω RC ωωωω+ ≈ + 1 j RC1 +r r jωRCr jCjCj0ω++0L++1 jCjCjωrCr r R jωL+ 11 1 R (1)( 1) jLωjLω ++ jωCr+ 1R1 . . jCωjCω ++ r r R The resonant frequency of the AFSS isR determined by TheThe resonant resonant frequency frequency of of the the AFSS AFSS is is de determineterminedd by 1 =fr = π . . (2) ffrr=1/1/ (2 (2π LC LC ) ). (2)( 2) rr2π √LCr TheThe AFSS AFSS absorber absorber wa was ssimulated simulated by by MATLAB MATLAB basedbased on an equivalentequivalent circuit circuit method method (ECM). (ECM). The AFSS absorber was simulated by MATLAB based on an equivalent circuit method (ECM). A planeA plane wave normally normally occurred occurred on on the the absorber absorber,, withwith thethe electric fieldfield polarized polarized along along the the y -yaxis-axis, as, as A planeshown wave in Figure normally 1b. R occurredis the resistance on the of absorber, the lumped with resistor. the electric Cr is fieldthe polarized of along the varactor, the y-axis, shown in Figure 1b. R is the resistance of the lumped resistor. Cr is the capacitance of the varactor, as shownwhich increases in Figure as1b. the R reverse is the resistance bias voltage of thedecreases. lumped Figure resistor. 2a showsC is the simulated capacitance reflectivity of the varactor, with which increases as the reverse bias voltage decreases. Figure 2a showsr the simulated reflectivity with whicha varying increases Cr and as thefixed reverse R = 2400 bias Ohms. voltage In this decreases. figure, the Figure resonance2a shows frequency the simulated decreases reflectivity as Cr increases, with a a varying Cr and fixed R = 2400 Ohms. In this figure, the resonance frequency decreases as Cr increases, varyingand theCr reflectivityand fixed R nadir= 2400 is controlled Ohms. In thisover figure,a wide thefrequency resonance band frequency from 410 decreases to 835 MHz as Cwhenr increases, Cr is and the reflectivity nadir is controlled over a wide frequency band from 410 to 835 MHz when Cr is andvaried the reflectivity from 1 to 5 nadir pF. is controlled over a wide frequency band from 410 to 835 MHz when C is varied from 1 to 5 pF. r varied from 1 to 5 pF.

(a) (b)

FigureFigure 2. The 2. The simulated simulated(a) results results for for varying varyingC rC:(r: a()a) by by the the equivalent equivalent circuitcircuit method(b) ( (ECM)ECM) or or (b (b) )by by a high frequency structure simulatora high (HFSS).frequency structure simulator (HFSS). Figure 2. The simulated results for varying Cr: (a) by the equivalent circuit method (ECM) or (b) by a high frequency structure simulator (HFSS).

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Materials 2019, 12, x FOR PEER REVIEW 11 of 13 The AFSS absorber was numerically simulated by a high-frequency structure simulator (HFSS) in orderThe to AFSS verify absorber the equivalent was numerically circuit model. simulated In Figureby a high2b,- thefrequency minimum structure value simulator of the reflectivity (HFSS) isin modulated order to verify over the a wideequivalent frequency circuit band model. from In Figure 406 to 2b, 824 the MHz minimum when value Cr varies of the reflectivity from 1 to 5is pF. Themodulated numerical over simulation a wide frequency suggests band that thefrom numerical 406 to 824 solution MHz when approaches Cr varies the from theoretical 1 to 5 pF. solution The verynumerical well. simulation suggests that the numerical solution approaches the theoretical solution very well. 2.3. Experiment and Results 2.3. Experiment and Results A sample of the proposed absorber was fabricated based on the design shown in Figure1. The AFSS patternA was sample constructed of the proposed on a FR4-printed absorber wa circuits fabricated board by based standard on the photoetching design shown techniques in Figure 1. and The was loadedAFSS withpattern resistors was constructed and varactors on a (NXP, FR4-printed BB131) bycircuit manual board welding by standard techniques. photoetching A photograph techniques of the AFSSand prototypewas loaded is shownwith resistors in Figure and3a, andvaractors Figure (NXP,3b shows BB131) the by topological manual welding structure techniques. of the unit cell.A Thephotograph AFSS board of measuresthe AFSS 500 prototype by 500 mm is shown and contains in Figure 100 3 (10a, and10) Figure dipole 3 elements.b shows the The topological FR4 substrate structure of the unit cell. The AFSS board measures 500 by 500× mm and contains 100 (10 × 10) dipole has a relative permittivity of 4.4 (1-j0.02) and a thickness of 1 mm. The AFSS prototype was pasted elements. The FR4 substrate has a relative permittivity of 4.4 (1-j0.02) and a thickness of 1 mm. The with a foam board with a thickness of 9 mm, so the total thickness of the absorber sample was 10 mm. AFSS prototype was pasted with a foam board with a thickness of 9 mm, so the total thickness of the Figure3c shows a schematic diagram of the circuit connection in the AFSS array. An element was absorber sample was 10 mm. Figure 3c shows a schematic diagram of the circuit connection in the divided into two parts by devices. The adjacent two subunits were connected back to back, together AFSS array. An element was divided into two parts by devices. The adjacent two subunits were vertically,connected and back then, to back a row, together of units vertical was connectedly, and then together, a row of by units a bias was line. connected Finally, together the red by array a bias was connectedline. Finally, to the the anode red array by the wa mains connected feeder to 1, andthe anode the blue by arraythe main was feeder connected 1, and to the the blue cathode array through was theconnected main feeder to the 2. cathode The adjacent through two the main rows feeder of varactors 2. The adjacent were welded two rows with of varactors the opposite were polarity welded to ensurewith the that opposite the same polarity polarity to was ensure connected that the together. same polarity As a result,was connected all the devices together. were As ina res parallelult, all and biasedthe devices at the samewere in voltage. parallel and biased at the same voltage.

