electronics

Article A 28 GHz Broadband Helical Inspired End-Fire and Its MIMO Configuration for 5G Pattern Diversity Applications

Hijab Zahra 1,*, Wahaj Abbas Awan 2 , Wael Abd Ellatif Ali 3 , Niamat Hussain 4,* , Syed Muzahir Abbas 1,5 and Subhas Mukhopadhyay 1

1 School of Engineering, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia; [email protected] (S.M.A.); [email protected] (S.M.) 2 Department of Integrated IT Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea; [email protected] 3 Department of Electronics & Communications Engineering, College of Engineering and Technology, Arab Academy for Science, Technology and Maritime Transport (AASTMT), Alexandria 1029, Egypt; [email protected] 4 Department of Computer and Communication Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Korea 5 BENELEC, Botany, Sydney, NSW 2019, Australia * Correspondence: [email protected] (H.Z.); [email protected] (N.H.)

Abstract: In this paper, an end-fire antenna for 28 GHz broadband communications is proposed with its multiple-input-multiple-output (MIMO) configuration for pattern diversity applications in 5G communication systems and the Internet of Things (IoT). The antenna comprises a simple

 geometrical structure inspired by a conventional planar helical antenna without utilizing any vias.  The presented antenna is printed on both sides of a very thin high-frequency substrate (Rogers

Citation: Zahra, H.; Awan, W.A.; Ali, RO4003, εr = 3.38) with a thickness of 0.203 mm. Moreover, its MIMO configuration is characterized W.A.E.; Hussain, N.; Abbas, S.M.; by reasonable gain, high isolation, good envelope correlation coefficient, broad bandwidth, and high Mukhopadhyay, S. A 28 GHz diversity gain. To verify the performance of the proposed antenna, it was fabricated and verified Broadband Helical Inspired End-Fire by experimental measurements. Notably, the antenna offers a wide −10 dB measured impedance Antenna and Its MIMO Configuration ranging from 26.25 GHz to 30.14 GHz, covering the frequency band allocated for 5G communication for 5G Pattern Diversity Applications. systems with a measured peak gain of 5.83 dB. Furthermore, a performance comparison with the Electronics 2021, 10, 405. https:// state-of-the-art mm-wave end-fire antennas in terms of operational bandwidth, electrical size, and doi.org/10.3390/electronics10040405 various MIMO performance parameters shows the worth of the proposed work.

Academic Editors: Reza K. Amineh Keywords: 28 GHz; end-fire antenna; millimeter-wave communication; IoT; Internet of things; and Paolo Baccarelli 5G antenna Received: 9 December 2020 Accepted: 3 February 2021 Published: 7 February 2021

1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in The fifth-generation (5G) of mobile communications is expected to revolutionize the published maps and institutional affil- manner in which communications take place globally. This is based on a user-centric iations. approach which will guarantee improved mobile broadband, and aims for a peak data rate of 20 Gbps [1]. Moreover, 5G technology is expected to upgrade the Internet of Things (IoT) capabilities of the current mobile network, which will be a transformation for future communication technologies [2]. Consequently, to achieve these highly promising features

Copyright: © 2021 by the authors. of 5G technology, antennas operating at the designated bands capable of supporting 5G Licensee MDPI, Basel, Switzerland. functionalities are required [3]. In addition, 5G antennas must support other features, such This article is an open access article as accurate beam coverage, and have the ability to meet the requirements of ever-changing distributed under the terms and scenarios [4]. In addition to reducing antenna losses to provide coverage with considerable conditions of the Creative Commons depth and width, precise control of the antenna pattern to direct the beam towards the Attribution (CC BY) license (https:// intended direction, and high spectral efficiency to allow the sharing of resources in the creativecommons.org/licenses/by/ time and frequency domain between several users, are highly desired [5]. 4.0/).

Electronics 2021, 10, 405. https://doi.org/10.3390/electronics10040405 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 405 2 of 15

The technology of the 5G frequency spectrum depends on three frequency bands (sub-1 GHz, sub-6 GHz, and millimeter waves), the most important frequency band of which is 28 GHz (n257) [6]. The 28 GHz band is considered to be the most tested band in the world, mainly due to the availability of large portions of the unutilized spectrum around it, and due to the global availability of this band [7]. Furthermore, this frequency is preferable for the reduced complexity of the devices because of the better propagation compared to other millimeter frequencies [8]. A considerable amount of research has been devoted to the design of antennas operating at 28 GHz using different optimization techniques [9–14]. One of the commonly used types is the , due to its advantage of design simplicity and its compactness, as demonstrated in [9,10]. Metasurface-based antennas have the advantages of wide bandwidth and high gain at the expense of bigger dimensions and profiles [11,12]. Antennas with defected ground structures (DGS) offer the advantages of wider operational bandwidth and compact size, but the disadvantage of complex geometrical structures [13–16]. Moreover, arrays of antennas provide high gain and wide bandwidth, however, the structures are complex with bigger dimensions [17,18]. Several designs have been reported for 5G mm-wave applications [19–21], in which the single element gain is in the range of 2dB to 6 dB. In addition to other , end-fire antennas are also well-known due to their numerous advantages, including compact size, wideband, and high gain for radar and sensing applications. Although a large number of studies have used frequency bands to obtain compact antennas with end-fire radiation patterns, few studies have been conducted using the mm-wave band spectrum. A dual-polarized end-fire antenna with an operational bandwidth of 3.03% was presented in [22], however, the antenna has a complex geometrical structure due to the presence of multiple vias and mesh grid structure. In [23], a broadband Yagi-Uda antenna is presented with a high gain of around 12 dB, but the antenna has a large dimension i.e., 5.6λC × 1.6λC. In [24,25], an SIW-fed antenna and planar helical antennas were proposed for V-band applications, respectively. Both presented works offer wide operational bandwidth of 11.67% and 25.5%, and high gain of 12 dB and 6.3 dB, respectively. However, these designs have a larger electrical size in conjunction with the presence of several vias that cause the structural complexity for mass production. Another interesting work was presented in [26], where a Yagi-Uda antenna with director and reflectors was designed to achieve a high gain of 7.82 dB and an impedance bandwidth of 12.5%. The antenna has a large overall size of 4.05λC × 1.89λC. Planar helical antennas have also been reported in [27,28], however, they also have complex designs with connecting via-holes. After an in-depth study of the state-of-the-art antennas, it can be concluded that the design of a compact size antenna with a low level of complexity and characterized by good mechanical robustness, which can be implemented on the same printed circuit board as other electronic devices, is needed for upcoming 5G communication systems. In this paper, an end-fire antenna is proposed for 28 GHz applications. The antenna is inspired by a planar helical shape for enhanced compactness to make it suitable for surface-mounted applications in 5G smart devices and IoT applications. The suggested antenna is characterized by a via-free simple geometrical structure, which significantly reduces the level of complexity combined with the advantages of wider bandwidth and higher gain. To fulfill the requirements of the multiple-input-multiple-output (MIMO) system, a single element was further utilized to design a two-element MIMO antenna characterized by a low envelope correlation coefficient (ECC), channel capacity loss (CCL), high mean effective gain (MEG), pattern diversity capability, and high efficiency. The remainder of the paper is organized as follows: antenna design is discussed in Section2, simulation and measurement results are discussed in Section3 and, finally, a conclusion is presented in Section4. Electronics 2021, 10,Electronics xElectronics FOR PEER2021 2021 REVIEW, 10, 10, 405, x FOR PEER REVIEW 3 of 15 33 of of 15 15

