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Article A Simple Linear-Type Negative Substrate Patch

Wei-Hua Hui, Yao Guo and Xiao-Peng Zhao *

Department of Applied Physics, School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710129, China; [email protected] (W.-H.H.); [email protected] (Y.G.) * Correspondence: [email protected]

Abstract: A microstrip (MPA) loaded with linear-type negative permittivity metama- terials (NPMMs) is designed. The simple linear-type metamaterials have negative permittivity at 1–10 GHz. Four groups of antennas at different bands are simulated in order to study the effect of linear-type NPMMs on MPA. The antennas working at 5.0 GHz are processed and measured. The measured results illustrate that the gain is enhanced by 2.12 dB, the H-plane half- beam width (HPBW) is converged by 14◦, and the effective area is increased by 62.5%. It can be concluded from the simulation and measurements that the linear-type metamaterials loaded on the substrate of MAP can suppress surface waves and increase forward well.

Keywords: metamaterials; negative permittivity; microstrip patch antenna; gain

  1. Introduction Citation: Hui, W.-H.; Guo, Y.; Zhao, Electromagnetic (EM) metamaterials are composed of periodic, subwavelength arti- X.-P. A Simple Linear-Type Negative ficial structures, which can be resonant or non-resonant units. A two-dimensional form Permittivity Metamaterials Substrate of metamaterials is called a metasurface. According to the uniform effective medium Microstrip Patch Antenna. Materials theory, the properties of these structures can be determined by the effective permittivity 2021, 14, 4398. https://doi.org/ and permeability. When an EM wave is incident on the metamaterials, they often show 10.3390/ma14164398 abnormal physical properties that natural materials do not possess, such as near zero refraction, negative permittivity, negative permeability, or double-negative. Metamaterials Academic Editor: George Kenanakis with a single negative property can suppress the propagation of EM waves. Metamaterials with double negative properties are called left-handed metamaterials (LHMs), obeying Received: 22 June 2021 the left-hand rule. Veselago firstly proposed the concept of metamaterials in 1968 [1]. Accepted: 29 July 2021 Pendy et al. proved the existence of left-handed metamaterials in the frequency Published: 6 August 2021 range in 2001. Since then, metamaterials have attracted a lot of attention in many fields, such as EM stealth cloak [2], perfect absorber [3–5], EM harvest [6–9], sensor, and Publisher’s Note: MDPI stays neutral devices [10–12]. with regard to jurisdictional claims in published maps and institutional affil- Microstrip patch antenna (MPA) has many advantages of light weight, small size, iations. easy processing, low profile, and easy to make dual-band or circular polarization antenna. However, MPA also has some disadvantages including narrow bandwidth, low efficiency, low gain, and narrow 3 dB axial ratio bandwidth, which restrict its integration with modern multifunctional devices [13]. In order to overcome these disadvantages of MPA, researchers have adopted metama- Copyright: © 2021 by the authors. terials with different EM properties to refine its performances [14–18]. Nasimuddin et al. Licensee MDPI, Basel, Switzerland. embedded two-dimensional rectangular-ring units into the substrate of a circularly polar- This article is an open access article distributed under the terms and ized patch antenna, so that the VSWR bandwidth of the antenna reached 35.6%, the 3 dB conditions of the Creative Commons axis ratio is increased by a factor of seven, and the gain is increased by about 3 dB [14]. Attribution (CC BY) license (https:// Singh et al. subsequently proposed a Fabry–Perot structure MPA based on a creativecommons.org/licenses/by/ double annular slot resonator unit cell superstrate, which increases the gain by 12.5 dB [15]. 4.0/). Gao et al. proposed a three-layer high (HRIM) superstrate