(a) (b) (c)

FigureFigure 3. 3.Photograph Photograph of of thethe fabricatedfabricated AFSS prototype: prototype: (a (a) )the the whole whole AFSS AFSS array, array, (b ()b the) the unit unit cell, cell, and and (c)(c a) schematica schematic diagram diagram of of the the bias bias circuit.circuit.

TheThe reflectivity reflectivity of of the the proposed proposed absorberabsorber over the the frequency frequency range range of of 300 300–1000–1000 MHz MHz for for various various biasbias voltages was was measured measured in the in anechoic the anechoic chamber chamber of the Science of the and Science Technology and onTechnology Electromagnetic on ScatteringElectromagnetic Laboratory. Scattering The measurement Laboratory. setupThe measurement for the compact setup range for system the compact is shown range in Figure system4. Twois identicalshown hornin Figure antennas 4. Two were identical utilized horn as antennas transmitting were and utilized receiving as transmitting devices, respectively. and receiving The devices, spherical wavesrespectively. emitted The by thespherical horn antennawaves emitted were reflected by the horn by aantenna parabolic were metal reflected reflector by abefore parabolic becoming metal a reflector before becoming a plane wave. A short test distance between the sample and reflector was plane wave. A short test distance between the sample and reflector was easily realized to meet the easily realized to meet the far-field conditions. According to the datasheet of BB131, the junction far-field conditions. According to the datasheet of BB131, the junction capacitance Cr changed from 1 capacitance Cr changed from 1 to 5 pF when the reverse bias voltage (Ur) varied from −5 to −30 V. The to 5 pF when the reverse bias voltage (Ur) varied from 5 to 30 V. The measured results are presented measured results are presented in Figure 5a. When the− bias −voltage was 0 V, the AFSS absorber had in Figure5a. When the bias voltage was 0 V, the AFSS absorber had no absorption. When the inverse no absorption. When the inverse bias voltage varied from −5 to −30 V, the resonant frequency moved bias voltage varied from 5 to 30 V, the resonant frequency moved to a higher frequency and covered to a higher frequency and− cover− ed a frequency band ranging from 415 to 822 MHz below −10 dB. The a frequency band ranging from 415 to 822 MHz below 10 dB. The total thickness was equivalent to total thickness was equivalent to only λ/72 of the lower− limit frequency and λ/36 of the higher limit onlyfrequency.λ/72 of theThe lowermeasured limit results frequency for various and λ/ 36reverse of the voltages higher limit(Ur) ranging frequency. from The −5 measuredto −30 V agree results for various reverse voltages (Ur) ranging from 5 to 30 V agree roughly with the simulated results roughly with the simulated results for a varying− capacitance− (Cr) from 1 to 5 pF. Both the measured

Materials 2019, 12, x FOR PEER REVIEW 11 of 13 and simulated results validate the tunability of the proposed absorber, providing a feasible method Materials 2019, 12, x FOR PEER REVIEW 11 of 13 for absorption in UHF applications. Figure 5b shows the simulated and measured absorption performance of the proposed absorber. and simulated results validate the tunability of the proposed absorber, providing a feasible method According to the datasheet of BB131, the capacitance of the varactor is approximately 1 pF when it is for absorption in UHF applications. biased at −30 V. In Figure 5b, the measured reflectivity is −14.1 dB at 822 MHz, and the reflectivity Figure 5b shows the simulated and measured absorption performance of the proposed absorber. simulated by the HFSS is −43.2 dB at 824 MHz. It is observed that the resonant frequencies of these According to the datasheet of BB131, the capacitance of the varactor is approximately 1 pF when it is two curves agree with each other, but the peak amplitude and bandwidth of the reflectivity are biased at −30 V. In Figure 5b, the measured reflectivity is −14.1 dB at 822 MHz, and the reflectivity different. This difference occurs, because the size of the experimental sample is limited while being simulated by the HFSS is −43.2 dB at 824 MHz. It is observed that the resonant frequencies of these set as an infinite periodic array in simulation. The measurement tolerance exists because of the edge two curves agree with each other, but the peak amplitude and bandwidth of the reflectivity are diffraction of the experimental sample and the fabrication tolerance. Moreover, the calculation different.Materials 2019 This, 12, 3989difference occurs, because the size of the experimental sample is limited while being5 of 12 tolerance exists because of the model complexity of the varactors when simulated by the HFSS. set as an infinite periodic array in simulation. The measurement tolerance exists because of the edge diffraction of the experimental sample and the fabrication tolerance. Moreover, the calculation for a varying capacitance (Cr) from 1 to 5 pF. Both the measured and simulated results validate the tolerancetunability exists of the because proposed of absorber,the model providing complexity a feasibleof the varactors method forwhen absorption simulated in by UHF the applications. HFSS. Device Under Test Reflector

Plane wave Device Under Test Reflector

Foam Plane wave

Foam Transmit and receive Platform antennas

Transmit and receive Platform antennas

Network analyzer Computer Network analyzer Figure 4. Schematic view of the compact range system for the reflectivity measurement. Computer Figure 4. Schematic view of the compact range system for the reflectivity measurement. Figure 4. Schematic view of the compact range system for the reflectivity measurement.

(a) (b)

FigureFigure 5. 5. (a(a) )Measured Measured results results for for the the absorber absorber with with various various voltages; voltages; (b (b) )the the simulated simulated and and measured measured absorptionabsorption performance performance.(a) . (b)

Figure 5. (a) Measured results for the absorber with various voltages; (b) the simulated and measured 3. DiscussionFigure5b and shows Analysis the simulated and measured absorption performance of the proposed absorber. Accordingabsorption to the performance datasheet. of BB131, the capacitance of the varactor is approximately 1 pF when it is The and volume loss density distributions were simulated by using the HFSS at a biased at 30 V. In Figure5b, the measured reflectivity is 14.1 dB at 822 MHz, and the reflectivity resonant3. Discussion frequency− and Analysis of 824 MHz with Cr =1 pF to illustrate the− absorbing mechanism of the proposed simulated by the HFSS is 43.2 dB at 824 MHz. It is observed that the resonant frequencies of these two absorber. The field distributions− are shown in Figure 6. It is observed that there is a high concentration curvesThe agree electric with field each and other, volume but the loss peak density amplitude distributions and bandwidth were simu oflated the reflectivityby using the are HFSS different. at a resonantThis difference frequency occurs, of 824 because MHz thewith size Cr =1 of pF the to experimental illustrate the sample absorbing is limited mechanism while of being the proposed set as an absinfiniteorber periodic. The field array distributions in simulation. are shown The measurement in Figure 6. It tolerance is observed exists that because there is of a thehigh edge concentration diffraction of the experimental sample and the fabrication tolerance. Moreover, the calculation tolerance exists because of the model complexity of the varactors when simulated by the HFSS.