2. Antenna Geometry2.2. Antenna Antenna and Design Geometry Geometry Methodology and and Design Design Methodology Methodology This section presentsThisThis section sectionthe details presents presents for the the the proposed details details for for design the the proposed proposed geometry, design design and geometry,the geometry, design and and the the design design evolution and impactevolutionevolution of design and and impact impact parameters. of of design design parameters. parameters.

2.1. Antenna Geometry2.1.2.1. Antenna Antenna Geometry Geometry The geometricalThe Theconfiguration geometrical geometrical of configuration configurationthe proposed of ofend-fire the the proposed propos antennaed end-fire end-fire is shown antenna antenna in Figure is shownis shown in Figurein Figure1 . 1. The antenna has a compact size, with a length (AX) and width (AY), which correspond 1. The antenna Thehas a antenna compact has size, a compact with a length size, with (AX) a and length width (AX (A) andY), which width correspond (AY), which correspond to to 1.36λ ×λC × 0.9λ λC, whereλ λC is the free-space wavelength at the central frequency of 28 GHz. to 1.36λC × 0.9λC1.36, whereC λ0.9C is theC, where free-spaceC is wavelength the free-space at the wavelength central frequency at the central of 28 frequencyGHz. of 28 GHz. The presented antennaTheThe presented presented is imprinted antennaantenna on both is imprinted imprinted sides of Rogers on on both both RO4003 sides sides of substrate Rogers of Rogers RO4003 with RO4003 a thick-substrate substrate with with a thick- a thicknessness (H) of (H) only of only0.203 0.203 mm, mm,dielectric dielectric constant constant (𝜀) of(ε 3.38,) of 3.38,and dissipation and dissipation factor factor(tan δ) ness (H) of only 0.203 mm, dielectric constant 𝜀 of 3.38, and dissipationr factor (tan δ) of 0.0027 [29]. The(tanof antenna0.0027δ) of 0.0027 [29]. was The [fed29]. antennausing The antenna a groundedwas fed was using fedcoplanar using a grounded waveguide a grounded coplanar (GCPW) coplanar waveguide tech- waveguide (GCPW) (GCPW) tech- nique. The radiatortechnique.nique. consists The The radiatorof a radiator planar consists helical consists of antennaa ofplanar a planar with helical helicalan antennainspired antenna with V-shaped withan inspired an struc- inspired V-shaped V-shaped struc- θ ture, where bothstructure,ture, arms where are where expanded both both arms by arms are an expanded areangle expanded of θ .by The an by backside angle an angle of ofθ of. theThe. Thesubstrate backside backside con-of the of thesubstrate substrate con- consistssists of a of partial a partial ground plane with with a length a length L3. A L hole. A holewith witha radius a radius of 1 mm of 1 was mm etched was sists of a partial ground plane with a length L3. A hole with a radius3 of 1 mm was etched from the substrateetchedfrom to insertthe from substrate the substratescrew to ofinsert the to insert theend-launch screw the screw of connector, the of end-launch the end-launch which connector, is used connector, to whichfeed which is used is used to feed to the proposed antenna,feedthe proposed the as proposed depicted antenna, antenna, in Figure as depicted as 1d. depicted in Figure in Figure 1d.1 d.

(a) (a) (b ) (b) ( c ) (c) ( d ) (d) Figure 1. Geometry of proposed antenna (a) top view, (b) bottom view, (c) side view, and (d) 3-D Figure 1. FigureGeometry 1. Geometry of proposed of proposed antenna antenna (a) top view,(a) top ( bview,) bottom (b) bottom view, (view,c) side (c) view, side view, and (andd) 3-D (d) model3-D with end- model with end-launchmodel connector. with end-launch connector. launch connector. 2.2. Antenna Designing2.2. Antenna and Radiation Designing Mechanism and Radiation Mechanism 2.2. Antenna Designing and Radiation Mechanism The design methodologyThe design and methodology various steps and involved various in steps the designinvolved of inthe the proposed design of the proposed The design methodology and various steps involved in the design of the proposed antenna are depictedantenna in Figureare depicted 2. The inante Figurenna design2. The comprisesantenna design of the comprises following ofthree the following three antenna are depicted in Figure2. The antenna design comprises of the following three steps. steps. steps.

Prototype-2 Proposed design Prototype-1 Prototype-1 Prototype-2 Proposed design

Figure 2. Design evolutionFigureFigure 2. 2. Designof Design the proposed evolution evolution planar of of the the helix proposed proposed antenna. planar planar helix helix antenna. antenna.

Step-1: Design ofStep-1:Step-1: a T-shapedDesign Design monopole of of a a T-shaped T-shaped antenna monopole monopole operating antenna antenna at 28 operatingGHz. operating at at 28 28 GHz. GHz. Step-2: ModifyingStep-2:Step-2: the conventionalModifying Modifying the the T-shaped conventional conventional antenna T-shaped T-shaped to improve antenna antenna impedance to to improve improve band- impedance impedance band- band- width and end-fire radiation. width and end-firewidth radiation. and end-fire radiation.

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Step-3: Design of helical inspired broadband antenna to cover 28 GHz band spectrum with an end-fire . First, a T-shaped GCPW fed was designed to resonate at the de- sired frequency (fc) of 28 GHz. The length (P1) of the monopole antenna was determined by using the following equation [30]: 𝑐 𝑃 = (1) 4 𝑓 𝜀

where c is the free-space speed of light which is approximately equal to 3 × 108 m s−1, and Electronics 2021, 10, 405 ɛeff is the effective dielectric constant of the substrate, which can be estimated 4by of 15using the following relation [30]: 𝜀 +1 𝜀 +1 𝑑 . (2) Step-3: Design of helical𝜀 inspired≈ broadband+ antenna1+12 to cover 28 GHz band spectrum with an end-fire radiation pattern. 2 2 ℎ