Materials 2021, 14, 4398. https://doi.org/10.3390/ma14164398 https://www.mdpi.com/journal/materials Materials 2021, 14, 4398 2 of 8

to enhance the performance of MPA. The gain and bandwidth are enhanced greatly [17]. Then, Borazjani et al. proposed a high-gain broadband MPA composed of four layers of negative refractive index metamaterials superstrate, and the gain is increased by about 10 dB [18]. Although these antennas have been modified notably by adopting various metamaterial superstrates, the high profile limits their practical application. In order to solve the limitation of these antennas’ high profile, our team used metama- terials with negative permeability as the substrate of the MPA to suppress surface waves and successfully improved the performance of MPA [19,20]. However, the effective band- width of negative permeability metamaterials is too narrow, which is only suitable for these antennas with a narrow bandwidth. Subsequently, Zhou et al. adopted a meander-line unit with near-zero permittivity to enhance the of the [21]. Yang et al. employed artificial magnetic conductors (AMCs) under the edge-fed patch antenna to improve the bandwidth and gain [22]. Cao et al. applied parasitic patches around the radiation patch on the substrate, and the effective bandwidth of the parasitic patches is close to 2 GHz [23]. Therefore, it is still a challenge to adopt single metamaterials to improve the EM performance of an ultra-wideband (UWB) or multi-band microstrip antennas. The with electrical continuity has negative permittivity in a wider band, which has great potential in the UWB antenna or multi-band antenna array. In this paper, in order to form an MPA with a negative permittivity substrate, simple linear-type metamaterials units are loaded around the center patch in an annular man- ner. Four patch antennas loaded with negative permittivity metamaterials (NPMMs) are simulated at the L-band, S-band, X-band, and C-band, respectively. A group of X-band antennas are fabricated and measured. The simulated and experimental results consistently reveal that the linear-type NPMMs can improve the performance of MPA effectively in a wide band.

2. Design of Negative Permittivity Metamaterials (NPMMs) Pendry shows that the effective permittivity of the cut-wire medium can be expressed as [24,25]: 2 2 ωp − ω ε(ω) = 1 − 0 (1) 2 2 ω − ω0 + iωΓ

where ωp is the plasma frequency, ω0 is the frequency, which is determined only by the of wire and is independent of the charge, effective mass, and density of , and Γ is system loss. For ω0 < ω < ωp, the permittivity is negative, and there is no EM mode in the structure. The cross-section of the metal wire obtained by the is rectangular, and its plasma frequency can be expressed as: c0 fp = q √ (2) g 2π lng/ mt/π

In this equation, g is the lattice constant, c0 is the in , m is the width of the metal line, and t is the thickness of the metal line. When the metal line metamaterials have electrical continuity, the resonance frequency ω0 is zero. Based on the above theory, the unit of the linear-type metamaterials is shown in Figure1a,b. The structure of the metamaterial is very simple, consisting of a substrate and double-sided etched metal . The substrate is a rectangular polytetrafluoroethylene (PTFE) (εr = 3.5, tan δ = 0.001). The structural parameters are shown as the following: n = 1 mm, g = 5.5 mm, m = 0.5 mm, t = 0.035 mm, h = 2 mm. Then, these parameters are introduced into Equation (2) to calculate the plasma frequency. The plasma frequency fp can be obtained as 10.49 GHz. MaterialsMaterials2021, 14, 2021 4398, 14, x FOR PEER REVIEW 3 of 8 3 of 8

(a) (b)

(c) (d)

Figure 1. Structure andFigure EM 1. propertiesStructure andof linear-type EM properties metamaterials of linear-type unit: metamaterials (a) top view; unit:(b) side (a) topview; view; (c) S11 (b) sideand view;S21; (d) permittivity. (c) S11 and S21; (d) permittivity.

