Design and Development of a Wideband Planar Yagi Antenna Using Tightly Coupled Directive Element

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Article Design and Development of a Wideband Planar Yagi Antenna Using Tightly Coupled Directive Element

Muhammad A. Ashraf 1,2 , Khalid Jamil 1,2, Ahmed Telba 1,*, Mohammed A. Alzabidi 2 and Abdel Razik Sebak 3

1 Department of Electrical Engineering, King Saud University, Riyadh 11421, Saudi Arabia; [email protected] (M.A.A.); [email protected] (K.J.) 2 Prince Sultan Defense Studies and Research Center (PSDSARC), Riyadh 13322, Saudi Arabia; [email protected] 3 Department of Computer and Electrical Engineering, Concordia University, Montreal, QC H3G 1M8, Canada; [email protected] * Correspondence: [email protected]

Received: 24 August 2020; Accepted: 29 October 2020; Published: 30 October 2020

Abstract: In this paper, a novel concept on the design of a broadband printed Yagi antenna for S-band wireless communication applications is presented. The proposed antenna exhibits a wide bandwidth (more than 48% fractional bandwidth) operating in the frequency range 2.6 GHz–4.3 GHz. This is achieved by employing an elliptically shaped coupled-directive element, which is wider compared with other elements. Compared with the conventional printed Yagi design, the tightly coupled directive element is placed very close (0.019λ to 0.0299λ) to the microstrip-fed dipole arms. The gain performance is enhanced by placing four additional elliptically shaped directive elements towards the electromagnetic field’s direction of propagation. The overall size of the proposed antenna is 60 mm 140 mm 1.6 mm. The proposed antenna is fabricated and its characteristics, × × such as reflection coefficient, radiation pattern, and gain, are compared with simulation results. Excellent agreement between measured and simulation results is observed.

Keywords: dipole arms; elliptic directive element; end-ﬁre; printed Yagi antenna

1. Introduction In many wireless communication and radar applications, such as TV, microwave imaging, wireless ad-hoc networks, point-to-point and multipoint links, digital array radars, and high-resolution direction tracking, radio frequency (RF) front-end systems are required to operate over a wide bandwidth [1–4]. For an efficient RF front-end operation, antenna elements offering low cost, light weight, moderate gain, easy integration, and low dispersion to wideband signals are extremely important [5]. To fulfill such requirements, many types of microstrip-based antennas have been proposed in the literature [6–10]. Commonly used broadband microstrip antennas include antipodal and slot-coupled Vivaldi antennas; log-periodic arrays; feedline–coupled, multilayer-stacked patch antennas; and quasi-Yagi antennas [11–16]. Generally, Vivaldi and log-periodic array antennas are capable of operating over an ultra-broad bandwidth, but they may not be suitable for some applications where signal bandwidth is smaller. Alternatively, multilayer-stacked patch antennas offer broad bandwidth and can be easily designed and tuned for different operating frequencies by modifying their dimensions. However, the design and implementation of multilayered antennas is complex and less cost-effective [14]. Due to their simple structure, easy integration with other microwave circuits and high gain, Yagi antennas are also suitable in wireless applications. However, typical microstrip Yagi antennas offer

Micromachines 2020, 11, 975; doi:10.3390/mi11110975 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 975 2 of 15 a narrow bandwidth (<10%) [17,18]. Research efforts on improving the performance of typical Yagi antennas by exploiting their fundamental design principles have been presented in the literature. As a result, a new antenna structure called quasi-Yagiwas proposed to improve the conventional Yagiantenna performance [19]. This antenna was first introduced by Huang in 1991. However, one critical limitation of the Quasi-Yagi antenna is its lower gain compared to the classic Yagi antennas. Several techniques for simultaneously enhancing the bandwidth and gain of microstrip Yagi antennas have been proposed in the literature [20–25]. The printed Yagi antenna consists of a dipole antenna acting as a feed, which is called driven element. The directive elements are printed toward the end-fire direction to enhance the gain of the antenna by directing the electromagnetic energy toward a specific direction. The front-to-back ratio and gain over a broad bandwidth or multiple bands can be improved by increasing the number of directive elements. However, this technique increases the overall size of the antenna. For example, a printed Yagi antenna comprising two different types of feeding networks (i.e., loop and dipole types) has been proposed in [26]. This antenna exhibits a fractional bandwidth of 23% and 16% with a gain of 4.7 and 3.65 dBi, respectively. However, the gain enhancement achieved by increasing the number of directive elements is limited as the multiple incorporated directive elements introduce a phase shift at the center frequency of operation in a particular band. Such degradation in antenna performance is not suitable for broadband applications [27]. A broadband Yagi antenna comprising modified bowtie driven dipole is presented in [28]. The presented antenna is very compact with an excellent fractional bandwidth of 42%. However, the maximum gain value is about 4 dBi as it has employed only one directive element to maintain the antenna’s compactness. In [29] a magnetic dipole quasi-Yagi antenna based on a dielectric resonator (DR) is presented. The gain of DR based Yagi antenna is found greater than the traditional ones. However, because of DR high quality factor property, the fraction bandwidth is limited to 3% only. A compact planar Moxon-Yagi composite antenna is presented in [30]. This hybrid antenna operate over dual bands with fractional bandwidth and gain values of 3.2% and 5 dBi, respectively. Similarly, a compact printed Yagi antenna based on half bow-tie monopole element, two director strips and finite ground plane is presented in [31]. The antenna operates at 2.31 GHz center frequency with an excellent fractional bandwidth of 63.5%. As only two directive elements are used, the gain performance of the antenna is compromised to 5.8 dBi. In Table1, we have compared the performance of our proposed antenna with those published in [28–31]. It can be inferred that in printed Yagi antenna designs, simultaneous improvement in gain, bandwidth and size is quiet uncommon.