whereFirst, ɛr is a the T-shaped dielectric GCPW constant fed monopole of the antennasubstrate, was d designedis the width to resonate of the atmonopole, the desired and h is thefrequency substrate (fc) thickness. of 28 GHz. The The bandwidth length (P1) of of the the monopole T-shaped antenna antenna was was determined 650 MHz, by ranging using the following equation [30]: from 27.78 GHz to 28.43 GHz, as shown in Figure 3. The antenna exhibits a bi-directional radiation pattern due to equal distribution cof current on both sides of the head of the T- P1 = √ (1) shaped antenna, as depicted in Figure 4,4 fwithc εe fa f maximum gain value of 3.96 dB, as illus- trated in Figure 5. The bandwidth of the T-shaped antenna was not enough to cover the c × 8 −1 globallywhere is allocated the free-space band speed spectrum of light of which 28 GHz is approximately with a range equalof 26.5–29.5 to 3 10 GHz.m s Moreover,, and the ε is the effective dielectric constant of the substrate, which can be estimated by using the reportedeff gain and radiation patterns were not suitable for 5G mm-wave communication following relation [30]: systems. − Thus, to further improveε the+ 1 bandwidthε + 1  of the antennad   0.5 and radiation characteristics, ε ≈ r + r 1 + 12 (2) the symmetric T-shapede fantenna f 2 designed2 in the firsth step was modified as an asymmetric T-shaped antenna, as depicted in Figure 2. The asymmetric structure changed the current where ε is the dielectric constant of the substrate, d is the width of the monopole, and distributionr on the surface and resulted in increased current flowing through the large h is the substrate thickness. The bandwidth of the T-shaped antenna was 650 MHz, armranging compared from 27.78 to the GHz short to 28.43arm of GHz, the asT-shaped shown inhead, Figure as3 depicted. The antenna in Figure exhibits 4. This a redis- tributionbi-directional of current radiation and pattern unbalanced due toequal geometrical distribution configuration of current onresulted both sides in more of the electro- magnetichead of the waves T-shaped bending antenna, in a asspecific depicted directio in Figuren and,4, with therefore, a maximum an end-fire gain value radiation of pat- tern,3.96 dBas, depicted as illustrated in inFigure Figure 5.5. Moreover, The bandwidth it was of the also T-shaped observed antenna that wasthe notimpedance enough band- widthto cover of thethe globally modified allocated T-shaped band antenna spectrum increased of 28 GHz from with 650 a range MHz of 26.5–29.5to 4300 MHz, GHz. with a rangeMoreover, of 26.9–31.1 the reported GHz, gain and and the radiationgain improved patterns from were 3.96 not GHz suitable to 5.11 for 5G dB, mm-wave as illustrated in Figurecommunication 3 and Figure systems. 5, respectively.

0

-5

-10

-15 | (dB) 11 |S -20 Prototype-1

-25 Prototype-2 Proposed -30 24 25 26 27 28 29 30 31 32 Frequency(GHz)

FigureFigure 3. |S11|| of of the variousvarious antennas antennas used used in thein th designe design of the of proposedthe proposed antenna. antenna. Thus, to further improve the bandwidth of the antenna and radiation characteristics, the symmetric T-shaped antenna designed in the first step was modified as an asymmetric T-shaped antenna, as depicted in Figure2. The asymmetric structure changed the current distribution on the surface and resulted in increased current flowing through the large arm compared to the short arm of the T-shaped head, as depicted in Figure4. This redistribution of current and unbalanced geometrical configuration resulted in more electromagnetic waves bending in a specific direction and, therefore, an end-fire radiation pattern, as depicted in Figure5. Moreover, it was also observed that the impedance bandwidth of the modified T-shaped antenna increased from 650 MHz to 4300 MHz, with a range Electronics 2021, 10, 405 5 of 15

Electronics 2021, 10, x FOR PEER REVIEWof 26.9–31.1 GHz, and the gain improved from 3.96 GHz to 5.11 dB, as illustrated in 5 of 15 Electronics 2021, 10, x FOR PEER REVIEW 5 of 15 Figures3 and5, respectively.

Prototype-IPrototype-I Prototype-II Prototype-II Proposed Proposed design design

Figure 4. Surface charge distribution (at 28 GHz) of various steps in the antenna design. FigureFigure 4. Surface 4. Surface charge charge distribution distribution (at 28 (at GHz) 28 GHz) of various of various steps steps in the in antenna the antenna design. design.

GainGain (dB) (dB) GainGain (dB) (dB) GainGain (dB) (dB) 6 6 6 6 6 6

-10 -10 -10 -10 -10 -10

Prototype-IPrototype-I Prototype-IIPrototype-II Prototype-III Prototype-III

Figure 5. Radiation pattern of various steps in the design of proposed antenna. FigureFigure 5. Radiation 5. Radiation pattern pattern of of various various steps steps in thethedesign design of of proposed proposed antenna. antenna.