3. Discussion and Analysis The electric field and volume loss density distributions were simulated by using the HFSS at a resonant frequency of 824 MHz with Cr =1 pF to illustrate the absorbing mechanism of the proposed absorber. The field distributions are shown in Figure6. It is observed that there is a high concentration Materials 2019, 12, 3989 6 of 12 Materials 2019, 12, x FOR PEER REVIEW 11 of 13 ofof electric electric field field around around the the gap gap where where the the resistor resistor and and varactor varactor are are loaded. loaded. According According to to the the volume volume loss densityloss density distribution, distribution, the absorber the absorber experiences experiences both dielectric both dielectric and resistive and resistive loss, which loss, both which contribute both tocontribute the total energyto the total loss. energy The loss loss. on The the loss AFSS on the is mainlyAFSS is caused mainly bycaused absorption by absorption from the from resistive the elements.resistive elements. Resonance Resonance occurs at occurs 824 MHz, at 824 where MHz, where the equivalent the equivalent circuit circuit yields yields a strong a strong electric electric field. Whenfield. R When= 2400 R Ohms= 2400, Ohms, the input the impedanceinput impedance of the absorberof the absorber matches matches perfectly perfectly with freewith space, free space and, the incidentand the wave incident enters wave inside enters of the inside designed of the structure, designed generating structure, multiple generating reflections. multiple Therefore, reflections. most ofTherefore, the energy most losses of arise the energy from absorption losses arise of thefrom dielectric absorption substrate, of the whiledielectric the substrate, paralleled while capacitance the paralleled capacitance Cr Cr alters the AFSS layer’s impedancealters the AFSS and, layer as a’s result, impedance the resonant and, as a frequency. result, the resonant Hence, the frequency. resonance frequencyHence, the of resonance the absorber frequency can be adjusted.of the absorber can be adjusted.

(a) (b)

FigureFigure 6. 6.The The simulated simulated resultsresults obtainedobtained by the the HFSS: HFSS: (a (a) )the the electric electric field field distribution distribution and and (b) ( bthe) the volumevolume loss loss density density distribution. distribution.

TheThe reflectivity reflectivity is is a a function function ofof certaincertain parameters such such as as the the AFSS AFSS shape, shape, resistor, resistor, thickness thickness of of thethe foam, foam, thickness thickness and and permittivity permittivity ofof thethe dielectric substrate substrate,, a andnd the the incident incident angles. angles. To Tostudy study the the influenceinfluence of of these these parameters parameters onon thethe reflectivity,reflectivity, the HFSS HFSS was was used used to to carry carry out out simulation simulation analysis analysis forfor each each parameter. parameter.

3.1.3.1. Analysis Analysis of of the the Designed DesignedStructure Structure withwith DiDifferentfferent AFSS Shapes Shapes AccordingAccording to to the the waveguide handbookhandbook [[35],35], L in in Equation Equation (2 (2)) is is mainly mainly determined determined by by the the shape shape ofof the the AFSS AFSS element. element. This This cancan bebe equivalentequivalent to the the reactance reactance component component and and eventually eventually affects affects the the resonanceresonance frequency frequency of of the the designed designed structure.structure. Figure Figure 7 showsshows three three AFSS AFSS cells cells with with different di fferent shapes, shapes, whose parameters are consistent with Figure 1a. According to the equivalent circuit formula, the whose parameters are consistent with Figure1a. According to the equivalent circuit formula, the L of shape 1 is largest, the L for shape 2 is smallest, and the L for shape 3 lies in the middle. inductance L of shape 1 is largest, the L for shape 2 is smallest, and the L for shape 3 lies in the middle. Figure 8 shows the simulated reflectivity for these three AFSS absorbers with R = 2400 Ohms and Cr Figure8 shows the simulated reflectivity for these three AFSS absorbers with R = 2400 Ohms and = 1 pF. The reflectivity nadir of the designed structure is −42.7 dB at 794 MHz for shape 1, −55.9 dB at Cr = 1 pF. The reflectivity nadir of the designed structure is 42.7 dB at 794 MHz for shape 1, 55.9 dB 902 MHz for shape 2, and −43.2 dB at 824 MHz for shape 3.− The relationship between the resonant− at 902 MHz for shape 2, and 43.2 dB at 824 MHz for shape 3. The relationship between the resonant frequencies for the three shapes− is fr1 < fr3 < fr2. frequenciesIt is easy for theto observe three shapes that the is resonantfr1 < fr3 < frequencyfr2. of the absorber decreases when the equivalent parameterIt is easy L toincreases, observe which that theis consistent resonant with frequency Equation of the(2). O absorbern the oth decreaseser hand, the when value the of equivalentL affects parameterthe instantaneous L increases, bandwidth which is of consistent the absorber with, Equationin that the (2). larger On thethe otherL, the hand, smaller the the value bandwidth. of L affects theTherefore, instantaneous it is necessary bandwidth to ensure of the that absorber, L is set into thatan appropriate the larger value the L, by the optimizing smaller thethe bandwidth.shape of Therefore,the AFSS it unit is necessary cell. to ensure that L is set to an appropriate value by optimizing the shape of the AFSS unit cell.

Materials 2019, 12, x FOR PEER REVIEW 11 of 13 Materials 2019, 12, 3989 7 of 12 Materials 2019, 12, x FOR PEER REVIEW 11 of 13

(a) (b) (c)

Figure 7. Different shapes for the AFSS unit cell: (a) shape 1, (b) shape 2, and (c) shape 3. (a) (b) (c)

Figure 7. DiffeDifferentrent shapes for the AFSS unit cell: ( a) shape 1 1,, ( b) shape 2, and (c) shape 3.3.