The simulationThe simulation software software high-frequency high-frequency structure stru simulatorcture simulator (HFSS) (HFSS) is employed is employed to to optimizeoptimize the linear-type the linear-type metamaterials. metamaterials. The softwareThe software uses uses the finitethe finite element element method method to to solvesolve the the EM EM model. model. The The electronic electronic boundariesboundaries are parallel parallel to to the the x-axis, x-axis, and and the the magnetic magneticboundaries boundaries are are parallel parallel to tothe the y-axis. y-axis. The The solution solution frequency frequency is 10 is 10GHz, GHz, the the solver solver accuracy accuracyis 0.001, is 0.001, and and the the maximum maximum numb numberer of passes of passes is 20. is 20.The The simulated simulated reflection reflection and and transmis- transmissionsion curves curves of of the the linear-type linear-type metamaterialsmetamaterials areare shown shown in in Figure Figure1c. 1c. It meansIt means that that the the transmissiontransmission forbidden forbidden band band is from is from 1 to 101 to GHz, 10 GHz, and EM and waves EM waves cannot cannot pass through pass through while thewhile metamaterials the metamaterials unit interval unit interval g is 3 mm, g is 5.53 mm, mm, 5.5 7 mm,mm, and7 mm, 9 mm. and 9 mm. In the studyIn the ofstudy metamaterials, of metamaterials, it is generally it is generally described described as effective as effective medium medium theory. theory. When theWhen the wavelength is much is largermuch thanlarger the than size the of size the structure,of the structure, the electromagnetic the electromagnetic wave willwave not will identify not identify its internal its structure,internal structure, so that the so properties that the properties of the metamaterials of the metamaterials can be describedcan be by described effective by permittivity effective permittivity and effective and permeability. effective permeability. According to According the above to the theory,above the permittivity theory, the ofpermittivity linear-type of metamaterials linear-type metamaterials can be calculated can be by calculated the scattering by the scat- parametertering extraction parameter method extraction [26,27 ].method The electromagnetic [26,27]. The electromagnetic wave is a perpendicular wave is a incident perpendicular to the linear-typeincident to metamaterials.the linear-type Themetamaterials. transmission The and transmission reflection can and be reflection obtained can by simu- be obtained lation.by The simulation. refractive indexThe refractive n and wave index impedance n and wave z can impedance be obtained z can by be the obtained transmission by the trans- matrixmission and the matrix S-parameter, and the and S-parameter, the effective and permittivity the effectiveε andpermittivity permeability ε andµ permeabilitycan be μ furthercan obtained be further by Equations obtained (5)by andEquations (6). (5) and (6).   1 1  1 2 2 n = cosn= cos1 − S11 1+ −S21 S +S (3) (3) kd 2Skd21 2S v u 2 2 u (1 + S11)(1− +S S21) −S z = ±t z=± (4) (4) 2 2 (1 − S11)(1− −S S21) −S n ε = n (5) z ε= (5) z µ = nz (6) μ=nz The real and imaginary parts of the permittivity calculated by the scattering parameter (6) extraction method are shown in Figure1d. It denotes that the real part of the permittivity is