Table 1. Comparison of wideband printed Yagi Antenna with proposed work.

Ref. Center Freq. (GHz) BW (%) Gain (Max) Dimensions (L W) × [28] 2.72 42 4 0.39λ 0.47λ 0 × 0 [29] 8.74 3 8.34 1.15λ 1.15λ 0 × 0 [30] 2.44 3.2 5 0.84λ 0.58λ 0 × 0 [31] 2.31 63.54 5.8 0.41 λ 0.41λ 0 × 0 Proposed 3.6 49.3 8.7 1.68λ 0.72λ 0 × 0

In this work, a moderate-size modified printed Yagi antenna is presented. This antenna is designed to exhibit high gain and wide bandwidth, simultaneously. The conventional printed Yagi antenna comprises a driven element followed by multiple directive elements of a typical rectangular shape. As the electromagnetic energy is coupled from the driven element to the directive element, the inter-element spacing must be in the range 0.15λ–0.3λ. Moreover, the typical rectangular shape can affect matching over a wide bandwidth due to rapid changes in the input impedance. Therefore, in the proposed printed Yagi antenna design, an elliptical shape for the dipole antenna and directive elements was selected. Additionally, contrary to the typical spacing (0.15λ to 0.3λ) used for the directive elements, the input impedance can be matched over a wider bandwidth by placing the first directive element in a closer proximity (i.e., between 0.019λ to 0.0299λ). Thus, the proposed antenna exhibits a Micromachines 2020, 11, 975 3 of 15 wide bandwidth (49.3% fractional bandwidth) covering the S-band from 2.6 GHz to 4.3 GHz. In the proposed design, the directive elements are printed on both the top and bottom sides of the substrate. The bottom-plane directive elements are placed by some offset with respect to the top-plane directive elements. This enhances the effective length toward the direction of propagation, resulting in enhanced gain without increasing the overall dimensions of the antenna.

2. Printed Yagi Antenna Design Principles Antenna design based on elliptical shaped microstrip structures has been emerged as unique and important category among the diﬀerent shaped because of its dual resonant feature and ﬂexibility in optimization. Design of an elliptical shape microstrip patch antenna has more degree of freedom as the resonant frequency is the function of its major axis and minor axis radii as following [32]: s e,o 15 q e,o = 11 f11 (1) πea r