Figure 8. The simulated results for the designed structure with different AFSS shapes. Figure 8. The simulated results for the designed structure with different AFSS shapes. 3.2. Analysis of the Designed Structure with Different Resistances

3.2. AnalysisFigure9Figure ofshows the 8. Design The the simulated simulateded Structure results reflectivity with for Different the designed of the Resistances proposed structure with absorber different with AFSS various shapes R. and fixed Cr = 1 pF. The reflectivity nadir of the designed structure is 8.5 dB at 827 MHz for R = 1000 Ohms, MaterialsFigure 2019, 129 ,shows x FOR PEER the simulated REVIEW reflectivity of the proposed− absorber with various R and fixed11 of 13Cr 3.2.31.5 Analysis dB at 824 of the MHz Design for Red= Structure2000 Ohms, with Different43.2 dB at Resistances 824 MHz for R = 2400 Ohms, 18.8 dB at 823 MHz −= 1 pF. The reflectivity nadir of the designed− structure is −8.5 dB at 827 MHz for R− = 1000 Ohms, −31.5 for R = 3000 Ohms, and 12.1 dB at 825 MHz for R = 4000 Ohms. dB atFigure 824 MHz 9 shows for R the = 2000 −simulated Ohms, reflectivity−43.2 dB at of824 the MHz proposed for R = absorber 2400 Ohms, with − 18.8various dB atR 823and MHzfixed forCr =R 1 = pF. 3000 The Ohms, reflectivity and −12.1 nadir dB of at the 825 designed MHz for structure R = 4000 isOhms. −8.5 dB at 827 MHz for R = 1000 Ohms, −31.5 dB at The824 simulatedMHz for R results = 2000 indicateOhms, − that43.2 RdB is at of 824 great MHz importance for R = 2400 for Ohms,the absorption −18.8 dB rate at 823; however, MHz for R Rhas = 3000 little Ohms, effect andon the −12.1 resonant dB at 825 frequency. MHz for The R = r4000eflectivity Ohms. nadir first decreases and then increases whenThe R increasessimulated from results 1000 indicate to 4000 that Ohms, R is andof great the absorptionimportance peak for the reaches absorption its extreme rate; however,point when R hasR = 2400little Ohms.effect on Therefore, the resonant the simulation frequency. analysis The reflectivity for the other nadir parameters first decreases in Section and 3.3then–3.6 increases is based whenon the R premise increases that from R = 10002400 toOhms 4000 and Ohms, Cr = and 1 pF the. absorption peak reaches its extreme point when R = 2400 Ohms. Therefore, the simulation analysis for the other parameters in Section 3.3–3.6 is based on the premise that R = 2400 Ohms and Cr = 1 pF.

Figure 9. The simulated results for the designed structure with different resistance (R) values. Figure 9. The simulated results for the designed structure with different resistance (R) values.

3.3. Analysis of the Designed Structure with Foam Materials of Different Thicknesses

Foam materials with different thicknesses were considered in the design structure. To observe the reflectivity, four different thicknesses—6, 9, 12, and 15 mm—were utilized for the proposed design. As observed in Figure 10, the reflectivity of the structure is −13.0 dB at 897 MHz for a 6 mm- thick foam material. Similarly, the value of the reflectivity is −43.2 dB at 824 MHz, corresponding to a 9 mm thickness. In addition, the reflectivity value is −16.3 dB at 772 MHz for a 12 mm-thick foam material. Finally, the reflectivity is −12.7 dB at 723 MHz for a foam thickness of 15 mm. Due to the different thicknesses of the foam material, the reflectivity was different at various resonant frequencies. The higher the thickness of the substrate materials, the lower the resonant frequency. Further, when the thickness is 9 mm, the reflectivity of the proposed absorber is the lowest, achieving a perfect impedance matching effect.

Figure 10. The simulated results for the designed structure with foam materials of different thickness.

3.4. Analysis of the Designed Structure with Different Thickness FR4 Substrate Materials

Materials 2019, 12, x FOR PEER REVIEW 11 of 13

Materials 2019, 12, 3989 8 of 12 Figure 9. The simulated results for the designed structure with different resistance (R) values.

3.3. AnalysisThe simulated of the Design resultsed Structure indicate thatwith RFoam is of Materials great importance of Different for Thicknesses the absorption rate; however, R has little effect on the resonant frequency. The reflectivity nadir first decreases and then increases Foam materials with different thicknesses were considered in the design structure. To observe when R increases from 1000 to 4000 Ohms, and the absorption peak reaches its extreme point when the reflectivity, four different thicknesses—6, 9, 12, and 15 mm—were utilized for the proposed R = 2400 Ohms. Therefore, the simulation analysis for the other parameters in Sections 3.3–3.6 is based design. As observed in Figure 10, the reflectivity of the structure is −13.0 dB at 897 MHz for a 6 mm- on the premise that R = 2400 Ohms and C = 1 pF. thick foam material. Similarly, the value ofr the reflectivity is −43.2 dB at 824 MHz, corresponding to 3.3.a 9 mm Analysis thickness. of the DesignedIn addition, Structure the reflectivity with Foam value Materials is − of16. Di3 ffdBerent at 772 Thicknesses MHz for a 12 mm-thick foam material. Finally, the reflectivity is −12.7 dB at 723 MHz for a foam thickness of 15 mm. Due to the differentFoam thicknesses materials with of dithefferent foam thicknesses material, were the consideredreflectivity in was the designdifferent structure. at various To observe resonant the reflectivity,frequencies. four different thicknesses—6, 9, 12, and 15 mm—were utilized for the proposed design. As observed in Figure 10, the reflectivity of the structure is 13.0 dB at 897 MHz for a 6 mm-thick The higher the thickness of the substrate materials, the −lower the resonant frequency. Further, foam material. Similarly, the value of the reflectivity is 43.2 dB at 824 MHz, corresponding to a 9 mm when the thickness is 9 mm, the reflectivity of the proposed− absorber is the lowest, achieving a perfect thickness. In addition, the reflectivity value is 16.3 dB at 772 MHz for a 12 mm-thick foam material. impedance matching effect. − Finally, the reflectivity is 12.7 dB at 723 MHz for a foam thickness of 15 mm. Due to the different − thicknesses of the foam material, the reflectivity was different at various resonant frequencies.