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Materials 2021, 14, 4398 4 of 8 The real and imaginary parts of the permittivity calculated by the scattering param- eter extractionThe real and method imaginary are shown parts inof Figurethe permittivity 1d. It denotes calculated that the by real the part scattering of the permit-param- etertivity extraction is negative method from 1are to shown10 GHz. in ThisFigure is basically1d. It denotes consistent that the with real the part result of the calculated permit- negative from 1 to 10 GHz. This is basically consistent with the result calculated according tivityaccording is negative to Equation from (2).1 to 10 GHz. This is basically consistent with the result calculated accordingto Equation to Equation (2). (2). 3.3. Microstrip Microstrip Patch Patch Antenna Antenna Loaded Loaded with with Negative Negative Permittivity Permittivity Metamaterials Metamaterials 3. Microstrip Patch Antenna Loaded with Negative Permittivity Metamaterials TheThe conventional conventional MPA MPA is is composed composed of of a aradiation radiation patch, patch, ground , plane, and and PTFE PTFE The conventional MPA is composed of a radiation patch, ground plane, and PTFE plate.plate. The The detailed detailed stru structurecture is is shown shown in in Figure Figure 2,2, the the parameters parameters of of the the antenna antenna are are given given plate. The detailed structure is shown in Figure 2, the parameters of the antenna are given asas follows: follows: L L= =14.5 14.5 mm, mm, W W = =20 20 mm, mm, gradL gradL = =30 30 mm, mm, gradW gradW = =30 30 mm, mm, subL subL = =115 115 mm, mm, as follows: L = 14.5 mm, W = 20 mm, gradL = 30 mm, gradW = 30 mm, subL = 115 mm, andand subW subW = =115 115 mm. mm. The The antenna antenna is is fed fed by by a a50 50 ΩΩ coaxialcoaxial probe probe with with a aradius radius of of 0.65 0.65 mm mm and subW = 115 mm. The antenna is fed by a 50 Ω coaxial probe with a radius of 0.65 mm alongalong the the x-direction x-direction away away from from the the patch patch center, center, and and the the length length of of L1 L1 is is 3.4 3.4 mm. mm. The The along the x-direction away from the patch center, and the length of L1 is 3.4 mm. The centercenter frequency frequency is is 5.0 5.0 GHz. GHz. The The bandwidth bandwidth is is 230 230 MHz MHz (4.89–5.12 (4.89–5.12 GHz). GHz). The The electric field center frequency is 5.0 GHz. The bandwidth is 230 MHz (4.89–5.12 GHz). The electric field distributiondistribution excited excited by by a surface a wave is shown is shown in Figure in Figure 3a. According3a. According to the to electric the electric field distribution excited by a surface wave is shown in Figure 3a. According to the electric field distribution,field distribution, the linear-type the linear-type NPMMs NPMMs units unitssurround surround the patch the patch in polar in polar coordinates, coordinates, as distribution,shownas shown in Figure in the Figure linear-type2; the2; thedistance distance NPMMs between between units th esu theinnermostrround innermost the metamaterials patch metamaterials in polar and coordinates, and the thecenter center asof shownpatchof patch R in is RFigure 34 is mm, 34 mm,2; andthe and distanceits itssize size isbetween about is about λ/2 thλe/. 2innermostThe. The value value metamaterialsof ofR Ris isoptimized optimized and tothe to ensure ensurecenter the theof patchnearestnearest R distanceis distance 34 mm, without without and its coupling couplingsize is about with with theλ/2 the patch.. The patch. valueThe The interval of interval R is ofoptimized the of themetamaterials metamaterials to ensure gthe is g nearest5.5is mm, 5.5 mm, distanceand andthe total thewithout totalwidth coupling width of the of five-loop with the five-loop the metamaterials patch. metamaterials The interval units is unitsof about the is metamaterials λ/4 about, whichλ/4 means, which g is 5.5thatmeans mm, the and thatsurface the the total surfacewave width passing wave of the passing through five-loop through quarter-wavelength metamaterials quarter-wavelength units metamaterials is about metamaterials λ/4, which can be means cansup- be thatpressedsuppressed the observably.surface observably. wave In passingorder In order to throughensure to ensure the quarter-wavelength the electrical electrical continuity continuity metamaterials under under the the limited limitedcan be length lengthsup- pressedofof the the metal metal observably. line, line, the the In distribution distribution order to ensure of the the metamaterials electrical continuity isis differentdifferent under fromfrom the the the limited x-y x-y direction direction length of ofother otherthe metal documents. documents. line, the distribution of the metamaterials is different from the x-y direction of other documents.

(a) (b) (a) (b) FigureFigure 2. 2.SchematicSchematic diagram diagram of the of the MPA MPA loaded loaded with with linear-type linear-type NPMMs. NPMMs. The blue The bluepart is part the is the FigurePTFEPTFE substrate, 2. substrate, Schematic the the centraldiagram central yellow yellow of the part partMPA is is loadedthe the co conventionalnventional with linear-type MPA. MPA. Five NPMMs. concentric The blue rings part areare is formedformed the by PTFE substrate, the central yellow part is the conventional MPA. Five concentric rings are formed bylinear-type linear-type metamaterials metamaterials units. units. (a ()a Top) Top view; view; (b ()b side) side view. view. by linear-type metamaterials units. (a) Top view; (b) side view.

(a) (b) (c) (d) (a) (b) (c) (d) Figure 3. Simulated electric field distribution: (a,c) MPA in the horizontal and vertical plane; (b,d) MPA loaded with FigureNPMMsFigure 3. 3.in Simulated Simulatedthe horizontal electric electric and field fieldvertical distribution: distribution: plane. (a (,ca),c MPA) MPA in in the the horizontal horizontal and and vertical vertical plane; plane; (b (,bd,)d )MPA MPA loaded loaded with with NPMMsNPMMs in in the the horizontal horizontal and and vertical vertical plane. plane.

