And Ku-Band Application

applied sciences

Article Compact Left-Handed Meta-Atom for S-, C- and Ku-Band Application

Md. Mehedi Hasan 1,*, Mohammad Rashed Iqbal Faruque 1,* and Mohammad Tariqul Islam 2,* ID 1 Space Science Centre (ANGKASA), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia 2 Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia * Correspondence: [email protected] (M.M.H.); [email protected] (M.R.I.F.); [email protected] (M.T.I.); Tel.: +60-102938061 (M.R.I.F.)

Received: 10 August 2017; Accepted: 10 October 2017; Published: 23 October 2017

Abstract: A new compact left-handed meta-atom for S-, C- and Ku-band applications is presented in this paper. The proposed structure provides a wide bandwidth and exhibits left-handed characteristics at 0◦, 90◦, 180◦ and 270◦ (xy-axes) rotations. Besides, the left-handed characteristics and wide bandwidth of 1 × 2, 2 × 2, 3 × 3 and 4 × 4 arrays are also investigated at the above-mentioned rotation angles. In this study, the meta-atom is designed by creating splits at the outer and inner square-shaped ring resonators, and a metal arm is placed at the middle of the inner ring resonator. The arm is also connected to the upper and lower portions of the inner ring resonator, and later, the design appears as an I-shaped split ring resonator. The commercially available, finite integration technique (FIT)-based electromagnetic simulator CST Microwave Studio is used for design and simulation purposes. The measured data comply well with the simulated data of the unit cell for 1 × 2, 2 × 2, 3 × 3 and 4 × 4 arrays at every rotation angle. Owing to the effective medium ratio (EMR) of 8.50 at 0◦ and 180◦ rotations, the proposed meta-atom structure is compact in size. Moreover, due to the quality factor of 82, the designed meta-atom is flexible for high-performance antenna, filter and sensor applications. Therefore, the meta-atom integrated antenna shows multi frequency bands with the highest peak gain of 5 dBi, which is used as the long distance radio communication frequency.

Keywords: compactness; left-handed characteristics; meta-atom; wide bandwidth

1. Introduction Electromagnetic meta-atoms are composed of artificial atoms, whose electric and magnetic response could be flexibly tailored to meet desired exotic EM properties. Benefiting from these novel properties, meta-atoms have produced many exotic effects in several applications, such as negative permeability, sub-wavelength imaging super-lens, electromagnetic absorbers [1], negative refraction [2], filter design [3], antenna performance enhancing, SAR reduction [4] and invisible cloaking [5]. The development of the meta-atoms utilizes the intrinsic loss of the system, with the help of structural design, to obtain a wide bandwidth or sharp resonance at a certain frequency. Material that exhibits either negative permittivity or negative permeability is called a single-negative meta-atom. When the values of permittivity and permeability are near zero over a specific frequency range, the material is specified as a near-zero refractive index meta-atom. Moreover, a material exhibiting negative permeability and negative permittivity simultaneously can be characterized as a double-negative or left-handed meta-atom. Although negative permittivity can be found in a few metals, negative penetrability is hard to discover. Therefore, the presence of both negative permeability and permittivity is exceptionally hard to acquire as the characteristic for left-handed materials. Recently, various composites based on metallic and dielectric structures that act as left-handed meta-atoms have been developed (Figure1)[6–12].

Appl. Sci. 2017, 7, 1071; doi:10.3390/app7101071 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 1071 2 of 19 materials. Recently, various composites based on metallic and dielectric structures that act as Appl. Sci. 2017, 7, 1071 2 of 21 left-handed meta-atoms have been developed (Figure 1) [6].