Figure 10. The simulated results for the designed structure with foam materials of different thickness. Figure 10. The simulated results for the designed structure with foam materials of different The higher the thickness of the substrate materials, the lower the resonant frequency. Further, thickness. when the thickness is 9 mm, the reflectivity of the proposed absorber is the lowest, achieving a perfect impedance3.4. Analysis matching of the Design effect.ed Structure with Different Thickness FR4 Substrate Materials

3.4. Analysis of the Designed Structure with Different Thickness FR4 Substrate Materials

Different thicknesses for the substrate materials were considered in the design structure. To observe the reflectivity, four different thicknesses—0.5, 1.0, 1.5, and 2.0 mm—were utilized for the proposed design. It can be observed in Figure 11 that the reflectivity of the structure is 29.6 dB at 846 MHz − for a FR4 substrate material with a thickness of 0.5 mm. Similarly, the value of the reflectivity is 43.2 dB at 824 MHz for a 1.0 mm thickness. In addition, the reflectivity value is 42.1 dB at 817 MHz − − for a 1.5 mm-thick substrate material. Finally, the reflectivity is 43.3 dB at 809 MHz for a thickness − of 2.0 mm. Due to the different thicknesses of the substrate material, the reflectivity was different at different resonant frequencies. The higher the thickness of the substrate materials, the lower the resonant frequency. Materials 2019, 12, x FOR PEER REVIEW 11 of 13

Different thicknesses for the substrate materials were considered in the design structure. To observe the reflectivity, four different thicknesses—0.5, 1.0, 1.5, and 2.0 mm—were utilized for the proposed design. It can be observed in Figure 11 that the reflectivity of the structure is −29.6 dB at 846 MHz for a FR4 substrate material with a thickness of 0.5 mm. Similarly, the value of the reflectivity is −43.2 dB at 824 MHz for a 1.0 mm thickness. In addition, the reflectivity value is −42.1 dB at 817 MHz for a 1.5 mm-thick substrate material. Finally, the reflectivity is −43.3 dB at 809 MHz for a thickness of 2.0 mm. Due to the different thicknesses of the substrate material, the reflectivity was different at different resonant frequencies. The higher the thickness of the substrate materials, the lower the resonant frequency. Materials 2019, 12, 3989 9 of 12

Figure 11. The simulated results for the designed structure with substrate materials of diFigurefferent 11. thicknesses. The simulated results for the designed structure with substrate materials of different thicknesses. 3.5. Analysis of the Designed Structure for Substrate Materials with Varying Permittivity 3.5. AnalysisVarying of permittivity the Designedfor Structure the substrate for Substrate materials Materials has with an eVaryingffect on Permittivity the electromagnetic wave absorption. Different dielectric constants (2.2, 3.3, 4.4, and 5.5) with the same dielectric loss tangent Varying permittivity for the substrate materials has an effect on the electromagnetic wave (0.02) were analyzed in the design structure. It can be observed in Figure 12 that the reflectivity of the absorption. Different dielectric constants (2.2, 3.3, 4.4, and 5.5) with the same dielectric loss tangent structure is 39.9 dB at 850 MHz, respectively, for a dielectric constant of 2.2. Likewise, the value of the (0.02) were− analyzed in the design structure. It can be observed in Figure 12 that the reflectivity of the reflectivity is 51.9 dB at 840 MHz for a dielectric constant of 3.3. In addition, the reflectivity value is Materialsstructure 2019 is, 12−−39., x 9FOR dB PEER at 850 REVIEW MHz , respectively, for a dielectric constant of 2.2. Likewise, the value11 of of13 43.2 dB at 824 MHz for a dielectric constant of 4.4, as well as 35.8 dB at 813 MHz for a dielectric −the reflectivity is −51.9 dB at 840 MHz for a dielectric constant of− 3.3. In addition, the reflectivity value constant of 5.5 for the substrate materials. is −43.2 dB at 824 MHz for a dielectric constant of 4.4, as well as −35.8 dB at 813 MHz for a dielectric constant of 5.5 for the substrate materials. Due to the varying permittivity of substrate material, the reflectivity was found to be different at different resonant frequencies. A minimum peak was achieved for substrate materials with a dielectric constant of 3.3. However, a maximum peak was achieved for substrate materials with a dielectric constant of 5.5. An obvious variation in the resonant frequency occurs: the higher the permittivity of the substrate material, the lower the resonant frequency.

Figure 12. The simulated results for the designed structure with substrate materials of varyingFigure permittivity.12. The simulated results for the designed structure with substrate materials of varying permittivity. Due to the varying permittivity of substrate material, the reflectivity was found to be different at di3.6.fferent Analysis resonant of the frequencies.Designed Structure A minimum with Different peak was Incident achieved Angles for substrate materials with a dielectric constant of 3.3. However, a maximum peak was achieved for substrate materials with a dielectric constantThe ofincidence 5.5. An obviousangle (θ variation) was varied in the from resonant 0 to 60° frequency in steps occurs:of 15° to the study higher the the reflectivity permittivity under of theoblique substrate incidence. material, the lower the resonant frequency. It is seen from Figure 13 that the minimum reflectivity values for the design structure are −43.24 3.6.dB at Analysis 824 MHz of the for Designed an incidence Structure angle with of 0°, Di ff−erent34.3 dB Incident at 829 Angles MHz for 15°, −23.3 dB at 829 MHz for 30°, −15.2 dB at 825 MHz for 45°, and −10.2 dB at 826 MHz for 60° under transverse electric (TE) The incidence angle (θ) was varied from 0 to 60◦ in steps of 15◦ to study the reflectivity under polarization. The lowest peaks for the reflectivity are −43.2 dB at 824 MHz for an incidence angle of oblique incidence. 0°, −41.0 dB at 871 MHz for an incidence angle of 15°, −28.3 dB at 918 MHz for an incidence angle of 30°, −18.6 dB at 998 MHz for an incidence angle of 45°, and −11.0 dB at 1093 MHz for an incidence angle of 60° under transverse magnetic (TM) polarization. In Figure 13, it is seen that the absorption decreases considerably when increasing the incidence angle under TE polarization, whereas the reduction is smaller for TM polarization, although the resonant frequency shifts rather considerably with θ. Under TE polarization, the electric field is always parallel to the AFSS array, so the impedance (Z) and equivalent inductance (L) of the AFSS is independent of incident angle. Thus, the resonant frequency remains stable with variation of the incident angle. Under TM polarization, there is an angle (θ) between the electric field and the AFSS array. The AFSS impedance for TM mode depends on the incident angle [36]. As the incident angle increases, the impedance decreases, and the equivalent inductance L decreases because of = . Therefore, as incident angle increases, the resonant frequency will increase for TM polarization. 𝑍𝑍 𝑗𝑗𝑗𝑗𝑗𝑗 Under TM polarization, the reflectivity is lowest at 824 MHz for an incidence angle of 0° because of the perfect matched impedance at normal incidence. The reflectivity at 824 MHz increases rapidly to the highest peak with variation of the incident angle because of the gradually mismatched impedance at oblique incidence. This problem can be solved by optimization of the element shape [37,38], which can increase the absorption bandwidth and improve oblique insensitivity.