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The electric field distribution of the MPA loaded with NPMMs in the horizontal di- The electric field distribution of the MPA loaded with NPMMs in the horizontal rection is shown in Figure 3b. The energy at the boundary of the PTFE plate is direction is shown in Figure3b. The energy diffraction at the boundary of the PTFE plate is obviously reduced. It means that the surface wave is well suppressed by the linear-type obviously reduced. It means that the surface wave is well suppressed by the linear-type metamaterials. The comparison of the electric field radiation in the vertical direction of metamaterials. The comparison of the electric field radiation in the vertical direction of the the two antennas is shown in Figure 3c,d. It can be seen that due to the linear-type two antennas is shown in Figure3c,d. It can be seen that due to the linear-type NPMMs, NPMMs, the wave-front along the z direction tends to change from spherical wave to the wave-front along the z direction tends to change from spherical wave to plane wave. plane wave. Thus, the directivity of the MPA is improved. According to the law of con- Thus, the directivity of the MPA is improved. According to the law of conservation of servation of energy, the total energy radiated by the MPA loaded with NPMMs remains energy, the total energy radiated by the MPA loaded with NPMMs remains unchanged, unchanged, and the reduction of the lateral radiation will inevitably lead to an increase in and the reduction of the lateral radiation will inevitably lead to an increase in the forward the forward radiation. Thereby, the gain is enhanced in the main radiation direction. The radiation. Thereby, the gain is enhanced in the main radiation direction. The maximum maximum volume at the edge of the substrate along the x-axis is decreased volume current density at the edge of the substrate along the x-axis is decreased from 2 from 0.40.4 to to0.04 0.04 A/m A/m, and2, and the themaximum maximum average average Poynting vector is reduced is reduced from from 23.27 23.27 to 2 to 5.09 W/m5.09 W/m. 2. The centerThe frequency center frequency and bandwidth and bandwidth of the MPA of the loaded MPA with loaded NPMMs with NPMMs are the same are the same as the conventionalas the conventional MPA. The MPA. gain The comparison gain comparison of the oftwo the antennas two antennas at 5.0 atGHz 5.0 is GHz shown is shown in in FigureFigure 4. The4 .gain The is gain increased is increased from 6.55 from to 6.55 8.98 to dB. 8.98 The dB. half-power The half-power beam width beam width(HPBW) (HPBW) of the H-planeof the H-plane is reduced is reduced from 64° from to 38°. 64◦ to 38◦.

(a) (b) Figure 4.Figure Simulated 4. Simulated gain comparison gain comparison (a) E-plane; (a) E-plane;(b) H-plane. (b) H-plane.

In orderIn to order verify to the verify broadband the broadband characteristics characteristics of linear-type of linear-type NPMMs, NPMMs, three groups three groups of MPAsof operating MPAs operating at the L-band, at the L-band,S-band, S-band,and C-band and C-bandare simulated are simulated simultaneously. simultaneously. The The substratesubstrate thickness thickness of all antennas of all antennas is 2 mm. is The 2 mm. center The center frequencies are 1.5 GHz, are 1.5 3.5 GHz, GHz, 3.5 GHz, and 8.4 GHz,and 8.4 respectively. GHz, respectively. Adding Addingthe X-band the X-bandantennas, antennas, Table 1 lists Table the1 lists main the parameters main parameters of the fourof the group four antennas, group antennas, including including gain, directivity, gain, directivity, and HPBW. and HPBW. Directivity Directivity is calcu- is calculated 41253 by Kraus approximation formula D = , where θE and θH are the HPBW of the E-plate lated by Kraus approximation formula D= θEθ,H where θ and θ are the HPBW of the and H-plate, respectively. It can be seen from the data that compared with the conventional E-plate antennas,and H-plate, the gainrespectively. is improved It can by 1.48–2.56be seen from dB, and the the data directivity that compared is improved with by the 0.15–6.28. conventionalAt 8.4 antennas, GHz, the inductancethe gain is improved of the feed by probe 1.48–2.56 is too dB, large, and which the directivity affects the is performance im- proved ofby this0.15–6.28. group At antennas. 8.4 GHz, As the a result, the linear-type of the feed NPMMs probe is dotoo not large, improve which the af- MAP at fects thethe performance C-band significantly. of this group However, antennas. better As a gain result, and the directivity linear-type enhancements NPMMs do not can still be improveobtained the MAP by at optimizing the C-band the significantly. parameters ofHowever, the MPA better and linear-type gain and directivity NPMMs unit. en- hancements can still be obtained by optimizing the parameters of the MPA and linear- Tabletype 1. Simulation NPMMs ofunit. the different frequency bands MPAs loaded with linear-type NPMMs.