FigureFigure 1.1. ExamplesExamples ofof severalseveral meta-atommeta-atom basedbased metamaterialmetamaterial (MTM)(MTM) structuresstructures throughthrough periodicperiodic repetitionrepetition ofof metallicmetallic andand dielectricdielectric elements:elements: ((aa)) double-fishnetdouble-fishnet negative-indexnegative-index metamaterialmetamaterial withwith severalseveral layers. layers; Reproduced (b) chiral metamaterial with permission fabricated from [Xiao, thro S.ugh et al.], stacked [Opt. Lett.];electron-beam published lithography; by [The Optical (c) Society], [2009] [6]; (b) chiral metamaterial fabricated through stacked electron-beam lithography. chiral metamaterial made using direct-laser writing and electroplating; (d) hyperbolic metamaterial From [Soukoulis, C.M. et al. Optical Metamaterials—More Bulky and Less Lossy. Science 2010, 330, made by electroplating hexagonal-hole-array templates; (e) metal-dielectric layered metamaterial 1633–1634]. Reprinted with permission from Soukoulis, C.M. [7]; (c) chiral metamaterial made using composed of coupled plasmonic waveguides; (f) Split ring resonators (SRRs) oriented in all three direct-laser writing and electroplating. From [Ganse, J.K. et al. Gold Helix Photonic Metamaterial dimensions; (g) wide-angle visible negative-index metamaterial based on a coaxial design; (h) as Broadband Circular Polarizer. Science 2009, 325, 1513–1515]. Reprinted with permission from connected cubic-symmetry negative-index metamaterial; (i) metal cluster-of-clusters visible-frequency AAAS [8]; (d) hyperbolic metamaterial made by electroplating hexagonal-hole-array templates. From magnetic metamaterial; (j) negative-index metamaterial composed of two sets of high-refractive-index [Ganse, J.K. et al. Gold Helix Photonic Metamaterial as Broadband Circular Polarizer. Science 2009, dielectric spheres arranged on a simple cubic lattice. NRI, negative refractive index. 325, 1513–1515]. Reprinted with permission from AAAS [8]; (e) metal-dielectric layered metamaterial composed of coupled plasmonic waveguides Reproduced from [Gao, J. et al. Experimental realization In 1968, Veselago first discussed the negative index material. He demonstrated that a negative of epsilon-near-zero metamaterial slabs with metal-dielectric multilayers. Appl. Phys. Lett. 2013, index material refraction would occur at negative angles, energy would flow in a direction opposite 103, 051111], with the permission of AIP Publishing [9]; (f) Split ring resonators (SRRs) oriented in all to the direction of the phase velocity and the Doppler effect would be reversed at a certain three dimensions. Reproduced from [Chen, Y. et al. Acoustic band gaps of three-dimensional periodic frequency [7]. In 1996, Pendry et al. projected their summary of a thin wire configuration, which polymer cellular solids with cubic symmetry. Appl. Phys. Lett. 2013, 114, 043521], with the permission exhibited a negative permittivity (ε), and in 1999, they presented the split ring resonator with a of AIP Publishing [10]; (g) wide-angle visible negative-index metamaterial based on a coaxial design. negativeFrom permeability [Soukoulis, C.M. (μ) et[8]. al. Owing Optical to Metamaterials—More the absence of such Bulky prop anderties Less in Lossy.natural Science materials, 2010, the 330, topic was 1633–1634].not particularly Reprinted interesting with permission to researchers from Soukoulis, until Smith C.M. and [7]; (hish) connectedcolleagues cubic-symmetry invented the first successfulnegative-index metamaterial metamaterial. in the Reproducedlaboratory. withIn 2000, permission Smith fromet al. [Ji. exhibited R. et al.], [Nanoscale];a material that published displayed negativeby [Royal permittivity Society of and Chemistry], permeability [2016] at [11 the]; (i )same metal time cluster-of-clusters with unusual visible-frequency natural properties magnetic [9]. In the last metamaterial.few years, multi-band Reproduced withmeta-atom permission arrays from [Wang,at different L. et al.], rotation [Adv. Mater.]; angles published with a bycompact [Wiley], size, left-handed[2011] [12 properties]; (j) negative-index and a wide metamaterial bandwidth composed have be ofcome two a sets promising of high-refractive-index research field dielectricfor specialists becausespheres very arranged few studies on a simple are cubicconcentrated lattice. NRI, on negative this sector. refractive In index.2017, ReproducedHasan et al. with suggested permission that a tri-bandfrom metamaterial [Wang, L. et al.], absorber [Adv. Mater.]; was developed published byby [Wiley], the square [2011] shape [12]. resonators with a circular ring in the middle of the square resonators. The designed metamaterial absorber illustrates a 4.71 GHz-wide bandwidthIn 1968, and Veselago applicable first discussedfor tri- (C-, the X- negative and Ku index-) band material. applications. He demonstrated The effective that medium a negative ratio indexwas 5.83, material and refraction the absorptions would occur were at respectively, negative angles, 82%, energy 67% wouldand 93% flow [1]. in aIn direction 2013, Mallik opposite et toal. theintroduced direction ofa therectangular phase velocity “U-shaped” and the Dopplerleft-handed effect metamaterial would be reversed for several at a certain orthogonal frequency array [13]. Instructure, 1996, Pendry and the et al. effective projected medium their summary ratio was of 1.99 a thin [10]. wire In 2014, configuration, Islam et al. which proposed exhibited a 30 a × negative 30-mm2 permittivity“H-shaped” (metamaterialε), and in 1999, for theymulti-band presented operatio the splitns, and ring the resonator resonance with frequencies a negative were permeability found at (S-,µ)[ 14C-,]. OwingX- and to theKu-bands. absence ofIn suchaddition, properties the inH- naturalshaped materials, structure the exhi topicbited was double-negative not particularly interestingcharacteristics, to researchers but the untileffective Smith medium and his ratio colleagues was invented3.64. However, the first the successful sensitivity metamaterial of these inmetamaterial the laboratory. was In 2000,only Smith13 [11]. et al.In exhibited 2015, Purushothaman a material that displayedet al. explained negative permittivitya “ring-shaped” and permeabilitymetamaterial at structure the same applicable time with as unusual a filter naturalin waveguides. properties Moreover, [15]. In the the last effect few on years, the resonance multi-band of meta-atomthe parameters, arrays such at different as the rotationring width, angles inner with and a compact outer ring size, gap, left-handed splitting propertiesof the ring, and substrate a wide bandwidthmaterial thickness have become and orientation a promising of the research substrate field material, for specialists was investigated because very in their few paper studies [3]. are In concentrated2015, Hossain on et this al. sector.introduced In 2017, a two Hasan “G-shaped” et al. suggested double thatnegative a tri-band (DNG) metamaterial metamaterial, absorber where was the developedunit cell and by thearrays square are shapedifferent resonators in size, with and a the circular metamaterial ring in the was middle suitable of the for square S- and resonators. C-band Theapplications. designed The metamaterial dimensions absorber of the present illustratesed metamaterial a 4.71 GHz-wide structure bandwidth were 12 and× 12 applicablemm2 [12]. In for 2015, tri- (C-, X- and Ku-) band applications. The effective medium ratio was 5.83, and the absorptions were Appl. Sci. 2017, 7, 1071 3 of 21 respectively, 82%, 67% and 93% [1]. In 2013, Mallik et al. introduced a rectangular “U-shaped” left-handed metamaterial for several orthogonal array structure, and the effective medium ratio was 1.99 [16]. In 2014, Islam et al. proposed a 30 × 30-mm2 “H-shaped” metamaterial for multi-band operations, and the resonance frequencies were found at S-, C-, X- and Ku-bands. In addition, the H-shaped structure exhibited double-negative characteristics, but the effective medium ratio was 3.64. However, the sensitivity of these metamaterial was only 13 [17]. In 2015, Purushothaman et al. explained a “ring-shaped” metamaterial structure applicable as a filter in waveguides. Moreover, the effect on the resonance of the parameters, such as the ring width, inner and outer ring gap, splitting of the ring, substrate material thickness and orientation of the substrate material, was investigated in their paper [3]. In 2015, Hossain et al. introduced a two “G-shaped” double negative (DNG) metamaterial, where the unit cell and arrays are different in size, and the metamaterial was suitable for S- and C-band applications. The dimensions of the presented metamaterial structure were 12 × 12 mm2 [18]. In 2015, Armghan et al. proposed a metamaterial based on split ring resonators to produce a negative refractive index at terahertz frequencies. The 2200 × 2200-nm2 structure exhibited left-handed characteristics at approximately 9.7 THz [2]. In 2016, Zhou et al. designed an 8.5 × 8.5-mm2 “double Z-shaped” left-handed metamaterial via coplanar electric and magnetic resonators, and the effective medium ratio was 4.80. The resonator structure was composed of two orthogonal Z-shaped metal strips, and the metamaterial exhibited resonance at 7.3, 8.1 and 9.4 GHz [19]. In 2015, Alam et al. suggested an 8 × 8-mm2 “hexagonal” DNG metamaterial, where the bandwidth was 1.75 GHz (from 1.68–3.43 GHz) and 0.96 GHz (from 5.04–6.0 GHz) [4]. In 2016, Yang et al. demonstrated a “ring-shaped” 5 × 5-mm2 meta-atom for wearable, flexible and stretchable microwave meta-skin, with clocking effects from 8–13 GHz and an effective medium ratio of six [5]. In 2016, Du et al. presented a left-handed metamaterial created by the combination of ferrite sheets and dielectric rods to investigate the electromagnetic properties for X-band applications [20]. In 2016. Hasan et al. exhibited a Z-shaped metamaterial with resonance at C- and X-bands, where the bandwidth was 3.61 GHz (from 3.48–7.09 GHz). Due to the effective medium ratio (EMR), more than four of the proposed metamaterials were compact in size, and the quality factor was 31. Moreover, the double-negative properties appeared at 8.79 GHz [21]. In 2016, Liu et al. presented a 5 × 5-mm2 split ring-shaped left-handed metamaterial developed using modified circular electric resonators, and it exhibited a dual-band for microwave devices and antenna applications with an effective medium ratio of 5.45 [22]. In 2017, Hasan et al. displayed a 10 × 10-mm2 a split S-shaped ring resonator that exhibited resonances at X-band with negative refractive index from 8.0–11.70 GHz and 11.78–14.0 GHz, i.e., the bandwidths cover 3.70 GHz and 2.22 GHz, respectively [23]. In this paper, the reported reformed I-shaped meta-atom unit cells with 1 × 2, 2 × 2, 3 × 3 and 4 × 4 array structures are analyzed at different rotation angles, including 0-degrees, 90-degrees, 180-degrees and 270-degrees. The unit cell shows resonances at S-, C- and Ku-bands, with a maximum negative refractive index (NRI) bandwidth of 3 GHz at 0-degree, 3.29 GHz at 90-degree, 2.94 GHz at 180-degree and 3.66 GHz at 270-degree rotation. Similarly, for 1 × 2, 2 × 2, 3 × 3 and 4 × 4 array structures, the NRI bandwidths are elaborately explained in the Results Section. The proposed unit cell and the 1 × 2, 2 × 2, 3 × 3 and 4 × 4 arrays exhibit left-handed or double-negative characteristics at 9.20 GHz. The meta-atom structure is compact in size because the effective medium ratio at 0-degree and 180-degree rotation is 8.50 and at 90-degree and 270-degree rotation is more than four. The dimensions of the designed meta-atom are 10 × 10 mm2, which is compact in size according to the effective medium ratio and comparison with [5,17,19,21–23]. The proposed meta-atom unit cell and arrays also exhibit more frequency bands with a wider bandwidth than [1,4,5,16,17,19–23]. Moreover, the proposed meta-atom is used for enhancing the antenna bandwidth, number of frequency bands and gain because the number of frequency bands, bandwidth and gain of the antenna can be improved by using the partial or slotted or fractal or meta-atom integrated ground plane. Therefore, after the ground plane is replaced by the proposed meta-atom, then the frequency bands and gain of the antenna are increased. In addition, by using these proposed designs, the side lobes can be reduced, Appl. Sci. 2017, 7, 1071 4 of 21 which makes the radiation pattern better. Besides, the size of the antenna becomes reduced because the desired performances of the antenna are obtained easily by using this proposed meta-atom within a compact size.

2. Construction of the Proposed Meta-Atom Structure In the proposed design, an orthogonal metal strip of copper (thickness, h = 0.035 mm and conductivity, σ = 5.8 × 107 S/m) is placed in such a way in the inner split square-shaped ring resonator to form an I-shaped structure. A diagram of the proposed unit cell is shown in Figure2 and the array structure is shown in Figure3. Two equal gaps in every ring produce the capacitive effect, symbolized by “g” and each metal strip is responsible for generating inductance. Epoxy resin ﬁber is used as Appl.substrate Sci. 2017 material, 7, 1071 and has a dielectric constant of 4.5 and a loss tangent of 0.02. Table1 reveals4 of the 19 dimensions of the recommended meta-atom unit cell. Table 1. Dimensions of the proposed meta-atom unit cell. Table 1. Dimensions of the proposed meta-atom unit cell. Parameters Dimensions (mm) SubstrateParameters length, a Dimensions10 (mm) SubstrateSubstrate length,width, ab 10 10 RingSubstrate resonator width, length, b l 108.0 RingRing resonator resonator length,width, lw 8.08.0 RingMetal resonator strip width, width, d w 8.00.5 Metal strip width, d 0.5 SplitsSplits width, gg 0.30.3 SubstrateSubstrate height, t t 1.61.6

(a) (b)

Figure 2. Meta-atom unit cell: (a) proposed structure, (b) fabricated structure. Figure 2. Meta-atom unit cell: (a) proposed structure, (b) fabricated structure.

Figure 3. Fabricated 150 × 200-mm2 (15 × 20 unit cells) array prototype of the proposed meta-atom structure.

Appl. Sci. 2017, 7, 1071 4 of 19

Table 1. Dimensions of the proposed meta-atom unit cell.

Parameters Dimensions (mm) Substrate length, a 10 Substrate width, b 10 Ring resonator length, l 8.0 Ring resonator width, w 8.0 Metal strip width, d 0.5 Splits width, g 0.3 Substrate height, t 1.6

(a) (b) Appl. Sci. 2017, 7, 1071 5 of 21 Figure 2. Meta-atom unit cell: (a) proposed structure, (b) fabricated structure.