e,o e o where, f11 is the dual resonant frequency (in GHz) of TM11 and TM11 modes with a and e correspond to physical semi major axis (in cm) and eccentricities, respectively. The eccentricities q (e = 1 (b/a)2) values depends on aspect ratio (b/a) correspond to, semi minor axis /semi major − e,o) axis [33]. The approximate values of Mathieu function (q11 can be determined by following set of equations discussed in [32]. It can be seen that dual resonant modes of an elliptic shape structure can be controlled by varying the aspect ratio (b/a) that can be optimized to exhibit larger bandwidth. In Yagi antennas, induced current exhibits progressive phase shift on the rest of parasitic elements which establishes an array structure suitable for supporting travelling wave. For the circuits printed over a dielectric material, it is essential to present a smooth transitions of induced electric field from the first directive element to the last directive element over the complete band of interest. However, abrupt discontinuity because of sharp edges of a rectangular shape geometry, the electric field lines bends at the edges which results in fringing electric fields. Therefore, to avoid sudden discontinuities along the whole structure, we selected elliptical shape geometry for our design. The results revealed by multiple electromagnetic simulations, we found the antenna’s elements with an elliptical shape geometry improves the overall performance especially the bandwidth thus support the travelling wave phenomenon better than its counterpart structure. The general design principles and analytical investigations related to printed Yagi antenna has been extensively discussed in the literature [34–36]. However, it is important to summarize key salient guidelines required to understand the design and physical operational mechanism of the antenna. The printed Yagi antenna requires a balanced feed to the double sided printed dipole elements. Therefore, a 50 Ω microstrip to a parallel strip line transition is required. Typically, printed Yagi structure exhibits, low input impedance and lower front to back ratio. These parameters are significantly controlled, respectively (i) the spacing between the active dipole arms and the ground plane called reflector and (ii) the size (typically the length) of the reflector. By increasing the size of reflector, the backward gain reduces resulting in an improved front-to-back ratio. Similarly, the input impedance increase by increasing the separation between the dipoles and the reflector. However, a significant drop in forward gain is observed for a reflector spacing more than 0.3λ. In travelling wave antennas, gain enhancement is the function of longitudinal dimensions thus proportional to the larger dimensions (length) of the antenna. In Yagi antennas gain increases by increasing the number of directive elements. However, practically, there is a limit beyond that additional directive element enhances very little to the gain value because of gradual reduction in induced electric field on the farthest directive elements. For our gain requirements (8 dBi~9 dBi), we found maximum four equally spaced directive elements are suitable resulting in overall antenna length to be 140 mm including the ground plane (reflector). As the guided wavelength is smaller than free space wavelength by a factor of square root of the relative permittivity of the substrate material, Micromachines 2020, 11, 975 4 of 15 the major design parameters such as dipole arm lengths, length and spacing between the directive elements are normalized accordingly. Considering the same number of directive elements, the initial length and width of the antenna without dielectric material was calculated to be 285 mm and 85 mm, respectively. It can be seen that the use of substrate material reduces the length by more than half. In summary, typical performance characteristics parameters of a printed Yagi antenna are affected by (i) reflector and feeder geometry (ii) spacing between reflector and the feeder and (iii) the total numbers of directive elements. Therefore, to achieve required performance it is essential to optimize the geometrical parameters of these structural variables.

3. Proposed Yagi Antenna Design The location and geometric parameter (length and width) of the directive elements is very important in designing a Yagi antenna element. The input impedance of the antenna element is mainly affected by the first parasitic directive element. We have exploited this fact to design a wideband Yagi antenna. At first, we designed a Yagi antenna using conventional design approach, where first parasitic directive element is placed at λg/4 from the active dipoles, towards end-fire direction as shown in Figure1. The layout diagram comprising top and bottom planes of the antenna structure are shown in Figure1a,b, respectively. The design procedure is initiated by the elliptically shaped driven elements of λg/4 length printed on both metallization planes of the substrate. The truncated ground plane layer acts as a reflector for the Yagi structure. The antenna is fed by a 50 Ω microstrip line printed on the ground plane. The plated-via holes are placed on the two sides (left and right) of the printed circuit board (PCB). These holes are capable of reducing the side-lobe levels (SLL) by concentrating the radiated fields toward the end-fire direction [37]. The antenna is designed on a FR4 substrate with relative permittivity, dissipation factor and thickness of 4.4, 0.025 and 1.6 mm, respectively. By placing directive elements toward the end-fire direction of the antenna, the electromagnetic energy can be concentrated and a broad bandwidth matched to 50 Ω input impedance can be achieved. Therefore, to achieve wide bandwidth, we introduced an additional parasitic directive element between the active dipole arms and first directive element called tightly coupled directive element. The first directive element is wider compared to the others, and it is placed in close proximity to the driven elements. The top and bottom plane drawings of the proposed design having additional directive element is shown in Figure1c and d , respectively. This placement is quite uncommon in conventional printed Yagi-antenna design. It is worth mentioning that the directive elements are placed on the top and bottom planes of the antenna. To achieve efficient electric-field coupling among the directive elements, the bottom plane directive elements are placed with some offset with respect to the top plane directive elements. The purpose is to enhance the effective width toward the direction of propagation. Critical antenna design parameters, such as the first directive-element spacing, length, width, the other directive-elements dimensions (lengths and widths), and the spacing between directive elements, are optimized using the full-wave electromagnetic simulation program CST Microwave Studio 2019 [38]. The final antenna dimensions used in the fabrication are presented in Table2.

Table 2. Optimized parameters of the proposed antenna in mm.

LWLf Lfg Lg rnf rnw 140 60 67 12.9 55.5 22.6 6.6

rmd rnd d1 d2 d3 d4 d5 24.6 4.3 15.47 15.27 13.95 7.2 6.77 MicromachinesMicromachines 20202020,, 1111,, x 975 55 of of 16 15

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

FigureFigure 1. 1. LayoutLayout diagram diagram of of the the Yagi Yagi antenna antenna ( (aa,,bb),), with with first ﬁrst directive directive element element at at λλgg/4/4 and and geometric geometric parametersparameters of of the the proposed proposed antenna design ( cc,,d),), with with tightly coupled directive element.