Gain HPBW Directivity Center MPA Loaded MPA Loaded MPA MPA Loaded Frequency g/mm with NPMMs R/mm MPA/dB with MPA with NPMMs NPMMs/dB θE θH θE θH ◦ ◦ ◦ ◦ 1.5 GHz 120 9 6.24 8.22 91 98 74 54 4.63 10.32 ◦ ◦ ◦ ◦ 3.5 GHz 45 7 6.40 8.96 92 92 74 52 4.87 11.15 ◦ ◦ ◦ ◦ 5 GHz 34 5.5 6.55 8.98 86 64 83 38 7.5 13.08 ◦ ◦ ◦ ◦ 8.4 GHz 22 3 6.82 8.3 88 60 96 54 7.81 7.96

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Table 1. Simulation of the different frequency bands MPAs loaded with linear-type NPMMs. Materials 2021, 14, x FOR PEER REVIEW 6 of 8

Gain HPBW Directivity Center Fre- MPA Loaded MPA Loaded MPA MPA Loaded quency R/mm g/mm MPA/dBTable 1. Simulation with of the different frequency bawithnds MPAs NPMMs loaded with MPA linear-type NPMMs. with NPMMs NPMMs/dB 𝛉 𝛉 𝛉 𝛉 Gain 𝐄 𝐇 HPBW𝐄 𝐇 Directivity 1.5 GHz 120 9 6.24 8.22 91° 98° 74° 54° 4.63 10.32 Center Fre- MPA Loaded MPA Loaded 3.5 GHz 45 7 6.40 8.96 92° MPA92° 74° 52° 4.87 MPA11.15 Loaded quency R/mm g/mm MPA/dB with with NPMMs MPA 5 GHz 34 5.5 6.55 8.98 86° 64° 83° 38° 7.5 with13.08 NPMMs NPMMs/dB 𝛉 𝛉 𝛉 𝛉 8.4 GHz 22 3 6.82 8.3 88° 𝐄 60° 𝐇 96° 𝐄 54° 𝐇 7.81 7.96 1.5 GHz 120 9 6.24 8.22 91° 98° 74° 54° 4.63 10.32 ° ° ° ° 3.5 GHz 45 4. 7 Measurement 6.40 Results 8.96 92 92 74 52 4.87 11.15 Materials5 GHz2021 , 14, 4398 34 5.5 6.55 8.98 86° 64° 83° 38° 7.5 13.08 6 of 8 A group of antennas working at the C-band are fabricated by the printed circuit 8.4 GHz 22 3 6.82 8.3 88° 60° 96° 54° 7.81 7.96 board technology according to the third part simulation parameters. The antennas are fed back by a 50 Ω sub miniature version A (SMA) connector, and the fabricated antennas are 4. Measurement Results shown4. Measurementin Figure 5. The Results return loss (S11) of the two antennas is measured by an integrated vector networkAA group analyzer ofof antennasantennas (AV3618), working working as shown at at the the in C-band FigureC-band are 6a. are fabricated The fabricated center by frequency the byprinted the printed of circuitthe two circuit board antennasboardtechnology is technology 5.01 GHz. according Theaccording relative to the to third bandwidths the third part simulationpart of simulationtwo antennas parameters. parameters. are both The 4.2% antennasThe antennasfrom are 4.91 fed are to back fed 5.12 backGHz.by a by 50The aΩ 50 S11sub Ω subtrough miniature miniature of the version conventionalversion A A (SMA) (SMA) MPA connector, connect is −22 or,dB, and and and the the the fabricatedfabricated S11 trough antennas of the are are MPAshownshown loaded in in withFigure Figure linear-type 55.. The return NPMMs loss (S11) is −18 of dB. the Comparedtwo antennas with is measured the simulation by an results, integrated the centervectorvector frequencynetwork network analyzer analyzermoves 10 (AV3618), (AV3618),MHz to ashigh as shownshown frequency. in inFigure Figure This 6a. 6is a.The mainly The center center caused frequency frequency by the of ma- the of two the chiningantennastwo error antennas andis 5.01 the is GHz. 5.01uneven GHz.The permittivity relative The relative bandwidths of the bandwidths PTFE of two plate. ofantennas twoThe measured antennas are both areS11 4.2% both and from band- 4.2% 4.91 from to width5.124.91 are GHz. to in 5.12good The GHz. agreement S11 The trough S11 with troughof the the conventional simulation of the conventional results, MPA whichis MPA −22 dB,proves is − and22 dB,that the and theS11 simulated thetrough S11 troughof the resultsMPAof based the loaded MPA on HFSS loadedwith arelinear-type with correct linear-type and NPMMs reliable. NPMMs is − 18 dB. is − Compared18 dB. Compared with the with simulation the simulation results, theresults, center the frequency center frequency moves 10 moves MHz 10to high MHz frequency. to high frequency. This is mainly This is caused mainly by caused the ma- by chiningthe machining error and error the uneven and the permittivity uneven permittivity of the PTFE of theplate. PTFE The plate.measured The S11 measured and band- S11 widthand bandwidth are in good are agreement in good agreement with the simulation with the simulationresults, which results, proves which that proves the simulated that the resultssimulated based results on HFSS based are on correct HFSS areand correct reliable. and reliable.