In theInIn the finalthe final final step, step, step, a single a a single single turn turn turn helix-inspir helix-inspired helix-inspired antennaed antenna antenna was was was formed, formed, formed, as depicted as depicted inin Fig- in Fig- ureFigure 2.ure Helix 2.2 Helix. Helix antennas antennas antennas are well are wellwell known known known for for theirfor their their en end-fired-fire end-fire radiation radiation radiation pattern pattern if if they they if satisfythey satisfy satisfy the the the conditions for the axial mode of operation and their broadband operation. However, conditionsconditions for thefor axialthe axial mode mode of operation of operation and andtheir their broadband broadband operation. operation. However, However, the the the conventional helix antenna requires a larger volume, therefore, planar helix antennas conventionalconventional helix helix antenna antenna requires requires a larger a larger volume, volume, therefore, therefore, planar planar helix helix antennas antennas are are arebecoming becoming popular popular [31]. [31]. The The equi equivalentvalent modelsmodels of of an an uncoiled uncoiled single single turn turn helix helix and and a a con- becomingconventional popular helix [31]. are depictedThe equi invalent Figure models6. The planar of an uncoiled configuration single of theturn helix helix antenna and a con- ventional helix are depicted in Figure 6. The planar configuration of the helix antenna uses ventionaluses the helix following are depicted parameters, in Figure as discussed 6. The planar in [32 ]:configuration the diameter of of the the helix helix antenna (D), the uses the following parameters, as discussed in [32]: the diameter of the helix (D), the circum- thecircumference following parameters, of the helix√ as (C =discussedπD), the spacing in [32]: between the diameter helix turns of the (x), helix the total (D), length the circum- of ference of the helix (C2 = πD),2 the spacing between helix−1 turnsx (x), the total length of the ferencethe single of the turn helix (LT (C= = CπD),+ xthe), andspacing the pitch between angle helix (θ = sin turnsL (x),). In the most total planar length helix of the T antennas,single turn the (L topT = and √ bottom𝐶 +𝑥 arms), and are the connected pitch angle by means (θ = ofsin a−1 via). to In form most a singleplanar turn, helix anten- single turn (LT = √𝐶 +𝑥 ), and the pitch angle (θ = sin−1 ). In most planar helix anten- however,nas, the thetop insertion and bottom of a via arms results are inconnected additional by unwanted means of a via to form which a may single affect turn, how- nas,antenna the top performance. and bottom Thus,arms toare overcome connected the by aforementioned means of a via challenge to form in a thesingle presented turn, how- ever, the insertion of a via results in additional unwanted inductance which may affect ever,work, the ainsertion via free single of a turnvia results helix antenna in additional was formed unwanted by placing inductance two arms on which the top may side affect antennaofantenna the performance. substrate. performance. A helix Thus, antennaThus, to overcome to only overcome radiates the aforementionedthe in theaforementioned end-fire direction challenge challenge if it in satisfies the in presentedthe the presented work, a via free single turn helix antenna was formed3 4by placing two arms on the top side work,following a via free relation single for turn operation helix inantenna the axial was mode: formed4 < Cby < placing3 [32]. Therefore, two arms the on selected the top side spacingof the substrate. between the A helix armsantenna is x only = λ andradiates the relation in the end-fire between direction the circumference if it satisfies the of the substrate. A helix antenna only radiates4 in the end-fire direction if it satisfies the andfollowing the free relation space wavelength for operation at thein the central axial frequency mode: of< C 28 < GHz [32]. was Therefore, chosen to the be selected following relation for operation in the axial mode: < C < [32]. Therefore, the selected C = 1, to satisfy the relation for the end-fire radiation pattern. The optimized pitch λspacing between the helix arms is x = and the relation between the circumference and spacingangle between was found the to helix be 30 ◦arms, which is wasx = selected and the after relation a detailed between numerical the circumference analysis. The and the free space wavelength at the central frequency of 28 GHz was chosen to be = 1, to theresultant free space antenna wavelength and its corresponding at the central S-parameter frequency resultsof 28 GHz are depicted was chosen in Figures to be2 and 3 =. 1, to The antenna exhibits broad impedance bandwidth of 3.8 GHz, ranging from 26 GHz to satisfysatisfy the relationthe relation for the for end-firethe end-fire radiation radiation pattern. pattern. The Theoptimized optimized pitch pitch angle angle was was found found to beto 30 be°, 30which°, which was was selected selected after after a detailed a detailed numerical numerical analysis. analysis. The Theresultant resultant antenna antenna and andits corresponding its corresponding S-parameter S-parameter results results are depictedare depicted in Figures in Figures 2 and 2 and3. The 3. Theantenna antenna exhibitsexhibits broad broad impedance impedance bandwidth bandwidth of 3.8 of GHz,3.8 GHz, ranging ranging from from 26 GHz 26 GHz to 29.8 to 29.8 GHz GHz for for |S11||S11| < −10 < dB,−10 coveringdB, covering the globallythe globally allocated allocated band band spectrum spectrum of 28 of GHz. 28 GHz. The Thenumerically numerically calculatedcalculated gain gain of the of helix-inspiredthe helix-inspired antenna antenna was wa reporteds reported to be to 5.92 be 5.92 dB withdB with an end-fire an end-fire radiationradiation pattern, pattern, as depicted as depicted in Figure in Figure 5. It 5.is Itimportant is important to note to note that that in all in three all three steps steps the the

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29.8 GHz for |S11| < −10 dB, covering the globally allocated band spectrum of 28 GHz. The numerically calculated gain of the helix-inspired antenna was reported to be 5.92 dB with an end-fire radiation pattern, as depicted in Figure5. It is important to note that dimensionin all three stepsof the the ground dimension plane of the remains ground unchan plane remainsged. The unchanged. optimized The optimizedparameters of the pro- posedparameters antenna of the are proposed as follows: antenna A areY = as15 follows: mm; A AXY == 10 15 mm; mm; ALX1 == 105 mm; mm; LL1 2= = 5 3 mm; mm; L3 = 7 mm; WL12 == 3 3 mm; mm; LW3 2= = 7 6 mm; mm; W P1 1= = 3 5 mm; mm; W P2 2= = 6 4 mm; mm; P1 d= = 5 0.7 mm; mm; P2 = f 4= mm; 0.45 d mm; = 0.7 θ mm; = 30°. f = 0.45 mm; θ = 30◦.

(a) (b) FigureFigure 6. 6. DemonstrationDemonstration of ( a)) conventional helix helix and and (b )(b equivalent) equivalent model model of single of single turn turn planar helix.planar helix. 2.3. Parametric Analysis 2.3. ParametricChanging theAnalysis diameter of the helix eventually changes the circumference of the helix, whichChanging in turn changes the diameter the wavelength of the ofhelix the resonatingeventually antenna, changes because the circumference the previously of the helix, C = λ whichselected in relation turn changes for the end-fire the wavelength radiation pattern of wasthe λresonating1. The change antenna, in wavelength because ( the) previously shifts the frequency, and the radiation pattern is also affected, due to the relation 3 < C < 4 . selected relation for the end-fire radiation pattern was = 1. The change4 3in wavelength Therefore, the diameter of the helix, which is equal to the length of the monopole, was not (λinvestigated.) shifts the Otherfrequency, key parameters and the of radiation the helix-inspired patte antennarn is also were affected, pitch angle, due the to total the relation < length of single helix turns, and spacing between helix turns. By changing the pitch angle, C < . Therefore, the diameter of the helix, which is equal to the length of the monopole, other parameters attain new values because they depend upon each other, therefore, the wasparametric not investigated. analysis of the Other pitch anglekey parameters is presented in of this the section. helix-inspired antenna were pitch an- ◦ ◦ gle, theBy total reducing length the pitchof single angle helix (θ) from turns, 30 to and 15 ,spacing x corresponds between to 1.1 helix mm andturns. LT By changing thecorresponds pitch angle, to 11 other mm. parameters In this case, attain the antenna new showsvalues poor because performance they depend in terms upon of each other, return loss, and the resonating frequency shifts to the higher side, whereas the targeted therefore,28 GHz band the was parametric notched from analysis the resonating of the pitch region, angle as depicted is presented in Figure 7in. Itthis can section. also be observedBy reducing that, in the this pitch case, angle the gain (θ of) from the antenna 30° to decreases 15°, x corresponds from 5.92 dB toto3.89 1.1 dBmm and LT cor- respondsand the back to 11 lobes mm. become In this more case, prominent, the antenna as illustrated shows in poor Figure performance8. In contrast, when in terms of return ◦ ◦ loss,the valueand the of θ resonatingwas increased frequency from 30 to shifts 45 , then to the x and higher LT correspond side, whereas to 2.43 mmthe andtargeted 28 GHz 11.27 mm, respectively. In this case, the resonating frequency of the antenna was slightly bandshifted was toward notched the lower from side, the withresonating the disadvantage region, as of reduceddepicted bandwidth in Figure and 7. poorIt can also be ob- servedimpedance that, matching, in this case, as depicted the gain in of Figure the7 ante. Moreover,nna decreases it was also from observed 5.92 dB that to the 3.89 dB and the backgain lobes of the antennabecome also more decreases prominent, from 5.92 as dB illustrat to 5.05 dB,ed andin Figure the radiation 8. In patterncontrast, of the when the value antenna is also distorted from purely end-fire to dual-beam, as depicted in Figure8. Thus, of θ was increased from 30° to 45°, then x and LT correspond to 2.43 mm and 11.27 mm, it can be concluded from the aforementioned discussion that the presented antenna shows respectively. In this case, the resonating frequency◦ of the antenna was slightly shifted to- the best performance with optimized values of θ = 30 , x = 1.67 mm, LT = 11.13 mm. ward the lower side, with the disadvantage of reduced bandwidth and poor , as depicted in Figure 7. Moreover, it was also observed that the gain of the antenna also decreases from 5.92 dB to 5.05 dB, and the radiation pattern of the antenna is also distorted from purely end-fire to dual-beam, as depicted in Figure 8. Thus, it can be concluded from the aforementioned discussion that the presented antenna shows the best performance with optimized values of θ = 30°, x = 1.67 mm, LT = 11.13 mm.