Materials 2019, 12, 3989 10 of 12

It is seen from Figure 13 that the minimum reflectivity values for the design structure are 43.24 dB − at 824 MHz for an incidence angle of 0 , 34.3 dB at 829 MHz for 15 , 23.3 dB at 829 MHz for ◦ − ◦ − 30 , 15.2 dB at 825 MHz for 45 , and 10.2 dB at 826 MHz for 60 under transverse electric (TE) ◦ − ◦ − ◦ polarization. The lowest peaks for the reflectivity are 43.2 dB at 824 MHz for an incidence angle of 0 , − ◦ 41.0 dB at 871 MHz for an incidence angle of 15 , 28.3 dB at 918 MHz for an incidence angle of 30 , − ◦ − ◦ 18.6 dB at 998 MHz for an incidence angle of 45◦, and 11.0 dB at 1093 MHz for an incidence angle of −Materials 2019, 12, x FOR PEER REVIEW − 11 of 13 60◦ under transverse magnetic (TM) polarization.

(a) TE (b) TM

FigureFigure 13.13. TheThe simulatedsimulated resultsresults forfor thethe designeddesigned structurestructure withwith didifferentfferent incidentincident angles:angles: ((aa)) TETE polarizationpolarization andand ((bb)) TMTM polarization.polarization.

4. ConclusionIn Figure 13s , it is seen that the absorption decreases considerably when increasing the incidence angle under TE polarization, whereas the reduction is smaller for TM polarization, although the resonantA tunable frequency absorber shifts rather containing considerably an AFSS with layerθ. was presented. The AFSS absorber was manufacturedUnder TE by polarization, loading resistors the electric and varactors field is always onto an parallel FSS array. to the A AFSSfeeding array, network so the was impedance designed (Z)so that and the equivalent varactors inductance were connected (L) of in the parallel. AFSS is The independent obtained data of incident show that angle. the operating Thus, the freq resonantuency frequencycan be tuned remains over stable a wide with frequency variation band of the from incident 415 angle.to 822 UnderMHz, TMwhile polarization, the reverse there bias is voltage an angle is (variedθ) between from the−5 to electric −30 V. field The andresults the obtained AFSS array. from The measurements AFSS impedance are in foragreement TM mode with depends numerical on theresults. incident The anglereflectivity [36]. Aswas the less incident than −10 angle dB, with increases, 90% of the the impedance EM energy decreases, being absorbed. and the In equivalent addition, inductancethe absorbing L decreasesstructure becausehad an ultrathin of Z = jwLthickness. Therefore, of 10 mm, as incident which was angle equivalent increases, to the only resonant λ/72 of frequencythe lower willlimit increase frequency. for The TM polarization.suggested absorber is appropriate for P-band microwave application. The simulated results for the electric field and volume loss density distributions indicate that Under TM polarization, the reflectivity is lowest at 824 MHz for an incidence angle of 0◦ because ofmost the perfectof the energy matched losses impedance arise from at normal absorption incidence. by the The dielectric reflectivity substrate. at 824 MHz A detailed increases analysis rapidly was to thealso highest carried peak out with on the variation basis ofof thethe incident AFSS shape, angle becauseresistor, of thickness the gradually of the mismatched foam, thickness impedance and atpermittivity oblique incidence. of the dielectric This problem substrate can be, and solved incident by optimization angles to study of the elementthe tunable shape absorber [37,38], design which canmethods. increase the absorption bandwidth and improve oblique insensitivity. Compared to other tunable absorbers [39,40], the proposed absorber has the advantage of easy 4.fabrication, Conclusions an ultrathin thickness, short response time, low absorbing frequency, and wide tunable bandwidth, which makes it promising for smart-skin applications. A tunable absorber containing an AFSS layer was presented. The AFSS absorber was manufactured by loading resistors and varactors onto an FSS array. A feeding network was designed so that the Funding: This work was supported by the National Natural Science Foundation of China (NSFC) under grant varactors were connected in parallel. The obtained data show that the operating frequency can be no. 61701448. tuned over a wide frequency band from 415 to 822 MHz, while the reverse bias voltage is varied fromAuthor5 Contributions: to 30 V. The resultsK.Q. and obtained L.L. conceived from measurements of and designed are in the agreement experiments; with Y numerical.L. performed results. the − − Theexperiments; reflectivity K.Q was. and less J.S. analyzed than 10 the dB, data; with K. 90%Q. and of J the.C. contributed EM energy reagents/materials/analysis being absorbed. In addition, tools; K the.Q. wrote the paper. − absorbing structure had an ultrathin thickness of 10 mm, which was equivalent to only λ/72 of the lowerConflicts limit of frequency.Interest: The The authors suggested declare absorber no conflict iss appropriateof interest. for P-band microwave application. The simulated results for the electric field and volume loss density distributions indicate that mostReferences of the energy losses arise from absorption by the dielectric substrate. A detailed analysis was also carried1. Knott out, onE.F the.; Shaeffer, basis of J.F.; the Tuley, AFSS M.T. shape, Radar resistor, Cross Section thickness; Artech of the House foam,: Boston, thickness MA, andUSA, permittivity 1993; p.64. of the2. dielectricHaupt, R substrate,.L. Scattering and from incident small anglesSalisbury to studyscreens the. IEEE tunable Trans. absorber Antennas design Propag methods.. 2006, 54, 1807–1810, doi:10.1109/TAP.2006.875921. 3. Munk, B.A.; Munk, P.; Pryor, J. On designing Jaumann and circuit analog absorbers (CA absorbers) for oblique angle of incidence. IEEE Trans. Antennas Propag. 2007, 55, 186–193, doi:10.1109/ TAP.2006.888395. 4. Zadeh, A.K.; Karlsson, A. Capacitive circuit method of fast and efficient design of wideband radar absorbers. IEEE Trans. Antennas Propag. 2009, 57, 2307–2314, doi:10.1109/TAP.2009.2024490. 5. Shantnu, R.; Tomas J.F. Design of absorber-lined chambers for EMC measurements using a geometrical optics approach. IEEE Trans. Electromagn. Compat. 1984, 26, 111–119.