(a) (b) Figure 5. Fabricated antennas. The black part is the PTFE substrate with a permittivity of 3.5, the white part is the patch antenna and metamaterial units. The white spot in the center is the solder left by the welded SMA connector. (a) MPA; (b) MPA loaded with linear-type NPMMs. (a) (b)

FigureTheFigure gain 5. 5. Fabricated Fabricatedof the two antennas. antennas.antennas The Theis blackmeasured black part part is in isthe the PTFE PTFEmicrowave substrate substrate anechoicwith with a permittivity a permittivitychamber ofat 3.5,of5.01 3.5, the the GHz.whitewhite The partradiation part is is the the patchpatterns patch antenna of two and antennas metamaterial are shown units.units. The in Figure whitewhite spot spot6b,c. inin The thethe centergaincenter isis ischangedthe thesolder solder left fromleftby 7.47 theby tothe welded 9.58 welded dB SMA with SMA connector. anconnector. increment (a) MPA;(a) MPA;of (2.12b) MPA(b dB.) MPA loaded loaded with with linear-type linear-type NPMMs. NPMMs.

The gain of the two antennas is measured in the microwave anechoic chamber at 5.01 GHz. The radiation patterns of two antennas are shown in Figure 6b,c. The gain is changed from 7.47 to 9.58 dB with an increment of 2.12 dB.

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(a) (b)

(a) (b)

(c) (d)

FigureFigure 6. Measured 6. Measured parameters parameters of the two of the antennas two antennas (a) return (a )loss return S11; loss (b) gain S11; in (b )the gain E-plane; in the (c E-plane;) gain in(c) the gain H-plane in the H-planegain; (d) gain; gains (d versus) gains frequency. versus frequency.

The gainThe gainis reduced of the by two 7.54 antennas dB and is10.79 measured dB in the in theH-plane microwave at 90° and anechoic 270°, respec- chamber at tively.5.01 The GHz. HPBW The of radiation the H-plane patterns is reduced of two fr antennasom 72° to are58°. shown The HPBW in Figure of the6b,c. E-plane The gain is is reducedchanged fromfrom 92° to 7.47 88°. to Due 9.58 dBto the with linear-type an increment metamaterials of 2.12 dB. are inconsistent with the electric field distribution along the y-axis in the horizontal plane, this beam is converged with little change. Consequently, the effect of suppressing the surface wave along the y- axis is poor. However, the HPBW of the H-plane (yoz) shrinks more. According to the Friis transmission formula [7,8], the effective area can be expressed as: G A λ (7) 4π