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Ɵ = 15˚; x = 1.10 mm; LT = 11 mm Ɵ = 30˚; x = 1.67 mm; LT = 11.13 mm

Ɵ =Ɵ 15 = ˚45; x˚ =; x1.10 = 2.43 mm; mm; LT = L11T =mm 11.27 mm 0 Ɵ = 30˚; x = 1.67 mm; LT = 11.13 mm

Ɵ = 45˚; x = 2.43 mm; LT = 11.27 mm 0-5

-10-5

-10-15 | (dB)

11 -15 -20 |S | (dB)

11 -20 |S -25

-25

-30 -30 -35 -35 24 25 26 27 28 29 30 31 32 24 25 26 27Frequency 28 29 (GHz) 30 31 32 Frequency (GHz)

Figure 7.7.Return Return loss loss of of antenna antenna for for various various values values of pitch of pitch angle angle (θ). (θ). Figure 7. Return loss of antenna for various values of pitch angle (θ).

GainGain (dB) (dB) GainGain (dB) (dB) Gain (dB)Gain (dB) 66 6 6 6 6

-10 -10 -10 -10 -10 -10

Ɵ = 15˚; x = 1.10 mm; LT = 11 mm Ɵ = 30˚; x = 1.67 mm; LT = 11.13 mm Ɵ = 45˚; x = 2.43 mm; LT = 11.27 mm Ɵ = 15˚; x = 1.10 mm; LT = 11 mm Ɵ = 30˚; x = 1.67 mm; L = 11.13 mm Ɵ = 45˚; x = 2.43 mm; L = 11.27 mm T T Figure 8. Radiation pattern and gain of the antenna for various values of pitch angle (θ). Figure 8. Radiation pattern and gain of the antenna for various values of pitch angle (θ). Figure 8. Radiation pattern and gain of the antenna for various values of pitch angle (θ). 3. Results and Discussion 3. Results and Discussion 3. Results and Discussion 3.1.3.1. Single Single Element Element 3.1. SingleToTo verify Element the numerical numerical findings, findings, a prototyp prototypee of the the proposed proposed antenna was fabricated fabricated andand tested testedTo verify as as shown shown the numerical in in the inset findings, of Figure a 99. .prototyp TheThe standardstandarde of the chemicalchemical proposed etchingetching antenna methodmethod was waswas fabricated adopted to fabricate the antenna, and an end-launch connector by South West Corp. was andadopted tested to fabricateas shown the in antenna, the inset and of an Figure end-launch 9. The connector standard by chemical South West etching Corp. method was was used to excite the antenna. It is important to mention that the end-launch connector has adoptedused to excite to fabricate the antenna. the antenna, It is important and an to end-launch mention that connector the end-launch by South connector West has Corp. was been used to demonstrate the concept due its better performance and ease of use in 28 usedbeen usedto excite to demonstrate the antenna. the concept It is important due its better to mention performance that and the ease end-launch of use in 28 connector GHz has GHzmm-wave mm-wave designs designs [33–35 [33–35].]. been used to demonstrate the concept due its better performance and ease of use in 28 3.1.1.GHz Returnmm-wave Loss designs [33–35]. Figure 9 presents the comparison between the simulated and measured return loss of3.1.1. the proposedReturn Loss antenna. It can be seen that the antenna exhibits an impedance bandwidth of 3.8 FigureGHz, ranging 9 presents from the26 GHz comparison to 29.8 GHz, between and the the measured simulated impedance and measured bandwidth return loss wasof the observed proposed to be antenna. 3.89 GHz, It canranging be seen from that 26.25 the GHz antenna to 30.14 exhibits GHz foran impedance|S11| < −10 dB.bandwidth Theof 3.8 small GHz, discrepancy ranging frombetween 26 numericallyGHz to 29.8 calc GHz,ulated and and the tested measured results impedanceoccurred due bandwidth to imperfections in fabrication and the measurement setup. was observed to be 3.89 GHz, ranging from 26.25 GHz to 30.14 GHz for |S11| < −10 dB. The small discrepancy between numerically calculated and tested results occurred due to imperfections in fabrication and the measurement setup.

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0

-5

-10

-15 | (dB) 11 |S -20

-25

-30 24 25 26 27 28 29 30 31 32 Frequency(GHz)

FigureFigure 9. 9.Simulated Simulated and and measured measured return return loss of loss the of proposed the proposed antenna. antenna.

3.1.1.3.1.2. Return Far-Field Loss Results FigureFigure9 presents 10 presents the comparison the far-field between results the of simulated the proposed and measured antenna. return Measurements loss of were the proposed antenna. It can be seen that the antenna exhibits an impedance bandwidth carried out using an anechoic chamber. It can be seen that antenna exhibits an end-fire of 3.8 GHz, ranging from 26 GHz to 29.8 GHz, and the measured impedance bandwidth 𝜙=0° wasradiation observed pattern to be 3.89in the GHz, principal ranging E-plane from 26.25 ( GHz), to where 30.14 GHzthe main for |S11| lobe

numerically calculated and300 measured radiation efficiency of the antenna60 was also reported <85% in the achieved-10 band, as depicted in Figure 10b.