Appl. Sci. 2017, 7, 1071 5 of 19 Figure 3. Fabricated 150 × 200-mm2 (15 × 20 unit cells) array prototype of the proposed Figure 3. Fabricated 150 × 200-mm2 (15 × 20 unit cells) array prototype of the proposed 3. Methodologymeta-atom structure. For numerical analysis of the proposed unit cell, the finite integration technique-based CST 3. Methodology Microwave Studio has been used. In the simulation technique, the unit-cell structure is placed betweenFor numericaltwo waveguide analysis ports, of the and proposed the electromagnetic unit cell, the ﬁnite waves integration are set technique-basedto propagate in CSTthe Microwavez-directions. Studio Perfect has electric been and used. magnetic In the simulationboundariestechnique, are considered the unit-cell in the x- structure and y-directions is placed betweenshown in two Figure waveguide 4. A frequency-domain ports, and the electromagnetic solver with a waves tetrahedral are set mesh to propagate is utilized in the for z-directions. simulation Perfectfrom 2–14 electric GHz. and magnetic boundaries are considered in the x- and y-directions shown in Figure4. A frequency-domain solver with a tetrahedral mesh is utilized for simulation from 2–14 GHz.

Figure 4. Simulation setup with boundary condition in CST-MWS. Figure 4. Simulation setup with boundary condition in CST-MWS.

The reﬂectionreflection (S 11()S and11) transmissionand transmission (S21) coefﬁcients (S21) coefficients are analyzed are to analyzed realize the electromagneticto realize the propertieselectromagnetic of the properties anticipated of structures the anticipated [24,25]. structures However, [18].S11 andHowever,S21 can S11 be and written S21 can as, be written as,

2 =1 − Γ z, (1) S = , (1) 11 1 − Γ2z2 = , (2) The effective permittivity (ε) can be calculated by,

( ) = × , ( )

( ) = × , (3) ( )

The effective permeability (μ) can be obtained from,

( ) = × , ( )

( ) = × , (4) ( )

Therefore, the refractive index (n) can be expressed as follows [19],

( ) = × , ( ) Appl. Sci. 2017, 7, 1071 6 of 21

1 − z2Γ S = , (2) 21 1 − Γ2z2 The effective permittivity (ε) can be calculated by,

2 (1 − S21 − S11) εr = × , jkd (1 + S21 + S11)

c (1 − S21 − S11) εr = × , (3) jπ f d (1 + S21 + S11) The effective permeability (µ) can be obtained from,

2 (1 − S21 + S11) µr = × , jkd (1 + S21 − S11)

c (1 − S21 + S11) µr = × , (4) jπ f d (1 + S21 − S11) Therefore, the refractive index (n) can be expressed as follows, v u( 2 2 ) 2 u (S21 − 1) − S11 nr = × t , jkd 2 2 (S21 + 1) − S11 v u( 2 2 ) c u (S21 − 1) − S11 nr = × t , (5) jπ f d 2 2 (S21 + 1) − S11 where k = ω/C and the angular frequency ω = 2πf. Measurement of the meta-atom is performed by the two horn antenna (frequency range from 700 MHz–18 GHz) in a semi-anechoic chamber, placed 1 m apart to satisfy the far-field condition. For measurement purposes, the fabricated prototypes utilize a horn antenna. A fabricated 150 × 200-mm2 (15 × 20 unit cells) array prototype, displayed in Figure3, is placed within two horn antennas (from 700 MHz–18 GHz) in a semi-anechoic chamber. The chamber is surrounded by a wedge tapered absorber. The incident electromagnetic waves travel over the prototype similar to the simulated geometry. The horn antennas are connected to the Agilent N5227A vector network analyzer to obtain the reflection and transmission coefficient of the I-shaped meta-atom unit cell. Moreover, for additional experimental accuracy, the designed structure is also measured by the waveguides method [26]. The measured results of the both methods are almost similar to the proposed meta-atom structure. In addition, in the waveguide method, the fabricated 10 × 10-mm2 unit cell is placed between the waveguide ports, which work as transmission and receiving terminals. A set of (WR284 (frequency range from 2–3.95 GHz), WR187 (frequency range from 3.95–5.85 GHz), WR137 (frequency range from 5.85–8.20 GHz), WR90 (frequency range from 8.20–12.40 GHz), WR62 (frequency range from 12.40–18 GHz)) waveguides to coaxial adapters is connected to the Agilent N5227A vector network analyzer via a semi-rigid cable for measuring the S-parameters of the operating frequency from 2–18 GHz. The waveguide experiment is performed in an open-space environment, and the measurement is performed accurately with an Agilent N4694-60001 for calibration and an Agilent N5227A as a vector network analyzer. The experimental setup for measurement purposes is shown in Figure5. Appl. Sci. 2017, 7, 1071 6 of 19

( ) = × , (5) ( ) where k = ω/C and the angular frequency ω = 2πf. Measurement of the meta-atom is performed by the two horn antenna (frequency range from 700 MHz–18 GHz) in a semi-anechoic chamber, placed 1 m apart to satisfy the far-field condition. For measurement purposes, the fabricated prototypes utilize a horn antenna. A fabricated 150 × 200-mm2 (15 × 20 unit cells) array prototype, displayed in Figure 3, is placed within two horn antennas (from 700 MHz–18 GHz) in a semi-anechoic chamber. The chamber is surrounded by a wedge tapered absorber. The incident electromagnetic waves travel over the prototype similar to the simulated geometry. The horn antennas are connected to the Agilent N5227A vector network analyzer to obtain the reflection and transmission coefficient of the I-shaped meta-atom unit cell. Moreover, for additional experimental accuracy, the designed structure is also measured by the waveguides method [20]. The measured results of the both methods are almost similar to the proposed meta-atom structure. In addition, in the waveguide method, the fabricated 10 × 10-mm2 unit cell is placed between the waveguide ports, which work as transmission and receiving terminals. A set of (WR284 (frequency range from 2–3.95 GHz), WR187 (frequency range from 3.95–5.85 GHz), WR137 (frequency range from 5.85–8.20 GHz), WR90 (frequency range from 8.20–12.40 GHz), WR62 (frequency range from 12.40–18 GHz)) waveguides to coaxial adapters is connected to the Agilent N5227A vector network analyzer via a semi-rigid cable for measuring the S-parameters of the operating frequency from 2–18 GHz. The waveguide experiment is performed in an open-space environment, and the measurement is performed accurately with an Agilent N4694-60001 for calibration and an Agilent N5227A as a vector network Appl. Sci. 2017, 7, 1071 7 of 21 analyzer. The experimental setup for measurement purposes is shown in Figure 5.

(a) (b)

Figure 5. Experimental setup for measuring the S11 and S21 parameters: (a) horn antenna method; (b) Figure 5. Experimental setup for measuring the S11 and S21 parameters: (a) horn antenna method; waveguide (WG) method. (b) waveguide (WG) method.

4. Circuit Model of the Proposed Meta-Atom 4. Circuit Model of the Proposed Meta-Atom The proposed meta-atom structure contains the inductive-capacitive circuit, so the resonance The proposed meta-atom structure contains the inductive-capacitive circuit, so the resonance frequency (f), frequency (f ), = 1 , (6) f = √ , (6) 2π LTCT The total inductance (LT) and capacitance (CT) for the proposed structure can be obtained The total inductance (LT) and capacitance (CT) for the proposed structure can be obtained from [27]. from [21]. Therefore, the total inductance (LT) can be obtained from, Therefore, the total inductance (LT) can be obtained from, q  2   2(10d + h)2 (2w + g) + l2  = × + LT τ µ0t 2 , (7)  (2w + g) (10d + h) 

However, the total capacitance (CT) can be calculated by: " # (2w + g) 2(10d + g + h) CT = ε0 ln , (8) π(10d + g + h)2 (a − l)

−7 −12 where the free-space permeability is µ0 = 4π × 10 H/m, the permittivity is ε0 = 8.854 × 10 F/m and the parametric constant is τ = 0.0001. For left-handed characteristics, the series and shunt branch are formed by the inductive and capacitive effect. The capacitive effect is maintained by gaps or splits, which are symbolized by C1,C2,C3,C4 and C5 in the circuit shown in Figure6. Conversely, the metal strips are accountable for the inductive effect and denoted by L1,L2,L3,L4,L5 and L6. When the width of the metal strip is increased, the inductive effect is also increased. As a result, owing to the inductive effect, the resonance frequency is decreased. Similarly, by decreasing the width and increasing the number of splits, the capacitive effect can be raised for the proposed unit cell, which swings the resonance frequency downwards [28]. However, the addition of further splits and metal arms in the meta-atom structure produces a small phase delay and increases the total inductive and capacitive effect. Appl. Sci. 2017, 7, 1071 7 of 19

( ) ( ) = × + , (7) ( ) ( )

However, the total capacitance (CT) can be calculated by:

( ) ( ) = , ( ) () (8) where the free-space permeability is μ0 = 4π × 10−7 H/m, the permittivity is ε0 = 8.854 × 10−12 F/m and the parametric constant is τ = 0.0001. For left-handed characteristics, the series and shunt branch are formed by the inductive and capacitive effect. The capacitive effect is maintained by gaps or splits, which are symbolized by C1, C2, C3, C4 and C5 in the circuit shown in Figure 6. Conversely, the metal strips are accountable for the inductive effect and denoted by L1, L2, L3, L4, L5 and L6. When the width of the metal strip is increased, the inductive effect is also increased. As a result, owing to the inductive effect, the resonance frequency is decreased. Similarly, by decreasing the width and increasing the number of splits, the capacitive effect can be raised for the proposed unit cell, which swings the resonance frequency downwards [22]. However, the addition of further splits and metal arms in the meta-atom structure produces a small phase delay and increases the total inductive and Appl. Sci. 2017, 7, 1071 8 of 21 capacitive effect.