Materials 2019, 12, 3989 11 of 12

Compared to other tunable absorbers [39,40], the proposed absorber has the advantage of easy fabrication, an ultrathin thickness, short response time, low absorbing frequency, and wide tunable bandwidth, which makes it promising for smart-skin applications.

Author Contributions: K.Q. and L.L. conceived of and designed the experiments; Y.L. performed the experiments; K.Q. and J.S. analyzed the data; K.Q. and J.C. contributed reagents/materials/analysis tools; K.Q. wrote the paper. Funding: This work was supported by the National Natural Science Foundation of China (NSFC) under grant no. 61701448. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Knott, E.F.; Shaeffer, J.F.; Tuley, M.T. Radar Cross Section; Artech House: Boston, MA, USA, 1993; p. 64. 2. Haupt, R.L. Scattering from small Salisbury screens. IEEE Trans. Antennas Propag. 2006, 54, 1807–1810. [CrossRef] 3. Munk, B.A.; Munk, P.; Pryor, J. On designing Jaumann and circuit analog absorbers (CA absorbers) for oblique angle of incidence. IEEE Trans. Antennas Propag. 2007, 55, 186–193. [CrossRef] 4. Zadeh, A.K.; Karlsson, A. Capacitive circuit method of fast and efficient design of wideband radar absorbers. IEEE Trans. Antennas Propag. 2009, 57, 2307–2314. [CrossRef] 5. Shantnu, R.; Tomas, J.F. Design of absorber-lined chambers for EMC measurements using a geometrical optics approach. IEEE Trans. Electromagn. Compat. 1984, 26, 111–119. 6. Zuo, W.Q.; Yang, Y.; He, X.X.; Zhan, D.W.; Zhang, D.F. A miniaturized metamaterial absorber for ultrahigh-frequency RFID system. IEEE Antennas Wireless Propag. Lett. 2017, 16, 329–332. [CrossRef] 7. Costa, F.; Genovesi, S.; Monorchio, A.; Manara, G. Low-cost metamaterial absorbers for sub-GHz wireless systems. IEEE Antennas Wireless Propag. Lett. 2014, 13, 27–30. [CrossRef] 8. Ye, D.; Wang, Z.; Wang, Z.; Xu, K.; Zhang, B.; Huangfu, J.; Li, C.; Ran, L. Towards experimental perfectly-matched layers with ultra-thin metamaterial surfaces. IEEE Trans. Antennas Propag. 2012, 60, 5164–5172. [CrossRef] 9. Li, H.; Yuan, L.H.; Zhou, B.; Shen, X.P.; Cheng, Q.; Cui, T.J. Ultrathin multiband gigahertz metamaterial absorbers. J. Appl. Phys. 2011, 110.[CrossRef] 10. Liu, T.; Cao, X.; Gao, J.; Zheng, Q.; Li, W.; Yang, H. RCS Reduction of waveguide slot with metamaterial absorber. IEEE Trans. Antennas Propag. 2013, 61, 1479–1484. [CrossRef] 11. Wakatsuchi, H.; Kim, S.; Rushton, J.J.; Sievenpiper, D.F. Circuit-based nonlinear metasurface absorbers for high surface currents. Appl. Phys. Lett. 2013, 102.[CrossRef] 12. Humberto, F.A.; Maria, E.C.; Fernando, L.H. A thin C-band polarization and incidence angle- insensitive metamaterial perfect absorber. Materials 2015, 8, 1666–1681. [CrossRef] 13. Cheng, Y.Z.; Cheng, Z.Z.; Mao, X.S.; Gong, R.Z. Ultra-thin multi-band polarization-insensitive microwave metamaterial absorber based on multiple-order responses using a single structure. Materials 2017, 10, 1241. [CrossRef][PubMed] 14. Chen, K.; Cui, L.; Feng, Y.J.; Zhao, J.M.; Jiang, T.; Zhu, B. Coding metasurface for broadband microwave scattering reduction with optical transparency. Opt. Express 2017, 25.[CrossRef][PubMed] 15. Landy, N.I.; Sajuyigbe, S.; Mock, J.J.; Smith, D.R.; Padilla, W.J. Perfect metamaterial absorber. Phys. Rev. Lett. 2008, 100.[CrossRef] 16. Zhang, L.B.; Zhou, P.H.; Chen, H.Y.; Lu, H.P.; Xie, J.L.; Deng, L.J. Ultra-thin wideband magnetic-type metamaterial absorber based on LC resonator at low frequencies. Appl. Phys. A 2015, 121, 233–238. [CrossRef] 17. Zhang, H.B.; Deng, L.W.; Zhou, P.H.; Zhang, L.; Cheng, D.M. Low frequency needlepoint-shape metamaterial absorber based on magnetic medium. J. Appl. Phys. 2013, 113, 013903. [CrossRef] 18. Yoo, Y.J.; Zheng, H.Y.; Kim, Y.J.; Rhee, J.Y.; Kang, J.H.; Kim, K.W.; Cheong, H.; Kim, Y.H.; Lee, Y.P. Flexible and elastic metamaterial absorber for low frequency, based on small-size unit cell. Appl. Phys. Lett. 2014, 105. [CrossRef] 19. Sun, L.K.; Cheng, H.F.; Zhou, Y.J.; Wang, J. Low-frequency and broad band metamaterial absorber: Design, fabrication and characterization. Appl. Phys. A 2011, 105, 49–53. [CrossRef] Materials 2019, 12, 3989 12 of 12

