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Gain-Enhanced Metamaterial Absorber-Loaded Monopole Antenna for Reduced Radar Cross-Section and Back Radiation

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Gain-Enhanced Metamaterial Absorber-Loaded Monopole Antenna for Reduced Radar Cross-Section and Back Radiation materials Article Gain-Enhanced Metamaterial Absorber-Loaded Monopole Antenna for Reduced Radar Cross-Section and Back Radiation Heijun Jeong 1, Yeonju Kim 1, Manos M. Tentzeris 2,* and Sungjoon Lim 1,* 1 School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, Seoul 06974, Korea; [email protected] (H.J.); [email protected] (Y.K.) 2 School of Electrical and Computer Engineering, College of Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA * Correspondence: [email protected] (M.M.T.); [email protected] (S.L.); Tel.: +82-2-820-5827 (S.L.) Received: 7 February 2020; Accepted: 5 March 2020; Published: 10 March 2020 Abstract: This paper proposes a gain-enhanced metamaterial (MM) absorber-loaded monopole antenna that reduces both radar cross-section and back radiation. To demonstrate the proposed idea, we designed a wire monopole antenna and an MM absorber. The MM absorber comprised lumped elements of subwavelength unit cells and achieved 90% absorbance bandwidth from 2.42–2.65 GHz. For low-profile configurations, the MM absorber was loaded parallel to and 10 mm from the monopole antenna, corresponding to 0.09 λ0 at 2.7 GHz. The monopole antenna resonated at 2.7 GHz with a 3.71 dBi peak gain and 2.65 GHz and 6.46 dBi peak gain, before and after loading the MM absorber, respectively. Therefore, including the MM absorber increased peak gain by 2.7 dB and reduced back radiation by 15 dB. The proposed antenna radar cross-section was reduced by 2 dB compared with a monopole antenna with an artificial magnetic conductor. Keywords: high gain monopole antenna; metamaterial absorber; back radiation reduction; RCS reduction 1. Introduction Metamaterials (MMs) are periodic structures with a nominally infinite number of artificial structures [1] designed to control permittivity and permeability. Hence, MMs have been employed for many electromagnetic applications, such as terahertz devices [2], frequency selective surfaces [3], and super lenses [4], and particularly for antenna applications, such as electrically small [5], high gain [6], and beam-scanning [7] antennas. Antenna gain can be increased with suitable reflectors, but the reflector must be placed a quarter-wavelength (λ/4) from the antenna to avoid cancelling the original and image currents, which is not ideal for low-profile antennas. To create a high-gain antenna with a low profile, we replaced the conventional reflector (similar to a perfect electric conductor (PEC)) with MMs, forming an artificial magnetic conductor (AMC) [8], or high impedance surface [9], due to its similar reflection coefficient as perfect magnetic conductors at resonance frequency [10–12]. High-gain antennas can be created using an AMC as the reflective ground [13–15], or superstrate [16–18]. Despite their high-gain and low-profile configuration, the antenna radar cross-section (RCS) with AMC is similar to that of conductive plates, due to the reflected wave from the AMC. At the same time, MM absorbers have been widely used to reduce RCS, as MM absorbers can absorb electromagnetic waves, with the result that many MM absorber-based antennas are used as radar-absorbing materials for radar applications [19–21]. However, most radar-absorbing materials focus on RCS reduction, rather than enhancing gain or reducing back radiation. Materials 2020, 13, 1247; doi:10.3390/ma13051247 www.mdpi.com/journal/materials MaterialsMaterials2020 2018, 13, 11, 1247, x FOR PEER REVIEW 22 of of 16 16 This study proposes adding an MM absorber to a monopole antenna to reduce RCS while simultaneouslyThis study proposesenhancing adding gain, reducing an MM absorber back radiation to a monopole, and offering antenna beam to reducereflection RCS. The while MM simultaneouslyabsorber incorporate enhancings lumped gain, reducing elements back for radiation,subwavelength and off eringunit cells beam and reflection. achieves The a MMlow profile absorber by incorporatesloading parallel lumped to elementsthe monopole for subwavelength antenna at a unit minimum cells and distance. achieves P aroposed low profile antenna by loading performance parallel towas the numerically monopole antenna and experimentally at a minimum compared distance. Proposedwith monopole, antenna monopole performance-with was-PEC, numerically and monopole and - experimentallywith-AMC antennas, compared exhibiting with monopole, higher gain, monopole-with-PEC, lower back radiation, and monopole-with-AMChigher front-to-back ratio antennas, (FBR), exhibitingand lower higher RCS. gain, lower back radiation, higher front-to-back ratio (FBR), and lower RCS. 2.2. Proposed Proposed Antenna Antenna Design Design WeWe simulated simulated the the proposed proposed antenna antenna for for electromagnetic electromagnetic analysis analysis using using ANSYS ANSYS High High Frequency Frequency StructureStructure Simulator Simulator (HFSS) (HFSS) software software (Version (Version 17.2, 17.2 ANSYS,, ANSYS Canonsburg,, Canonsburg PA,, PA USA)., USA Figure). Figure1 shows 1 shows the proposedthe proposed and three and referencethree reference antennas, antennas, where thewhere proposed the proposed antenna comprisedantenna comprised a monopole a monopole antenna andantenna MM absorberand MM (Figure absorber1d). (Fig A quarter-wavelengthure 1d). A quarter vertical-wavelength monopole vertical antenna monopole was designed antenna forwas alldesigned antennas for with all antennas 2.7 GHz resonancewith 2.7 GHz and resonance geometrical and parameters geometric asal parameters shown in Table as shown1. We assumedin Table 1. 