FigureFigure 6. 6. EquivalentEquivalent circuit circuit model model of of the the proposed proposed meta-atom meta-atom structure. structure.

In the above equivalent circuit, the current I (from the source) is divided into I1, I2, I3, I4, I5 and I6, In the above equivalent circuit, the current I (from the source) is divided into I1,I2,I3,I4,I5 and I6, where I = I1 + I2 + I3 + I4 + I5 + I6 (Kirchhoff’s first law). If the applied voltage is V and the charge in the where I = I1 + I2 + I3 + I4 + I5 + I6 (Kirchhoff’s ﬁrst law). If the applied voltage is V and the charge in capacitor is Q, then according to Kirchhoff’s second law, the differential equations for the the capacitor is Q, then according to Kirchhoff’s second law, the differential equations for the loops loops are [23], are [29], From the loop ABEA in the circuit model, From the loop ABEA in the circuit model, IR + + × =V, Q dI1 IR + + L1 × = V, After the Laplace transformation, C1 dt

After the Laplace transformation, IR + + =V, (9) I From the loop BEDB in the circuit IRmodel,+ 1 + L SI = V, (9) SC 1 1 1 + × − − × − =0, From the loop BEDB in the circuit model, After the Laplace transformation, Q dI Q dI Q 1 4 +L1 × − − L4 × − = 0, + − − − =0, C1 dt C4 dt C3 After the Laplace transformation,

I1 I4 I3 + L1SI1 − − L4SI4 − = 0, SC1 SC4 SC3 1 I1 I4 I3 − − + S(L1 I1 − L4 I4) = 0, (10) S C1 C4 C3 From the loop CEFC in the circuit model,

dI2 Q dI5 Q Q L2 × + − L5 × − − = 0, dt C2 dt C5 C3

After the Laplace transformation,

I2 I5 I3 + L2SI2 − − L5SI5 − = 0, SC2 SC5 SC3 1 I2 I3 I5 − − + S(L2 I2 − L5 I5) = 0, (11) S C2 C3 C5 Appl. Sci. 2017, 7, 1071 9 of 21

From the loop DED in the circuit model,

dI4 Q dI6 L4 × + − L6 × = 0, dt C4 dt

After the Laplace transformation,

I4 L4SI4 + − L6SI6 = 0, SC4

I4 + S(L4 I4 − L6 I6) = 0, (12) SC4 From the loop FEF in the circuit model,

Q dI5 dI6 + L5 × − L6 × = 0, C5 dt dt

After the Laplace transformation,

I5 L5SI5 + − L6SI6 = 0, SC5

I5 + S(L5 I5 − L6 I6) = 0, (13) SC5

Similarly, the current passes through L6,

dI L × 6 = 0, 6 dt After Laplace transformation, L6SI6 = 0, (14) Therefore, from Equations (9)–(14), the corresponding current and ﬁeld calculation can be obtained for the proposed unit cell.

5. Results Analysis The current densities of the unit cell and 1 × 2 array at 12.94 GHz are shown in Figure7a,c. Due to the dissimilar geometry of the meta-atom structure, the currents flow in opposite directions. In the resonator, opposite currents are flowing in the inner and outer surfaces, which causes the stop band at this frequency by cancelling the field. However, at 12.80 GHz, a strong electric field is also observed for the unit cell and 1 × 2 array structure in Figure7b,d. Appl. Sci. 2017, 7, 1071 10 of 21 Appl. Sci. 2017, 7, 1071 9 of 19

(a) (b)

(c)

(d)

FigureFigure 7. 7.Surface Surface current current at at 12.94 12.94 GHz GHz of: of:( (aa)) singlesingle unitunit cell and ( (cc)) 1 1 ×× 2 2array array structure. structure. Numerical Numerical electricelectric ﬁeld field distribution distribution inin thethe XYXY planeplane atat 12.9412.94 GHzGHz for: ( (bb) )single single unit unit cell cell and and (d ()d )1 1× ×2 2 arrayarray structure. structure.

5.1.5.1. Unit Unit Cell Cell Analysis Analysis

In Figure 8a,b, both the numerical and experimental reflection (S11) and transmission (S21) In Figure8a,b, both the numerical and experimental reflection ( S11) and transmission (S21) coefficients are shown. In Figure 8a, the simulation and experimental resonance frequencies of the coefficients are shown. In Figure8a, the simulation and experimental resonance frequencies of reflection coefficient are almost the same at 3.7 GHz and 8.6 GHz of the unit cell. In addition, the the reflection coefficient are almost the same at 3.7 GHz and 8.6 GHz of the unit cell. In addition, measured resonance of the reflectance of the array structure at 4.21 GHz (magnitude of 23 dB) and the measured resonance of the reflectance of the array structure at 4.21 GHz (magnitude of 23 dB) and 8.60 GHz (magnitude of 12 dB) is shown in the same Figure 8a. Figure 8b shows the triple-band 8.60 GHz (magnitude of 12 dB) is shown in the same Figure8a. Figure8b shows the triple-band resonance of the simulation at 3.48 GHz, 7.60 GHz and 12.84 GHz in the transmittance curve. The resonancemeasured of results the simulation exhibit resonance at 3.48 GHz,at 3.53 7.60 GHz GHz (S-band), and 12.84 7.54 GHzGHz in(C-band) the transmittance and 12.94 GHz curve. The(Ku-band) measured in Figure results 8b exhibit for the resonance unit cell. The at values 3.53 GHz of the (S-band), resonances 7.54 at GHz3.53 GHz, (C-band) 7.54 GHz and and 12.94 12.94 GHz (Ku-band)GHz are − in14.95 Figure dB, 8−b17.56 for dB the and unit −29.91 cell. dB, The respectively. values of Moreover, the resonances the measured at 3.53 transmittance GHz, 7.54 GHz of Appl. Sci. 2017, 7, 1071 11 of 21

Appl. Sci. 2017, 7, 1071 10 of 19 and 12.94 GHz are −14.95 dB, −17.56 dB and −29.91 dB, respectively. Moreover, the measured thetransmittance array structure of the exhibits array structure the resonance exhibits freque the resonancency peaks frequencyat 4.0 GHz peaks (magnitude at 4.0 GHz of 16 (magnitude dB), 7.41 GHzof 16 (magnitude dB), 7.41 GHz of (magnitude21 dB), 8.10 ofGHz 21 (magnitude dB), 8.10 GHz of (magnitude17.50 dB), 10.30 of 17.50 GHz dB), (magnitude 10.30 GHz of (magnitude12 dB) and 12.41of 12 dB)GHz and (magnitude 12.41 GHz of (magnitude 23 dB) in Figure of 23 dB)8b. inHowever, Figure8b. owing However, to the owing measurement to the measurement or fabrication or errors,fabrication the measured errors, the result measured frequencies result frequenciesare slightly areshifted slightly and shifted the magnitude and the magnitudeextended compared extended withcompared the simulated with the results. simulated results.

(a) (b)

Figure 8. Magnitude (in dB) of: (a) the reflection coefficient (S11) and (b) the transmission coefficient (S21). Figure 8. Magnitude (in dB) of: (a) the reflection coefficient (S11) and (b) the transmission coefficient (S21). Figure 9 shows the unit cell amplitudes of the permittivity, permeability and refractive index at 0-degree,Figure 90-degree,9 shows the180-degree unit cell and amplitudes 270-degree of the rota permittivity,tions. Figure permeability 9a demonstrations and refractive the negative index permeabilityat 0-degree, 90-degree,from 7.85–14 180-degree GHz (bandwidth and 270-degree of 6.15 rotations.GHz), negative Figure permittivity9a demonstrations from 4.60–7.70 the negative GHz (bandwidthpermeability of from 3.10 7.85–14 GHz) and GHz 8.30 (bandwidth to 10.36 GHz of 6.15 (bandwidth GHz), negative of 2.06 permittivity GHz) and fromnegative 4.60–7.70 refractive GHz index(bandwidth from 4.50 of 3.10to 7.50 GHz) GHz and (bandwidth 8.30 to 10.36 of 3.0 GHz GHz), (bandwidth 7.80 to 10.10 of GHz 2.06 GHz)(bandwidth and negative of 2.30 GHz) refractive and 12.60–14index from GHz 4.50 (bandwidth to 7.50 GHz of 1.40 (bandwidth GHz) at ofzero-degree 3.0 GHz), rotation. 7.80 to 10.10 In addition, GHz (bandwidth Figure 9b ofdisplays 2.30 GHz) the permeabilityand 12.60–14 from GHz (bandwidth7.79–14 GHz of (bandwidth 1.40 GHz) at of zero-degree 6.21 GHz), rotation. permittivity In addition, from 4.11–7.53 Figure9b GHz displays and 8–10.20the permeability GHz. However, from 7.79–14 the refractive GHz (bandwidth index is from of 6.21 3.96–7.25 GHz), GHz permittivity and 7.68–9.60 from 4.11–7.53 GHz at 90-degree GHz and rotation.8–10.20GHz. Furthermore, However, in the Figure refractive 9c, the index unit iscell from at 180-degree 3.96–7.25 GHz rotation and 7.68–9.60exhibits permeability GHz at 90-degree from 7.90–14rotation. GHz, Furthermore, permittivity in Figurefrom 94.66–7.12c, the unit GHz cell and at 180-degree 8.38–10.39 rotation GHz and exhibits a refractive permeability index fromfrom 4.53–7.477.90–14 GHz, GHz and permittivity 7.80–10.13 from GHz. 4.66–7.12 Additionally, GHz andpermeability 8.38–10.39 from GHz 7.80–14 and aGHz, refractive permittivity index from 4.11–7.534.53–7.47 GHz GHz and and 8.03–10.17 7.80–10.13 GHz GHz. and Additionally, a refractive permeability index from 3.59–7.25 from 7.80–14 GHz GHz, and 7.68–9.59 permittivity GHz from are shown4.11–7.53 in GHzFigure and 9d 8.03–10.17for 270-degree GHz rotation and a refractive of the proposed index from meta-atom 3.59–7.25 unit GHz cell. and Here, 7.68–9.59 it is revealed GHz are thatshown there in Figure is a variation9d for 270-degree between rotation the effective of the proposedmedium meta-atomparameters unit because cell. Here, the properties it is revealed of thatthe effectivethere is a variationmedium betweenparameters the effectiveare mostly medium affected parameters by the becausepolarization the properties owing to of the the effectiveinternal architecturemedium parameters of the mate are atom. mostly affected by the polarization owing to the internal architecture of the mate atom.