20. Khuyen, B.X.; Tung, B.S.; Yoo, Y.J.; Kim, Y.J.; Lam, V.D.; Yang, J.G.; Lee, Y.P. Ultrathin metamaterial-based perfect absorber for VHF and THz bands. Curr. Appl. Phys. 2016, 16, 1009–1014. [CrossRef] 21. Zuo, W.Q.; Yang, Y.; He, X.X.; Mao, C.Y.; Liu, T. An ultrawideband miniaturized metamaterial absorber in the ultrahigh-frequency range. IEEE Antennas Wireless Propag. Lett. 2017, 16, 928–931. [CrossRef] 22. Rozanov, K.N. Ultimate thickness to bandwidth ratio of radar absorbers. IEEE Trans. Antennas Propag. 2000, 48, 1230–1234. [CrossRef] 23. Papasimakis, N.; Luo, Z.; Shen, Z.X.; De Angelis, F.; Di Fabrizio, E.; Nikolaenko, A.E.; Zheludev, N.I. Graphene in a . Opt. Express 2010, 18, 8353–8359. [CrossRef][PubMed] 24. Kowerdziej, R.; Garbat, K.; Walczakowski, M. Nematic mixtures dedicated to thermally tunable terahertz devices. Liq. Cryst. 2018, 45, 1040–1046. [CrossRef] 25. Ricci, M.C.; Xu, H.; Prozorov, R.; Zhuravel, A.P.; Ustionov, A.V.; Anlage, S.M. Tunability of superconducting . IEEE Trans. Appl. Supercond. 2007, 17, 918–921. [CrossRef] 26. Barnes, A.; Despotakis, A.; Wright, P.V.; Wong, T.C.P.; Chambers, B.; Anderson, A.P. Control of microwave reflectivities of polymer electrolyte--polyaniline composite materials. Electrochimica Acta 1998, 43, 1629–1632. [CrossRef] 27. Tennant, A.; Chambers, B. A single-layer tuneable microwave absorber using an active FSS. IEEE Microwave Wireless Compon. Lett. 2004, 14, 46–47. [CrossRef] 28. Li, J.L.; Jiang, J.J.; He, Y.; Xu, W.H.; Chen, M.; Miao, L.; Bie, S.W. Design of a tunable low-frequency and broadband radar absorber based on active frequency selective surface. IEEE Antennas Wireless Propag. Lett. 2016, 15, 774–777. [CrossRef] 29. Zhang, Q.; Shen, Z.X.; Wang, J.P.; Lee, K.S. Design of a switchable microwave absorber. IEEE Antennas Wireless Propag. Lett. 1158, 11, 1158–1161. [CrossRef] 30. Xu, W.H.; He, Y.; Kong, P.; Li, J.L.; Xu, H.B.; Miao, L.; Bie, S.W.; Jiang, J.J. An ultra-thin broadband active frequency selective surface absorber for ultrahign-frequency applications. J. Appl. Phys. 2015, 118, 184903. [CrossRef] 31. Cui, T.J.; Qi, M.Q.; Wan, X.; Zhao, J.; Cheng, Q. Coding metamaterials, digital metamaterials and programmable metamaterials. Sci. Appl. 2014, 3.[CrossRef] 32. Qi, K.N.; Wang, Y.F.; Hou, X.Y.; Chen, J.W. Analysis and optimization of active tunable absorber. J. Beijing Univ. Aeronaut. Astronaut. 2015, 41, 1853–1858. [CrossRef] 33. Mias, C.; Yap, J.H. A varactor-tunable high impedance surface with a resistive-lumped-element biasing grid. IEEE Trans. Antennas Propag. 2007, 55, 1955–1962. [CrossRef] 34. Zhao, J.; Cheng, Q.; Chen, J.; Qi, M.Q.; Jiang, W.X.; Cui, T.J. A absorber using varactor diodes. New J. Phys. 2013, 15, 043049. [CrossRef] 35. Marcuvitz, N. Waveguide Handbook; McGraw-Hill: New York, NY, USA, 1986; p. 280. 36. Luukkonen, O.; Simovski, C.; Granet, G.; Goussetis, G.; Lioubtchenko, D.; Raisanen, A.V.; Tretyakov, S.A. Simple and accurate analytical model of planar grids and high-impedance surfaces comprising metal strips or patches. IEEE Trans. Antennas Propag. 2008, 56, 1624–1632. [CrossRef] 37. Zhang, C.; Cheng, Q.; Yang, J.; Zhao, J.; Cui, T.J. Broadband metamaterial for optical transparency and microwave absorption. J. Appl. Phys. 2017, 110, 143511. [CrossRef] 38. Ghosh, S.; Srivastava, K.V. Polarization-insensitive single-/dual-band tunable absorber with independent tuning in wide frequency range. IEEE Communication Antennas Propag. 2017, 65, 4903–4908. [CrossRef] 39. Wang, L.; Xia, D.; Fu, Q.; Wang, Y.; Ding, X.; Yang, B. Thermally tunable ultra-thin metamaterial absorber at P band. J. Electromagn. Appl. 2019, 33, 1406–1415. [CrossRef] 40. Kowerdziej, R.; Jaroszewicz, L. Tunable dual-band liquid crystal based near- perfect metamaterial absorber with high-loss metal. Liq. Cryst. 2019, 46, 1568–1573. [CrossRef]

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