7 copper conductivity = 5.8 10 S/m, and FR-47 substrate dielectric constant = 3.9 and loss tangent = 0.02. We assumed copper conductivity× = 5.8 × 10 S/m, and FR-4 substrate dielectric constant = 3.9 and loss An air radiation box was used for the radiation boundary with dimension 158.5 110.8 157 mm3. tangent = 0.02. An air radiation box was used for the radiation boundary with dimension× ×158.5 × 110.8 A×sub-miniature 157 mm3. A sub version-miniature A (SMA) version connector A (SMA) was connector included was to provideincluded excitation, to provide and excitation, the wave and port the waswave assigned port was to theassigned coaxial to transmission the coaxial transmission line. line. (a) (b) (c) (d) FigureFigure 1. 1.Monopole Monopole antennas: antennas: ( a()a) reference reference antenna antenna 1: 1: bare, bare, (b ()b reference) reference antenna antenna 2: 2: perfect perfect electric electric conductorconductor (PEC), (PEC), ( c()c) reference reference antenna antenna 3: 3: artificial artificial magnetic magnetic conductor conductor (AMC), (AMC), and and ( d()d) proposed proposed antenna:antenna metamaterial: metamaterial (MM) (MM) absorber. absorber. MaterialsMaterials2020 2018, 13, 11, 1247, x FOR PEER REVIEW 3 of3 of 16 16 Table 1. Proposed monopole antenna parameters. Table 1. Proposed monopole antenna parameters. Parameter Value (mm) Description Parameter Value (mm) Description L1 25 Wire monopole length L1 Rm 252 Wire monopole Wire diameter monopole length Rm Ws 1172 FR-4 substrate Wire monopole width diameter Ws Ls 117117 FR-4 substrate FR-4 length substrate width L 117 FR-4 substrate length s Hs 0.8 FR-4 substrate thickness Hs 0.8 FR-4 substrate thickness 2.1. Metamaterial Absorber Design 2.1. Metamaterial Absorber Design Figure 2 shows the MM absorber unit cell that will be loaded on the monopole antenna in parallel. HighFigure MM2 showsabsorptivity the MM can absorber be ach unitieved cell by that minimizing will be loaded reflection on the monopole Γ(ω) and antenna transmission in parallel. T(ω) Highcoefficient, MM absorptivity with total canabsorptivity be achieved by minimizing reflection G(!) and transmission T(!) coefficient, with total absorptivity A(!A)ω=11 G (!ω) TT(!ω) (1)(1) − − T(T(ω)!) can can be be made made zero zero by by fully fully covering covering the the MM MM absorber absorber bottom bottom plane; plane; and and under under normal normal incidence,incidence,G (Γ(ω)!) can can made made zero zero by matchingby matching MM MM impedance impedance (ZM) (Z ofM that) of forthat free for space free (Zspace0 = 377 (Z0W =), 377 since Ω), since Z0 ZM G(!) = ZZ− (2) 0M Γ(ω) Z0 + ZM (2) ZZ0M The MM unit cell was designed based on inductive-capacitive (LC) resonance. Resonance frequencyThe dependsMM unit oncell 1/ pwasLC, designed hence decreases based on with inductive increasing-capacitive inductance (LC) (L). resonance. Thus, four Resonance 17 nH chipfrequency inductors depends were on loaded 1/√LC, on hence square decreases split-ring with resonator increasing gaps inductance to miniaturize (L). Thus, the four unit 17 cell nH [ 22chip]. inductors were loaded on square split-ring resonator gaps to miniaturize the unit cell [22]. The The conductive pattern was designed on FR-4 substrate, with dielectric constant ("r) = 3.9; loss tangent tanconductiveδ = 0.02; andpattern dimensions was designed l = 3.3 on mm, FR- g4 =substrate,0.5 mm, with w = 0.5dielectric mm, c =constant4 mm, and(εr) = h 3.9;=5.6 loss mm. tangent tan δ = 0.02; and dimensions l = 3.3 mm, g = 0.5 mm, w = 0.5 mm, c = 4 mm, and h =5.6 mm. (a) (b) FigureFigure 2. 2.Proposed Proposed MM MM absorber absorber unit unit cell: cell (: a()a) top top and and ( b(b)) perspective perspective view. view. Figure3 shows MM absorber equivalent circuit and simulated absorptivity, where Z 0 and Zd Figure 3 shows MM absorber equivalent circuit and simulated absorptivity, where Z0 and Zd represent free space and dielectric substrate impedance, respectively. Top conductive patterns can be represent free space and dielectric substrate impedance, respectively. Top conductive patterns can be represented as series R, L, and C; where R represents conductive pattern resistance, L represents the represented as series R, L, and C; where R represents conductive pattern resistance, L represents the sum of inductance due to conductive patterns (Ld) and chip inductors (Lc); and C is capacitance due to sum of inductance due to conductive patterns (Ld) and chip inductors (Lc); and C is capacitance due the gap between unit cells.
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