(a) (b) Appl. Sci. 2017, 7, 1071 10 of 19 the array structure exhibits the resonance frequency peaks at 4.0 GHz (magnitude of 16 dB), 7.41 GHz (magnitude of 21 dB), 8.10 GHz (magnitude of 17.50 dB), 10.30 GHz (magnitude of 12 dB) and 12.41 GHz (magnitude of 23 dB) in Figure 8b. However, owing to the measurement or fabrication errors, the measured result frequencies are slightly shifted and the magnitude extended compared with the simulated results.

(a) (b)

Figure 8. Magnitude (in dB) of: (a) the reflection coefficient (S11) and (b) the transmission coefficient (S21).

Figure 9 shows the unit cell amplitudes of the permittivity, permeability and refractive index at 0-degree, 90-degree, 180-degree and 270-degree rotations. Figure 9a demonstrations the negative permeability from 7.85–14 GHz (bandwidth of 6.15 GHz), negative permittivity from 4.60–7.70 GHz (bandwidth of 3.10 GHz) and 8.30 to 10.36 GHz (bandwidth of 2.06 GHz) and negative refractive index from 4.50 to 7.50 GHz (bandwidth of 3.0 GHz), 7.80 to 10.10 GHz (bandwidth of 2.30 GHz) and 12.60–14 GHz (bandwidth of 1.40 GHz) at zero-degree rotation. In addition, Figure 9b displays the permeability from 7.79–14 GHz (bandwidth of 6.21 GHz), permittivity from 4.11–7.53 GHz and 8–10.20 GHz. However, the refractive index is from 3.96–7.25 GHz and 7.68–9.60 GHz at 90-degree rotation. Furthermore, in Figure 9c, the unit cell at 180-degree rotation exhibits permeability from 7.90–14 GHz, permittivity from 4.66–7.12 GHz and 8.38–10.39 GHz and a refractive index from 4.53–7.47 GHz and 7.80–10.13 GHz. Additionally, permeability from 7.80–14 GHz, permittivity from 4.11–7.53 GHz and 8.03–10.17 GHz and a refractive index from 3.59–7.25 GHz and 7.68–9.59 GHz are shown in Figure 9d for 270-degree rotation of the proposed meta-atom unit cell. Here, it is revealed that there is a variation between the effective medium parameters because the properties of the effective medium parameters are mostly affected by the polarization owing to the internal Appl. Sci. 2017, 7, 1071 12 of 21 architecture of the mate atom.

Appl. Sci. 2017, 7, 1071 11 of 19 (a) (b)

Figure 9. Amplitude of the permittivity, permeability and refractive index at: (a) 0°, (b) 90°, (c) 180°, Figure 9. Amplitude of the permittivity, permeability and refractive index at: (a) 0◦,(b) 90◦,(c) 180◦, (d) 270° rotations. (d) 270◦ rotations.

According to the left-handed or double-negative characteristics, the refractive index will be According to the left-handed or double-negative characteristics, the refractive index will be negative, if the unit cell permittivity and permeability appear negative simultaneously. From negative, if the unit cell permittivity and permeability appear negative simultaneously. From Figure9, Figure 9, the unit cell structure exhibits left-handed characteristics at every rotation angle at 9.20 the unit cell structure exhibits left-handed characteristics at every rotation angle at 9.20 GHz, GHz, which are shown in Table 2. There is a variation of the effective medium ratio between which are shown in Table2. There is a variation of the effective medium ratio between zero-degrees zero-degrees and 90-degrees. Actually, the electric and magnetic fields are closely related and and 90-degrees. Actually, the electric and magnetic ﬁelds are closely related and propagate as propagate as an electromagnetic wave. The electromagnetic wave is applied to the proposed an electromagnetic wave. The electromagnetic wave is applied to the proposed meta-atom in the meta-atom in the Z-direction. After application of the electromagnetic wave, the inner particles of Z-direction. After application of the electromagnetic wave, the inner particles of the meta-atom the meta-atom structure start oscillating up and down, as well as toward the vibration. However, at structure start oscillating up and down, as well as toward the vibration. However, at time t = 0, time t = 0, T/2, there is a maximum separation of charge, where the negative charges are at the top T/2, there is a maximum separation of charge, where the negative charges are at the top and the and the positive charges are at the bottom, and a maximum amplitude is produced at lower positive charges are at the bottom, and a maximum amplitude is produced at lower frequencies. frequencies. At t = T/4, 3T/4, there is a maximum charge separation, either positive or negative, and At t = T/4, 3T/4, there is a maximum charge separation, either positive or negative, and the amplitude the amplitude at higher frequencies. at higher frequencies. Table 2. Real values of ε, μ and n at 9.20 GHz. Table 2. Real values of ε, µ and n at 9.20 GHz. EMR, effective medium ratio. Structure Rotation Angle ε μ n EMR (/) Meta-Atom Type Structure Rotation0° Angle −4.3ε −16.1 µ n−8.4 EMR (8.5λ/a ) Meta-Atom Type 90°0◦ −−2.34.3 −−17.816.1 −8.4−6.4 8.54.0 Unit cell 90◦ −2.3 −17.8 −6.4 4.0 Left-Handed Unit cell 180° −4.5 −16.1 −8.5 8.5 Left-Handed 180◦ −4.5 −16.1 −8.5 8.5 270° −2.3 −17.8 −6.4 4.0 270◦ −2.3 −17.8 −6.4 4.0

5.2. Meta-Atom Array Configurations The performances of the 1 × 2, 2 × 2, 3 × 3 and 4 × 4 open array configurations are analyzed in this section under 0°, 90°, 180° and 270° rotation structures.

5.2. Meta-Atom Array Conﬁgurations The performances of the 1 × 2, 2 × 2, 3 × 3 and 4 × 4 open array conﬁgurations are analyzed in this section under 0◦, 90◦, 180◦ and 270◦ rotation structures.

5.2.1. 1 × 2 Array Analysis Figure 10 shows the effective medium parameter amplitudes at the different rotations of the 1 × 2 array geometry. The amplitudes of the permittivity, permeability and refractive index are plotted in the same Figure for every rotation. Figure 10a shows permittivity from 4.64–7.70 GHz (bandwidth of 3.06 GHz) and 8.36–10.36 GHz (bandwidth of 2 GHz), permeability from 7.89–14 GHz (bandwidth of 6.11 GHz) and a refractive index from 4.50–7.47 GHz, 7.80–10.11 GHz and 12.62–14 GHz at zero-degree rotation. Figure 10b exhibits permittivity from 4–7.19 GHz (bandwidth of 3.19 GHz) and 7.55–10.02 GHz (bandwidth of 2.47 GHz), permeability from 7.60–14 GHz (bandwidth of 6.40 GHz) and a refractive index from 3.86–6.86 GHz (bandwidth of 3 GHz) and 7.56–9.45 GHz (bandwidth of 1.89Appl. GHz) Sci. 2017 at, 7,90-degree 1071 rotation. Furthermore, in Figure 10c at 180-degree rotation, the12unit of 19 cell displays permittivity from 3.34–3.54 GHz (bandwidth of 0.20 GHz), 4.58–7.72 GHz (bandwidth of 180-degree rotation, the unit cell displays permittivity from 3.34–3.54 GHz (bandwidth of 0.20 GHz), 3.14 GHz) and 8.42–10.41 GHz (bandwidth of 1.99 GHz); permeability from 7.90–14 GHz (bandwidth of 4.58–7.72 GHz (bandwidth of 3.14 GHz) and 8.42–10.41 GHz (bandwidth of 1.99 GHz); permeability 6.10 GHz); and a refractive index from 4.42–7.48 GHz, 7.83–10.11 GHz and 12.54–14 GHz. Additionally, from 7.90–14 GHz (bandwidth of 6.10 GHz); and a refractive index from 4.42–7.48 GHz, permittivity7.83–10.11 of GHz 4.01–7.19 and 12.54–14 GHz (bandwidth GHz. Additionally, of 3.18 GHz) perm andittivity 7.55–10.02 of 4.01–7.19 GHz GHz (bandwidth (bandwidth of of 2.47 3.18 GHz); permeabilityGHz) and from7.55–10.02 7.55–14 GHz GHz (bandwidth (bandwidth of 2.47 of GH 6.45z); GHz); permeability and a refractive from 7.55–14 index GHz from (bandwidth 3.87–6.84 of GHz, 7.59–9.456.45 GHz); GHz and and a refractive 11–11.25 index GHz from are shown 3.87–6.84 in GHz, Figure 7.59–9.45 10d for GHz 270-degree and 11–11.25 rotation GHz ofare the shown proposed in meta-atomFigure 10d unit for cell. 270-degree At 9.20 GHz, rotation the permittivity, of the prop permeabilityosed meta-atom and unit refractive cell. At index 9.20 curvesGHz, displaythe negativepermittivity, peaks. Aspermeability a result, the and array refractive structure index appearances curves display as double-negativenegative peaks. As meta-atoms a result, the are array shown in Tablestructure3. appearances as double-negative meta-atoms are shown in Table 3.