where G is the gain of antenna, and λ is wavelength. The A of conventional MPA is 16 2 2 cm , and the A of MPA loaded with NPMMs is 26 cm . Therefore, the aperture efficiency is also improved. From the above analysis, it can be drawn that the linear-type NPMMs can greatly improve the performance of MPA. Figure 6d shows the gain comparison of 11 frequencies measured from 4.91 to 5.12 GHz. Since linear-type metamaterials have negative permeability at 1–10 GHz, when the surface wave passes through the linear-type metamaterials, a resonance forbidden gap appears. It can be seen from Figure 6d that the gain of these 11 points is increased obvi- ously. Therefore, the surface wave can be minimized to improve the performance of MAP well.

5. Conclusions In order to adopt metamaterials to improve the EM performance of UWB or multi- band microstrip antennas, this paper proposes a novel MPA based on linear-type NPMMs substrate. The simulations and measurements consistently manifest that without chang- ing the center frequency and bandwidth of the conventional MPA, the gain is improved by 2.12 dB. In addition, the HPBW of the H-plate is reduced by 14° and the effective area is increased by 62.5%. The linear-type metamaterials have small machining error due to its simple structure. This MPA loaded with NPMMs is easy to manufacture by a printed circuit board, and it can be conveniently applied to aircraft conformal antennas, wireless energy harvesting (WEH) systems, and so on.

Author Contributions: Conceptualization, W.-H.H. and Y.G.; methodology, X.-P.Z.; writing—orig- inal draft preparation, W.-H.H.; writing—review and editing, W.-H.H. and Y.G.; supervision, X.- P.Z.; project administration, X.-P.Z.; funding acquisition, X.-P.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 11674267, 51272215 and the National Key Scientific Program of China, grant number No.2012CB921503. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable.

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The gain is reduced by 7.54 dB and 10.79 dB in the H-plane at 90◦ and 270◦, respectively. The HPBW of the H-plane is reduced from 72◦ to 58◦. The HPBW of the E-plane is reduced from 92◦ to 88◦. Due to the linear-type metamaterials are inconsistent with the electric field distribution along the y-axis in the horizontal plane, this beam is converged with little change. Consequently, the effect of suppressing the surface wave along the y-axis is poor. However, the HPBW of the H-plane (yoz) shrinks more. According to the Friis transmission formula [7,8], the effective area can be expressed as:

G 2 A = λ (7) e 4π

2 where G is the gain of antenna, and λ is wavelength. The Ae of conventional MPA is 16 cm , 2 and the Ae of MPA loaded with NPMMs is 26 cm . Therefore, the aperture efficiency is also improved. From the above analysis, it can be drawn that the linear-type NPMMs can greatly improve the performance of MPA. Figure6d shows the gain comparison of 11 frequencies measured from 4.91 to 5.12 GHz. Since linear-type metamaterials have negative permeability at 1–10 GHz, when the surface wave passes through the linear-type metamaterials, a resonance forbidden gap appears. It can be seen from Figure6d that the gain of these 11 points is increased obviously. Therefore, the surface wave can be minimized to improve the performance of MAP well.

5. Conclusions In order to adopt metamaterials to improve the EM performance of UWB or multi- band microstrip antennas, this paper proposes a novel MPA based on linear-type NPMMs substrate. The simulations and measurements consistently manifest that without changing the center frequency and bandwidth of the conventional MPA, the gain is improved by 2.12 dB. In addition, the HPBW of the H-plate is reduced by 14◦ and the effective area is increased by 62.5%. The linear-type metamaterials have small machining error due to its simple structure. This MPA loaded with NPMMs is easy to manufacture by a printed circuit board, and it can be conveniently applied to aircraft conformal antennas, wireless energy harvesting (WEH) systems, and so on.

Author Contributions: Conceptualization, W.-H.H. and Y.G.; methodology, X.-P.Z.; writing—original draft preparation, W.-H.H.; writing—review and editing, W.-H.H. and Y.G.; supervision, X.-P.Z.; project administration, X.-P.Z.; funding acquisition, X.-P.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 11674267, 51272215 and the National Key Scientific Program of China, grant number No.2012CB921503. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest.

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