-20 E-Plane Simulated 3.2. MIMO Antenna E-Plane Measured -30 MIMO communication systems have gained considerable attention duringH-Plane Simulated the past decade due to the advantage of enhanced data throughput under the effectsH-Plane of signal Measured inter- ference, multiple paths,-20 and signal fading. Consequently, to satisfy the basic requirements

of the MIMO communication-10 system, i.e., a MIMO antenna, the proposed antenna was further transformed into a MIMO240 antenna by placing two identical120 elements orthogonal to each other. The fabricated0 prototype of the MIMO antenna was used to test the various MIMO performance parameters, including scattering parameters (i.e., both reflection and transmission coefficients),10 envelope correlation coefficient (ECC), channel capacity loss 180 (CCL), pattern diversity, diversity gain (DG), and mean effective gain (MEG). (a)

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0

-5

-10

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-25

-30 24 25 26 27 28 29 30 31 32 Frequency(GHz) Figure 9. Simulated and measured return loss of the proposed antenna.

3.1.2. Far-Field Results Figure 10 presents the far-field results of the proposed antenna. Measurements were carried out using an anechoic chamber. It can be seen that antenna exhibits an end-fire radiation pattern in the principal E-plane (𝜙=0°), where the main lobe is pointed toward θ = −105°. A similar pattern is also observed in the H-plane (𝜙 = 90°), where the main lobe is pointed toward θ = −135°. A good agreement between simulated and measured radia- tion patterns was observed for both E- and H-planes. The antenna possesses a simulated

Electronics 2021, 10and, 405 measured peak gain of < 5.83 dB in the operational bandwidth, and the numerically 9 of 15 calculated and measured radiation efficiency of the antenna was also reported <85% in the achieved band, as depicted in Figure 10b.

(dB) 0 10

0

300 60 -10

-20 E-Plane Simulated E-Plane Measured -30 H-Plane Simulated H-Plane Measured -20

-10 240 120

0

Electronics 2021, 10, x FOR PEER REVIEW 10 9 of 15 180 (a)

6.00 90

86 5.50

82 5.00 78 Gain Simulated Peak Gain (dB) 4.50 Gain Measured Efficiency Simulated 74 Efficiency Measured Efficiency Radiation (%)

4.00 70 24 25 26 27 28 29 30 31 32 Frequency (GHz) (b)

Figure 10. ComparisonFigure 10. Comparisonbetween simulated between and simulated measured and measured(a) radiation (a) radiation pattern at pattern 28 GHz, at 28 and GHz, (b and) (b) gain and radiationgain and efficiency. radiation efficiency.

3.2. MIMO Antenna3.2.1. Scattering Parameters A comparison between the simulated and measured value of reflection (|S |) and MIMO communication systems have gained considerable attention during the past11 transmission coefficients (|S12|) is presented in Figure 11. It can be observed that the decade due simulatedto the advantage and measured of enhanced reflection data bandwidths throughput are from un 26–29.8der the GHz effects and of 26.25–30.14 signal in- GHz, terference, multiplerespectively. paths, The and simulated signal value fading. of |S Consequently,12| was observed to to satisfy be <− 30the dB basic over require- the complete ments of theoperational MIMO communication region, and the system, measured i.e., value a MIMO of |S12 |antenna, was observed the proposed to be less antenna than −31 dB. was further Moreover,transformed the into minimum a MIMO value antenna of −57 by dB placing was observed two identical at the frequency elements of orthog- 27.61 GHz. onal to eachThe other. small The discrepancy fabricated between prototype numerically of the calculatedMIMO antenna and tested was results used occurredto test the due to various MIMOimperfections performance in fabrication parameters, and including the measurement scattering setup. parameters (i.e., both reflec- tion and transmission coefficients), envelope correlation coefficient (ECC), channel capac- ity loss (CCL), pattern diversity, diversity gain (DG), and mean effective gain (MEG).

3.2.1. Scattering Parameters

A comparison between the simulated and measured value of reflection (|S11|) and transmission coefficients (|S12|) is presented in Figure 11. It can be observed that the sim- ulated and measured reflection bandwidths are from 26–29.8 GHz and 26.25–30.14 GHz, respectively. The simulated value of |S12| was observed to be <−30 dB over the complete operational region, and the measured value of |S12| was observed to be less than −31 dB. Moreover, the minimum value of −57 dB was observed at the frequency of 27.61 GHz. The small discrepancy between numerically calculated and tested results occurred due to im- perfections in fabrication and the measurement setup.

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0 0 S11 Simulated S11S11 Simulated Measured -10 S11S12 Measured Simulated -10 S12S12 Simulated Measured S12 Measured -20 -20

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-40 -40 S-Parameters (dB) S-Parameters

S-Parameters (dB) S-Parameters -50 -50

-60 -60 24 25 26 27 28 29 30 31 32 24 25 26 27 28 29 30 31 32 Frequency(GHz) Frequency(GHz) FigureFigure 11.11. Simulated and measured measured scattering scattering paramete parametersrs of of the the multiple-input-multiple-output multiple-input-multiple-output Figure(MIMO) 11. Simulatedantenna. and measured scattering parameters of the multiple-input-multiple-output (MIMO)(MIMO) antenna. antenna. 3.2.2.3.2.2. EnvelopeEnvelope CorrelationCorrelation CoefficientCoefficient (ECC)(ECC) 3.2.2. Envelope Correlation Coefficient (ECC) TheThe envelopeenvelope correlationcorrelation coefficientcoefficient (ECC)(ECC) definesdefines howhow oneone antennaantenna isis independentindependent The envelope correlation coefficient (ECC) defines how one antenna is independent inin itsits performanceperformance withwith respectrespect toto anotheranother antenna.antenna. Ideally,Ideally, thethe valuevalue ofof ECCECC shouldshould bebe 0,0, inhowever,however, its performance for for real real cases, withcases, respect ECC ECC < < 0.5to0.5 another isis acceptable.acceptable. antenna. ECCECC Ideally, cancan bebe the calculatedcalculated value of usingusing ECC theshouldthe followingfollowing be 0, however, for real cases, ECC < 0.5 is acceptable. ECC can be calculated using the following relation,relation, providedprovided inin [[36]:36]: relation, provided in [36]: ∗ ∗ ∗|S∗ 𝑆 +S∗∗ 𝑆2| 𝐸𝐶𝐶 = ||SS11 S𝑆12 ++SS21 S𝑆22| (3) ECC𝐸𝐶𝐶= = (1 − |𝑆| −|𝑆| )(1 − |𝑆| −|𝑆| ) (3)(3)  2 2 2 2 (1− −|S |𝑆|| −−|𝑆|S || )(1− |𝑆|S | −|𝑆− |S|| ) For the presented antenna,1 the11 simulated21 value1 of22 ECC is <0.013,12 whereas the meas- For the presented antenna, the simulated value of ECC is <0.013, whereas the meas- ured value was observed to be <0.005, as depicted in Figure 12a. The value of the proposed For the presented antenna, the simulated value of ECC is <0.013, whereas the measured uredantenna value is was due observed to low mutual to be <0.005, coupling as depicted and thus in one Figure element 12a. Thehas valueless impact of the proposedon the per- antennavalue was is due observed to low tomutual be <0.005, coupling as depicted and thus in one Figure element 12a. has The less value impact of the on proposed the per- formance of the other element of the MIMO antenna. formanceantenna of is the due other to low element mutual of couplingthe MIMO and antenna. thus one element has less impact on the performance of the other element of the MIMO antenna.