(a) (b)

FigureFigure 10. 10.Amplitude Amplitude of of the the permittivity, permittivity, permeability permeability and refractive refractive index index at: at: (a) ( a0°,) 0(◦b,() 90°,b) 90 (c◦) ,(180°,c) 180 ◦, (d) 270(d) ◦270°rotation rotation of of the the 1 ×1 ×2 2 array array structure.structure.

Table 3. Real values of ε, μ and n at 9.20 GHz.

Structure Rotation Angle ε μ n EMR (/) Meta-Atom Type 0° −4.3 −16.1 −8.4 8.6 90° −1.8 −18.6 −5.9 4.3 1 × 2 Array Left-Handed 180° −4.6 −16.1 −8.6 8.6 270° −1.8 −18.6 −5.9 4.3

5.2.2. 2 × 2 Array Analysis The same methodology is applied to investigate the 2 × 2 array configuration at 0-degree, 90-degree, 180-degree and 270-degree rotations. It is visible from Figure 11a that there is a negative value from 7.84–14 GHz for permeability; a negative value from 4.56–7.79 GHz, 8.13–8.69 GHz and 8.92–10.36 GHz for permittivity; and a negative value from 4.48–6.68 GHz, 7.80–10.22 GHz and 12.22–14 GHz for the refractive index at zero-degree rotation. In addition, Figure 11b exhibits Appl. Sci. 2017, 7, 1071 14 of 21

Table 3. Real values of ε, µ and n at 9.20 GHz.

Structure Rotation Angle ε µ n EMR (λ/a) Meta-Atom Type 0◦ −4.3 −16.1 −8.4 8.6 90◦ −1.8 −18.6 −5.9 4.3 1 × 2 Array Left-Handed 180◦ −4.6 −16.1 −8.6 8.6 270◦ −1.8 −18.6 −5.9 4.3

5.2.2. 2 × 2 Array Analysis The same methodology is applied to investigate the 2 × 2 array conﬁguration at 0-degree, 90-degree, 180-degree and 270-degree rotations. It is visible from Figure 11a that there is a negative value from 7.84–14 GHz for permeability; a negative value from 4.56–7.79 GHz, 8.13–8.69 GHz and 8.92–10.36 GHz for permittivity; and a negative value from 4.48–6.68 GHz, 7.80–10.22 GHz and 12.22–14 GHz for the refractive index at zero-degree rotation. In addition, Figure 11b exhibits negative values from 7.74–14 GHz for permeability; negative values from 3.96–8.63 GHz and 8.74–10.13 GHz for permittivity; and negative values from 4.31–6.48 GHz, 6.98–7.52 GHz and 7.76–9.64 GHz for the refractive index at 90-degree rotation. Furthermore, in Figure 11c at 180-degree rotation, the unit cell displays negative permeability from 7.85–14 GHz; negative permittivity from 4.59–7.74 GHz and 8.94–10.60 GHz; and a negative refractive index from 4.48–6.70 GHz, 7.82–10.28 GHz and 12.27–14 GHz. Additionally, negative permeability from 7.77–14 GHz, negative permittivity from 4–10.16 GHz and a negative refractive index from 4.40–6.52 GHz and 7.77–9.69 GHz are shown in Figure 11d for 270-degree rotation of the proposed meta-atom unit cell. As the size of the structure increases, the resonance frequencies decrease, and the number of resonances also increases. The overall size of the structure is responsible for creating a larger inductive effect. With increasing inductance, the resonance frequency shifts to lower frequencies. In addition, splits and gaps are formed by the capacitive effect. As the number of unit cells varies, the number of splits and gaps between the resonators increases with the overall size of the unit cell. With the increasing number of splits and gaps, more capacitance and resonance points are formed. In addition, if the inductive and capacitive effects increase, the inductance and capacitance minimize each other and create more stop bands and resonance points. Moreover, the resonances also depend on the material’s internal structure. When EM-waves propagate through a material, its electric and magnetic ﬁelds oscillate in a sinusoidal pattern, and their velocity depends on the material’s electrical conductivity, which in turn depends on the material’s internal structure. The relative speed of electrical signals traveling through a material varies according to how they interact with its internal structure. Moreover, from Table4, the effective medium ratio at zero-degree and 180-degree rotation is 8.6 and 4.5 for 90-degree and 270-degree rotation. Appl. Sci. 2017, 7, 1071 13 of 19 negative values from 7.74–14 GHz for permeability; negative values from 3.96–8.63 GHz and 8.74–10.13 GHz for permittivity; and negative values from 4.31–6.48 GHz, 6.98–7.52 GHz and 7.76–9.64 GHz for the refractive index at 90-degree rotation. Furthermore, in Figure 11c at 180-degree rotation, the unit cell displays negative permeability from 7.85–14 GHz; negative permittivity from 4.59–7.74 GHz and 8.94–10.60 GHz; and a negative refractive index from 4.48–6.70 GHz, 7.82–10.28 GHz and 12.27–14 GHz. Additionally, negative permeability from 7.77–14 GHz, negative permittivity from 4–10.16 GHz and a negative refractive index from 4.40–6.52 GHz and 7.77–9.69 GHz are shown in Figure 11d for 270-degree rotation of the proposed meta-atom unit cell. As the size of the structure increases, the resonance frequencies decrease, and the number of resonances also increases. The overall size of the structure is responsible for creating a larger inductive effect. With increasing inductance, the resonance frequency shifts to lower frequencies. In addition, splits and gaps are formed by the capacitive effect. As the number of unit cells varies, the number of splits and gaps between the resonators increases with the overall size of the unit cell. With the increasing number of splits and gaps, more capacitance and resonance points are formed. In addition, if the inductive and capacitive effects increase, the inductance and capacitance minimize each other and create more stop bands and resonance points. Moreover, the resonances also depend on the material’s internal structure. When EM-waves propagate through a material, its electric and magnetic fields oscillate in a sinusoidal pattern, and their velocity depends on the material's electrical conductivity, which in turn depends on the material's internal structure. The relative speed of electrical signals traveling through a material varies according to how they interact with its internal structure. Moreover, from Table 4, the effective medium ratio at zero-degree and 180-degree rotation is 8.6 and 4.5 for 90-degreeAppl. Sci. 2017 and, 7, 270-degree 1071 rotation. 15 of 21

(a) (b)

FigureFigure 11. 11. AmplitudeAmplitude of of the the permittivity, permittivity, permeability permeability and and refractive refractive index index at: at: ( (aa)) 0°, 0◦ ,((bb) )90°, 90◦ ,((c)c )180°, 180◦ , (d(d) )270° 270◦ rotationsrotations of of the the 2 2 ×× 2 2array array structure. structure.

Table 4. Real values of ε, µ and n at 9.20 GHz.

Structure Rotation Angle ε µ n EMR (λ/a) Meta-Atom Type 0◦ −6.5 −16.1 −10.4 8.6 90◦ −2.6 −17.5 −6.8 4.5 2 × 2 Array Left-Handed 180◦ −7.1 −16.1 −10.8 8.6 270◦ −2.9 −17.2 −7.2 4.5

5.2.3. 3 × 3 Array Analysis From Figure 12a, the frequency spans of the negative effective permittivity, permeability and refractive index of the array structure are from 4.58–7.72 GHz and 8.42–10.40 GHz; 7.90–14 GHz; and 4.42–7.48 GHz, 7.83–10.10 GHz and 12.54–14 GHz, respectively. Moreover, in Figure 12b at 90-degree rotation, the frequency spans of the permittivity, permeability and refractive index are from 4–7.19 GHz and 7.55–10.02 GHz; 7.55–14 GHz; and 3.86–6.86 GHz and 7.56–9.45 GHz, respectively. At 180-degree rotation, the frequency spans are from 4.58–7.72 GHz and 8.42–10.41 GHz (permittivity); 7.90–14 GHz (permeability); and 4.42–7.48 GHz, 7.83–10.11 GHz and 10.80–11.32 GHz (refractive index), as explained in Figure 12c. However, Figure 12d shows that the negative permittivity, permeability and refractive index are sequentially from 4–7.19 GHz and 7.55–10 GHz; 7.55–14 GHz; 3.87–6.84 GHz and 7.59–9.45 GHz and left handed properties in Table5. Appl. Sci. 2017, 7, 1071 14 of 19

Table 4. Real values of ε, μ and n at 9.20 GHz.

Structure Rotation Angle ε μ n EMR (/) Meta-Atom Type 0° −6.5 −16.1 −10.4 8.6 90° −2.6 −17.5 −6.8 4.5 2 × 2 Array Left-Handed 180° −7.1 −16.1 −10.8 8.6 270° −2.9 −17.2 −7.2 4.5

5.2.3. 3 × 3 Array Analysis From Figure 12a, the frequency spans of the negative effective permittivity, permeability and refractive index of the array structure are from 4.58–7.72 GHz and 8.42–10.40 GHz; 7.90–14 GHz; and 4.42–7.48 GHz, 7.83–10.10 GHz and 12.54–14 GHz, respectively. Moreover, in Figure 12b at 90-degree rotation, the frequency spans of the permittivity, permeability and refractive index are from 4–7.19 GHz and 7.55–10.02 GHz; 7.55–14 GHz; and 3.86–6.86 GHz and 7.56–9.45 GHz, respectively. At 180-degree rotation, the frequency spans are from 4.58–7.72 GHz and 8.42–10.41 GHz (permittivity); 7.90–14 GHz (permeability); and 4.42–7.48 GHz, 7.83–10.11 GHz and 10.80–11.32 GHz (refractive index), as explained in Figure 12c. However, Figure 12d shows that the negative permittivity, permeability and refractive index are sequentially from 4–7.19 GHz and 7.55–10 GHz; Appl. Sci. 2017, 7, 1071 16 of 21 7.55–14 GHz; 3.87–6.84 GHz and 7.59–9.45 GHz and left handed properties in Table 5.

(a) (b)

Figure 12. Amplitude of the permittivity, permeability and refractive index at: (a) 0°, (b) 90°, (c) 180°, Figure 12. Amplitude of the permittivity, permeability and refractive index at: (a) 0◦,(b) 90◦,(c) 180◦, (d) 270° rotations of the 3 × 3 array structure. (d) 270◦ rotations of the 3 × 3 array structure.

Table 5. Real values of ε, µ and n at 9.20 GHz.

Structure Rotation Angle ε µ n EMR (λ/a) Meta-Atom Type 0◦ −4.6 −16.2 −8.7 8.5 90◦ −1.8 −18.6 −5.9 4.0 3 × 3 Array Left-Handed 180◦ −4.6 −16.1 −8.6 8.5 270◦ −1.8 −18.6 −5.9 4.0

5.2.4. 4 × 4 Array Analysis In Figure 13, the effective medium parameters are depicted at 0◦, 90◦, 180◦ and 270◦ rotations. From Figure 14a, the permeability is from 7.79–14 GHz, covering almost 6.21 GHz in bandwidth; the permittivity is from 4.46–7.58 GHz, covering approximately 3.12 GHz in bandwidth; and the refractive index is from 4.44–6.54 GHz and 7.73–9.86 GHz, covering almost 2.10 GHz and 2.13 GHz for zero-degree rotation. In Figure 14d, the permeability ranges from 7.74–14 GHz, covering almost 6.26 GHz in bandwidth; the permittivity ranges from 4–7.53 GHz and 7.90–10 GHz, covering bandwidths of approximately 3.53 GHz and 2.10 GHz; and the refractive index ranges from 5–7.20 GHz (2.20 GHz in bandwidth) and 7.71–9.63 GHz (1.92 GHz in bandwidth) for 270-degree rotation. Appl. Sci. 2017, 7, 1071 15 of 19

Table 5. Real values of ε, μ and n at 9.20 GHz.

Structure Rotation Angle ε μ n EMR (/) Meta-Atom Type 0° −4.6 −16.2 −8.7 8.5 90° −1.8 −18.6 −5.9 4.0 3 × 3 Array Left-Handed 180° −4.6 −16.1 −8.6 8.5 270° −1.8 −18.6 −5.9 4.0

5.2.4. 4 × 4 Array Analysis In Figure 13, the effective medium parameters are depicted at 0°, 90°, 180° and 270° rotations. From Figure 14a, the permeability is from 7.79–14 GHz, covering almost 6.21 GHz in bandwidth; the permittivity is from 4.46–7.58 GHz, covering approximately 3.12 GHz in bandwidth; and the refractive index is from 4.44–6.54 GHz and 7.73–9.86 GHz, covering almost 2.10 GHz and 2.13 GHz for zero-degree rotation. In Figure 14d, the permeability ranges from 7.74–14 GHz, covering almost Appl. Sci. 2017, 7, 1071 17 of 21 6.26 GHz in bandwidth; the permittivity ranges from 4–7.53 GHz and 7.90–10 GHz, covering bandwidths of approximately 3.53 GHz and 2.10 GHz; and the refractive index ranges from 5–7.20 GHz (2.20 GHz in bandwidth) and 7.71–9.63 GHz (1.92 GHz in bandwidth) for 270-degree Therefore, it is notable that the bandwidth of the permeability is always more than 6 GHz for all of the rotation. Therefore, it is notable that the bandwidth of the permeability is always more than 6 GHz rotationfor angles. all of the The rotation array structureangles. The also array shows structure left-handed also shows characteristics left-handed characteristics at 9.20 GHz, at with9.20 GHz, an effective mediumwith ratio an of effective more thanmedium 8 for ratio 0-degree of more andthan 180-degree 8 for 0-degree and and 4 180-degree for 90-degree and 4 and for 90-degree 270-degree and rotation, as shown270-degree in Table rotation,6. as shown in Table 6.

(a) (b)

Figure 13. Amplitude of the permittivity, permeability and refractive index at: (a) 0°, (b) 90°, (c) 180°, ◦ ◦ ◦ Figure 13.(d) Amplitude270° rotations of of thethe 4 permittivity, × 4 array structure. permeability and refractive index at: (a) 0 ,(b) 90 ,(c) 180 , (d) 270◦ rotations of the 4 × 4 array structure.

Table 6. Real values of ε, µ and n at 9.20 GHz.

Structure Rotation Angle ε µ n EMR (λ/a) Meta-Atom Type 0◦ −3.3 −17.3 −7.6 8.5 90◦ −2.1 −18.2 −6.1 4.0 4 × 4 Array Left-Handed 180◦ −3.5 −17.1 −7.8 8.4 270◦ −1.7 −18.9 −5.7 4.0

f Q = 0 , (15) ∆ f It can be observed that the Q-factor is high, and this means that the reported meta-atom has high sensibility. The value of the Q-factor depends on the sharpness of the transmission and the location of resonant frequency. The reported antenna has been designed to use for long distance Appl. Sci. 2017, 7, 1071 16 of 19

Table 6. Real values of ε, μ and n at 9.20 GHz.

Structure Rotation Angle ε μ n EMR (/) Meta-Atom Type 0° −3.3 −17.3 −7.6 8.5 90° −2.1 −18.2 −6.1 4.0 4 × 4 Array Left-Handed 180° −3.5 −17.1 −7.8 8.4 270° −1.7 −18.9 −5.7 4.0

The sensitivity of the proposed meta-atom has been explained in terms of the Q-factor. The bandwidth of the meta-atom is inversely related to the quality factor (Q-factor). The calculation of the Q factor has been performed using the 3-dB bandwidth of resonant frequency (Δf) and the resonant frequency (f0) that has a resonance peak under −10 dB. = , (15) Appl. Sci. 2017, 7, 1071 18 of 21 It can be observed that the Q-factor is high, and this means that the reported meta-atom has high sensibility. The value of the Q-factor depends on the sharpness of the transmission and the telecommunicationlocation of resonant (C-band). frequency. This The antenna reported is made antenna of threehas been layers; designed they areto use radiating for long patch, distance antenna substratetelecommunication and ground plane. (C-band). The This ground antenna plane is made is printed of three on layers; the lower they are part radiating of the substrate, patch, antenna where on the uppersubstrate portion and ground of the plane. substrate, The ground the radiating plane is patchprinted is on printed the lower using part a of microstrip the substrate, line where feeding in Figureon 14 thea,b. upper The portion width of and thelength substrate, of thethe radiating microstrip patch line is are printed stable using with a microstrip a view to line attaining feeding the in 50 Ω inputFigure impedance. 14a,b. The The width port and of the length microstrip of the microstrip feed line line is attached are stable to with a Sub-Miniature a view to attaining Version the 50 A Ω (SMA) connector.input impedance. To attain the The adequate port of the wide microstrip bandwidth, feed line multi is attached frequency to bandsa Sub-Miniature and high Version performances, A (SMA) connector. To attain the adequate wide bandwidth, multi frequency bands and high the ground plane of the proposed antenna has been replaced by the meta-atoms. Moreover, after performances, the ground plane of the proposed antenna has been replaced by the meta-atoms. replacing the antenna ground plane by the meta-atoms, then the SMA connector is connected with the Moreover, after replacing the antenna ground plane by the meta-atoms, then the SMA connector is partialconnected ground with integrated the partial with ground the meta-atoms integrated with shown the meta-atoms in Figure 14 shownc,d. in Figure 14c,d.