0.05 1.2 0.05 Simulated 1.2 Simulated Simulated Simulated Measured 1.0 Measured 0.04 Measured 1.0 Measured 0.04 0.8 0.03 0.8 0.03

0.6 ECC

0.6 ECC 0.02 0.02 0.4 CCL (bits/s/Hz) CCL 0.4 CCL (bits/s/Hz) CCL 0.01 0.01 0.2 0.2 0.00 0.0 0.00 24 25 26 27 28 29 30 31 32 0.0 24 25 26 27 28 29 30 31 32 24 25 26Frequency 27 28 29(GHz) 30 31 32 24 25 26Frequency 27 28 (GHz) 29 30 31 32 Frequency (GHz) Frequency (GHz) (a) (b) (a) (b) Figure 12. Simulated and measured: (a) envelope correlation coefficient (ECC) and (b) channel capacity loss (CCL). FigureFigure 12. 12. SimulatedSimulated and and measured: measured: (a ()a )envelope envelope correlation correlation coefficient coefficient (ECC) (ECC) and and (b (b) )channel channel capacity capacity loss loss (CCL). (CCL). 3.2.3. Channel Capacity Loss (CCL) 3.2.3. Channel Capacity Loss (CCL) Channel capacity loss (CCL) refers to the losses which may occur in the system due Channel capacity loss (CCL) refers to the losses which may occur in the system due to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable

Electronics 2021, 10, 405 11 of 15

Electronics 2021, 10, x FOR PEER REVIEW 11 of 15 3.2.3. Channel Capacity Loss (CCL) Channel capacity loss (CCL) refers to the losses which may occur in the system due to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable value valuefor a system.for a system. CCL of CCL any of MIMO any MIMO antenna antenna can be calculatedcan be calculated using the using following the following relation [36re-]: lation [36]: 2  CCL = −log2(1−|Sii| − Sij 2 (4) 𝐶𝐶𝐿 = −𝑙𝑜𝑔 (1−|𝑆| −|𝑆| ) (4) ForFor the the presented presented antenna, antenna, the the value value of of ii == jj == 1, 1, 2. 2. Therefore, Therefore, the the simulated simulated value value of of thethe MIMO MIMO antenna antenna is is<0.2 <0.2 bits/s/Hz, bits/s/Hz, whereas whereas the measured the measured value value was observed was observed to be <0.12 to be bits/s/Hz,<0.12 bits/s/Hz, as demonstrated as demonstrated in Figure in Figure12b. Both 12b. simulated Both simulated and measured and measured values values are less are thanless thanthe acceptable the acceptable range range of CCL, of CCL,thus the thus presented the presented MIMO MIMO antenna antenna has a simple has a simple effect oneffect the ondiversity the diversity performance performance of the system. of the system.

3.2.4.3.2.4. Pattern Pattern Diversity Diversity BecauseBecause the the antenna antenna radiates radiates in in the the end-fi end-firere direction, direction, the the MIMO MIMO antenna antenna elements elements werewere placed placed orthogonal orthogonal to to each each other other to toachiev achievee pattern pattern diversity. diversity. Figure Figure 13 illustrates 13 illustrates the patternthe pattern diversity diversity performance performance of the of MIMO the MIMO antenna antenna when wheneither eitherport-1 port-1 or port-2 or port-2were were excited. When port-1 is excited, the antenna shows a peak value of the radiation excited. When port-1 is excited, the antenna shows a peak value of the radiation pattern pattern located at θ = −105◦ in the E-plane, and the maximum beam was observed in located at θ = −105° in the E-plane, and the maximum beam was observed in the H-plane the H-plane pointed toward θ = −115◦. When port-2 is excited, the antenna exhibits an pointed toward θ = −115°. When port-2 is excited, the antenna exhibits an end-fire radia- end-fire radiation pattern pointed toward θ = 115◦ in the principal E-plane, whereas for the tion pattern pointed toward θ = 115° in the principal E-plane, whereas for the principal principal H-plane, the antenna maximum beam was directed toward θ = −110◦. Thus, the H-plane, the antenna maximum beam was directed toward θ = −110°. Thus, the pattern of pattern of the proposed antenna can be switched from the +x-axis to the −y-axis depending the proposed antenna can be switched from the +x-axis to the −y-axis depending upon the upon the user requirements. user requirements.

(dB) (dB) 0 0 10 10 0 0 300 60 -10 -10 300 60 E-Plane Simulated -20 E-Plane Measured -20 -30 H-Plane Simulated -30 -20 H-Plane Measured -20 -10 240 120 -10 240 120 0 0 10 10 180 180 (a) (b) FigureFigure 13. 13. SimulatedSimulated and and measured measured radiation radiation patterns patterns for for ( (aa)) port-1 port-1 and and ( (bb)) port-2. port-2.

3.2.5.3.2.5. Diversity Diversity Gain Gain (DG) (DG) DiversityDiversity gain gain (DG) (DG) is isone one of ofthe the key key parameters parameters for any for anyMIMO MIMO antenna antenna system system and isand defined is defined as the as loss the occurring loss occurring in transmission in transmission power power when when the diversity the diversity scheme scheme is per- is formed.performed. The TheDG DGcancan be calculated be calculated by byusing using the the ECC ECC of of the the MIMO MIMO antenna antenna by by following following relationrelation [37]: [37]: p DG = 10 1 − ECC2 (5) DG = 101 − ECC (5) The ideal case, in which ECC is 0, results in DG = 10 dB. Consequently, for the real case,The the ideal ECC shouldcase, in bewhich very ECC small is so 0, that results DG in must DG be = approximately10 dB. Consequently, equal to for 10 the dB. real For case,the presented the ECC should case, the be simulatedvery small results so that show DG must that thebe approximately antenna exhibits equal diversity to 10 dB. gain For of the>9.998 presented dB, whereas case, the the simulated measured results results show depict that DG the >9.999 antenna dB, exhibits as depicted diversity in Figure gain 14 ofa. >9.998Thus, dB, the lowwhereas value the of measured ECC eventually results resultsdepict DG in a >9.999 high value dB, as of depicted DG, which in Figure makes 14a. the Thus,proposed the low antenna value a strongof ECC candidate eventually for result diversitys in a applications. high value of DG, which makes the proposed antenna a strong candidate for diversity applications.