(a) (b)(c)(d)

Figure 14. The geometry of the: (a) antenna transparent 3D view; (b) ground plane without Figure 14. The geometry of the: (a) antenna transparent 3D view, (b) ground plane without integrated integrated meta-atom back view; (c) antenna transparent 3D view of integrated meta-atom with meta-atom back view, (c) antenna transparent 3D view of integrated meta-atom with ground plane, ground plane; (d) ground plane replaced by integrated meta-atom, back view. (d) ground plane replaced by integrated meta-atom, back view. From Figure 15a, the resonance of the return loss of the meta-atom antenna are respectively at 4.22,From 6.30, Figure 7.22, 15 11.34a, the and resonance 13.68 GHz, of whereas the return the bandwidth loss of the are meta-atom sequentially antenna 130 MHz are (from respectively 4.20 to at 4.22,4.33 6.30, GHz), 7.22, 120 11.34 MHz and (from 13.68 6.26 GHz, to 6.38 whereas GHz), 200 the MHz bandwidth (from 7.10 are to sequentially 7.30 GHz), 2.17 130 GHz MHz (from (from 9.93 4.20 to to 12.10 GHz) and 0.9 GHz (from 13.10 to 14 GHz). On the contrary, the designed antenna without 4.33 GHz), 120 MHz (from 6.26 to 6.38 GHz), 200 MHz (from 7.10 to 7.30 GHz), 2.17 GHz (from 9.93 integrating the meta-atom of the ground plane has the resonance at 5.67 and 7.79 GHz. Therefore, to 12.10 GHz) and 0.9 GHz (from 13.10 to 14 GHz). On the contrary, the designed antenna without after integrating the proposed meta-atom by replacing the ground plane, the antenna resonance integratingpeaks and the frequency meta-atom bands of the have ground been planeimproved. has theIn ad resonancedition, in atFigure 5.67 15b, and from 7.79 GHz.the gain Therefore, curve, it after integratingcan also the be commented proposed meta-atom that the antenna by replacing with the themeta-atom ground has plane, achieved the antenna an average resonance gain of almost peaks and frequency2.85 dB bands at the haveresonance been bandwidth improved. regions. In addition, The maximum in Figure peak 15 gainb, from is around the gain 5 dB. curve, it can also be commented that the antenna with the meta-atom has achieved an average gain of almost 2.85 dB at the resonanceAppl. Sci. bandwidth 2017, 7, 1071 regions. The maximum peak gain is around 5 dB. 17 of 19

(a) (b)

FigureFigure 15. Antenna 15. Antenna performances: performances: (a ()a return) return loss loss ((SS1111);), ( (bb) )gain; gain; without without and and with with the themeta-atom meta-atom at at the groundthe ground plane. plane.

Table 7 depicts the quality factors of the proposed meta-atom and that reported in [11,17]. In [11], the sensitivity was 14, and in [17], it was 31; but for the proposed structure, the sensitivity is 82, which is more than the previous one. Besides, for the high quality factor, making of the designed meta-atom is flexible for high-performance filters and sensor applications. Table 8, represents a comparison of the frequency bands and effective medium ratios of the proposed design with the previous design. For the compactness of the meta-atom, an effective medium ratio is an important factor. The proposed meta-atom unit cell exhibits an effective medium ratio (EMR) of 8.50, which is greater than that of the other unit cells. As a result, the designed structure is more suitable and compact in size. Moreover, the proposed meta-atom structure is applicable for S-, C- and Ku-band applications.

Table 7. Quality factor at the resonant frequency.

References Resonant Frequency (f0) Quality Factor (Q) Islam et al. [11] 2.74 GHz 14 Hasan et al. [17] 7.53 GHz 31 Proposed Meta Atom 3.53 GHz 82

Table 8. Performance comparison of the proposed meta-atom with the reported previous research.

References Year Dimensions (mm2) Frequency Band EMR Mallik et al. [10] 2013 25 × 25 C-Band 1.99 Islam et al. [11] 2014 30 × 30 Multi Band 3.64 Zhou et al. [13] 2015 8.5 × 8.5 X-Band 4.80 Yang et al. [5] 2016 5.0 × 5.0 X-Band 6.0 Hasan et al. [15] 2016 10 × 10 C- and X-Band 4.0 Liu et al. [16] 2016 5.0 × 5.0 X-Band 5.45 Hasan et al. [17] 2017 10 × 10 X-Band 2.5 Proposed Structure 2017 10 × 10 S-, C- and Ku-Band 8.50

From Table 9, Mallik et al. discussed 1 × 2 and 2 × 2 arrays at an orthogonal position [10]. Islam et al. presented 2 × 2 array structures for double-negative characteristics and multi-band applications [11]. Hossain et al. showed a 2 × 2 array configuration under open and interconnect conditions. In their framework, they analyzed the effect on resonances by changing the dimensions of the arrays [12]. In this study, the left-handed characteristics, compactness, frequency band and bandwidth of 1 × 2, 2 × 2, 3 × 3 and 4 × 4 arrays are analyzed by 0°, 90°, 180° and 270° rotations. Appl. Sci. 2017, 7, 1071 19 of 21

Table7 depicts the quality factors of the proposed meta-atom and that reported in [ 17,23]. In [17], the sensitivity was 14, and in [23], it was 31; but for the proposed structure, the sensitivity is 82, which is more than the previous one. Besides, for the high quality factor, making of the designed meta-atom is ﬂexible for high-performance ﬁlters and sensor applications. Table8, represents a comparison between the frequency bands and effective medium ratios of the proposed design with the previous design. For the compactness of the meta-atom, an effective medium ratio is an important factor. The proposed meta-atom unit cell exhibits an effective medium ratio (EMR) of 8.50, which is greater than that of the other unit cells. As a result, the designed structure is more suitable and compact in size. Moreover, the proposed meta-atom structure is applicable for S-, C- and Ku-band applications.

Table 7. Quality factor at the resonant frequency.

References Resonant Frequency (f0) Quality Factor (Q) Islam et al. [17] 2.74 GHz 14 Hasan et al. [23] 7.53 GHz 31 Proposed Meta Atom 3.53 GHz 82

Table 8. Performance comparison of the proposed meta-atom with the reported previous research.

References Year Dimensions (mm2) Frequency Band EMR Mallik et al. [16] 2013 25 × 25 C-Band 1.99 Islam et al. [17] 2014 30 × 30 Multi Band 3.64 Zhou et al. [19] 2015 8.5 × 8.5 X-Band 4.80 Yang et al. [5] 2016 5.0 × 5.0 X-Band 6.0 Hasan et al. [21] 2016 10 × 10 C- and X-Band 4.0 Liu et al. [22] 2016 5.0 × 5.0 X-Band 5.45 Hasan et al. [23] 2017 10 × 10 X-Band 2.5 Proposed Structure 2017 10 × 10 S-, C- and Ku-Band 8.50

From Table9, Mallik et al. discussed 1 × 2 and 2 × 2 arrays at an orthogonal position [16]. Islam et al. presented 2 × 2 array structures for double-negative characteristics and multi-band applications [17]. Hossain et al. showed a 2 × 2 array conﬁguration under open and interconnect conditions. In their framework, they analyzed the effect on resonances by changing the dimensions of the arrays [18]. In this study, the left-handed characteristics, compactness, frequency band and bandwidth of 1 × 2, 2 × 2, 3 × 3 and 4 × 4 arrays are analyzed by 0◦, 90◦, 180◦ and 270◦ rotations.

Table 9. Array structural comparison with the reported references.

References Conﬁguration Array Structure Mallik et al. in 2013 [16] U-Shaped 2 × 2 orthogonal array Islam et al. in 2014 [17] H-Shaped 1 × 2, 2 × 2 array Hossain et al. in 2015 [18] G-Shaped 2 × 2 open and interconnect array Proposed Structure I-Shaped 1 × 2, 2 × 2, 3 × 3, 4 × 4 arrays at 0◦, 90◦, 180◦ and 270◦ rotation.

6. Conclusions A new compact left-handed meta-atom for S-, C- and Ku-band applications has been designed, investigated, fabricated and measured in this paper. The unit cell and array structure exhibited left-handed characteristics with wide bandwidths in the major portions of S-, C- and Ku-bands at 0◦, 90◦, 180◦ and 270◦ rotations. For the design, simulation and calculation of the S-parameters of the proposed meta-atom prototype, CST Microwave Studio has been used. Both the horn antenna and waveguide methods have been used for measurement purposes to obtain greater accuracy in the measured results. In addition, S-band is applicable for weather radar monitoring and microwave Appl. Sci. 2017, 7, 1071 20 of 21 devices. C-band is relevant in radio telecommunication and electromagnetic cloaking, and the Ku-band is signiﬁcant for satellite applications.

Acknowledgments: This work was supported by the Research Universiti Grant, Arus Perdana Code: AP-2015-007. Author Contributions: Md. Mehedi Hasan made substantial contributions to the conception, design and analysis. Mohammad Rashed Iqbal Faruque participated in revising the article critically for important intellectual contents. Mohammad Tariqul Islam provided necessary instructions for experimental purposes. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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