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10.000 -6.0

-6.2 9.998

-6.4 9.996

-6.6 9.994 MEG(dB) -6.8 Diversity Gain (dB) Diversity Gain 9.992 Simulated -7.0 Simulated Measured Measured 9.990 -7.2 24 25 26 27 28 29 30 31 32 24 25 26 27 28 29 30 31 32 Frequency (GHz) Frequency (GHz) (a) (b) Figure 14. Simulated and measured (a) diversity gain and (b) mean effective gain. Figure 14. Simulated and measured (a) diversity gain and (b) mean effective gain.

3.2.6. Mean Effective Gain (MEG) 3.2.6. Mean Effective Gain (MEG) Another key parameter for diversity applications is mean effective gain (MEG) of the Another key parameter for diversity applications is mean effective gain (MEG) of MIMO antenna, which is defined as the mean of received power by a system inside a the MIMO antenna, which is defined as the mean of received power by a system inside fading environment. The value of MEG can be calculated using the following relation, a fading environment. The value of MEG can be calculated using the following relation, given in [37]: given in [37]: " l # MEG = 0.5 × 𝜇 = 0.5 × 1 − |𝑆 | (6) MEG = 0.5 × µir = 0.5 × 1 − ∑ Sij (6) j=1 For the proposed MIMO antenna l = 2, i = j = 1, 2, where μir represents the efficiency of theFor element the proposed of the MIMO MIMO antenna antenna underl = 2, obi =servation.j = 1, 2, where Practically,µir represents the acceptable the efficiency value ofof MEG the element should ofbe the less MIMO than − antenna3 dB. It can under be observed observation. that, Practically, for both simulation the acceptable and meas- value urementof MEG shouldresults, bethe less MEG than value−3 dB.is within It can the be observedacceptable that, range, for as both depicted simulation in Figure and mea-14b. Thus,surement the results,good agreement the MEG valuebetween is within simulated the acceptable and measured range, results as depicted of various in Figure perfor- 14b. manceThus, the parameters good agreement of the MIMO between antenna simulated valida andtes measured the potential results of ofthe various proposed performance work for MIMOparameters and diversity of the MIMO applications. antenna validates the potential of the proposed work for MIMO and diversity applications. 3.3. Comparison with State-of-the-Art Works 3.3. Comparison with State-of-the-Art Works The presented antenna is compared in Table 1 with the state-of-the-art mm-wave The presented antenna is compared in Table1 with the state-of-the-art mm-wave end-fire antennas to show the worth of this design. It can be noted that the presented end-fire antennas to show the worth of this design. It can be noted that the presented antenna exhibits a compact size compared to [23–26], although the antenna presented in antenna exhibits a compact size compared to [23–26], although the antenna presented [22] has a compact size, but has limited bandwidth and low gain. Conversely, the gain in [22] has a compact size, but has limited bandwidth and low gain. Conversely, the gain values reported by [23–26] were higher than those of the current study, but they exhibit values reported by [23–26] were higher than those of the current study, but they exhibit bigger dimensions, a via-based design [24–26], and less bandwidth [24,26]. Moreover, to bigger dimensions, a via-based design [24–26], and less bandwidth [24,26]. Moreover, to provide a closer examination of MIMO performance, a comparison of the proposed MIMO provide a closer examination of MIMO performance, a comparison of the proposed MIMO antenna with other 28 GHz MIMO antennas is presented in Table 2. The proposed MIMO antenna with other 28 GHz MIMO antennas is presented in Table2. The proposed MIMO antenna exhibits very low mutual coupling, and low values of ECC and CCL, and a high antenna exhibits very low mutual coupling, and low values of ECC and CCL, and a high value of DG was also observed. Therefore, it can be deduced from the aforementioned value of DG was also observed. Therefore, it can be deduced from the aforementioned discussiondiscussion that that the the presented presented end-fire end-fire ante antennanna outperforms the the related work due to its compactcompact dimension, widewide operationaloperational bandwidth,bandwidth, and and moderate moderate gain gain value, value, in in addition addition to toits its via-free via-free design, design, good good MIMO MIMO antenna antenna performance performance parameters, parameters, and and pattern pattern diversity diversity for forMIMO MIMO applications. applications.

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Table 1. Comparison of the proposed antenna with state-of-the-art end-fire antennas.

Dimension Fractional Refs. Peak Gain (dB) Via Free Design (λC × λC) Bandwidth (%) [22] 0.45 × 0.33 3.0 3.2 No [23] 5.6 × 1.6 16.7 11.8 Yes [24] 3.1 × 1.7 11.7 12.0 No [25] 2.9 × 1.35 25.5 6.3 No [26] 4.05 × 1.89 12.5 7.8 No Proposed (Single 1.36 × 0.9 14.0 5.9 Yes element)

Table 2. Comparison of proposed MIMO antenna with related works for 28 GHz applications.

Dimensions Mutual Coupling ECC/CCL DG (dB)/MEG Pattern Refs. No. of Ports (mm × mm × mm) (dB) (bits/s/Hz) (dB) Diversity [3] 20 × 20 × 7.608 4 <−25 <0.005/N.R. N.R./N.R. No [12] 31.7 × 53 × 4 2 <−20 <0.120/N.R. >9.40/N.R. No [13] 12.7 × 50.8 × 0.8 4 <−22 <0.150/N.R. N.R./N.R. No [18] 30 × 35 × 0.76 4 <−20 <0.010/<0.40 >9.96/<−6.6 No [36] 40 × 75 × 0.76 4 <−22 <0.015/<0.25 >9.83/<−6.6 No [37] 20.4 × 20.4 × 0.5 4 <−30 <0.015/<0.19 >9.91/N.R. No Proposed 15 × 25 × 0.203 2 <−30 <0.005/<0.12 >9.95/<−6.0 Yes

4. Conclusions In this paper, a planar helix-inspired wideband antenna for 28 GHz 5G applications was presented. The antenna geometry was extracted from a grounded-CPW fed T-shaped antenna by transforming it into an asymmetric antenna. Then, the concept of the planar helix antenna was utilized to improve the performance of the antenna. The via-free antenna offers a compact size of 1.36λC × 0.9λC while exhibiting wideband measured impedance bandwidth of 3.89 GHz (26.25–30.14 GHz), with an end-fire radiation pattern having a maximum measured gain of 5.83 dB and maximum radiation efficiency of 88%. A single element was also utilized to design a MIMO antenna system, in which both elements were placed orthogonal to each other to achieve pattern diversity. The MIMO antenna showed parameter measurements of ECC < 0.005, CCL < 0.12 bits/s/Hz, and DG > 9.99 over the entire resonating bandwidth. The performance of the presented antenna was compared with state-of-the-art end-fire antennas and it could be observed that antenna outperforms related works due to its better performance.

Author Contributions: H.Z., W.A.A., W.A.E.A., N.H., S.M.A. and S.M., all authors equally con- tributed for developing methodology, performance analysis and drafting the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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