4.4. Parametric Parametric Study Study and and Analysis Analysis of of Geometrical Geometrical Antenna Antenna Parameters Parameters InIn a a standard standard travelling-wave travelling-wave printed printed Yagi Yagi ante antenna,nna, the the effect effect of various geometrical antenna antenna parametersparameters such such as as (i) (i) driven driven el elements’ements’ width width and and length length (ii) (ii) refle reflector’sctor’s spacing, spacing, width width and and length length (iii)(iii) different different or or equal equal length length directive directive elements elements (iv) (iv) number number of ofdirective directive elements elements and and (v) (v)different different or equalor equal width width directive directive elements, elements, on oncritical critical perfor performancemance parameter, parameter, has has been been extensively extensively studied studied in thein theliterature literature [34–36]. [34–36 In]. Inour our design, design, at atfirst first step step we we optimized optimized each each geometric geometric parameter parameter of of the the standardstandard printed printed Yagi’s Yagi’s elements elements to to their their values values resulting resulting the the optimum optimum performance performance of of the the antenna. antenna. ForFor simplicity, simplicity, we do do not not report report parametric parametric studie studiess of of these these well-known well-known geometric geometric variables variables and and their their effecteffect onon antenna’santenna’s performance. performance. Instead, Instead, in this in section,this section, we analyze we analyze the effect the of geometriceffect of parametersgeometric parametersof the proposed of the tightly proposed coupled tightly directive coupled element directiv one the element gain and on reflectionthe gain and coe ffireflectioncient characteristics coefficient characteristicsof the antenna. of By the multiple antenna. full-wave By multiple simulation full-wave analysis, simulation we studied analysis, the e ffweect studied of length, the width effect and of length,separations width (r andnf, r nwseparations, d4 and d (r5 innf, r Tablenw, d42 )and of thed5 in tightly Table coupled 2) of the directive tightly coupled element directive on the gain element and onbandwidth the gain ofand the bandwidth proposed antenna. of the proposed The length antenn and widtha. The oflength the elliptic and width shape of directive the elliptic element shape are directivepresented element by their are major presented and minor by thei axisr major diameters, and minor rmd axisand diameters, rnw, respectively. rmd and r Atnw, first,respectively. by varying At first,major by and varying minor axismajor diameters and minor of the axis said diameters directive element, of the thesaid variation directive inreflection element, coetheffi variationcient and inreflectiongain values coefficient are studied and as showngain values in Figure are studied2. The majoras shown and in the Figure minor 2. axis The diameters’ major and valuesthe minor are axisvaried diameters’ over a range values 8.2–32.8 are varied mm and over 4.4–8.8 a range mm, 8.2–32.8 respectively. mm and In 4.4–8.8 Figure2 mm,a, the respectively. major axis diameter In Figure is 2a,changed the major while axis keeping diameter the minoris changed axis diameter while keepin constantg the to minor a value axis of 4.4diameter mm and constant so on to to 6.6 a value mm and of 4.48.8 mm mm and as shown so on into Figure6.6 mm2b,c, and respectively. 8.8 mm as shown It can bein seenFigure that 2b,c, the respectively. printed Yagi antennaIt can be experiencesseen that the a printedsignificant Yagi change antenna in experiences bandwidth anda significant gain by varyingchange majorin bandwidth axis diameter and gain more by thanvarying the major minor axis axis diameterdiameter more values. than By the analyzing minor axis these diameter results, valu the optimumes. By analyzing value of these major re andsults, minor the optimum axis diameters value ofare major acquired and tominor be 24.6 axis mm diameters and 6.6 are mm, acquired respectively. to be In24.6 the mm next and simulation 6.6 mm, setup,respectively. the position In the of next the simulationproposed directive setup, the element position is optimized.of the proposed In Figure directive3, it can element be seen is thatopti translatingmized. In Figure the said 3, directiveit can be seenelement that awaytranslating from the the active said directive dipoles critically element eawayffects thefrom reflection the active coe dipolesfficient thuscritically the gaineffects values the reflectionover the proposedcoefficient spectrum. thus the Conversely,gain values while over thethe directive proposed element spectrum. is placed Conversely, closer to while the dipole the directivearms, the element proposed is antennaplaced closer still exhibited to the dipole a wide arms, bandwidth the proposed performance. antenna still exhibited a wide bandwidth performance.

MicromachinesMicromachines 20202020, 1,111, x, 975 6 of6 15of 16

(a)

(b)

(c)

FigureFigure 2. 2.ParametricParametric study study of of proposed proposed antenna antenna structural parametersparameters based based on on reﬂection reflection coe coefficientﬃcient andand gain gain performance performance analysis analysis with with ( (aa)) r rnwnw == 4.44.4 mm, mm, ( (bb)) r rnwnw = =6.66.6 mm mm and and (c (c) )r rnwnw = =8.88.8 mm. mm.

Micromachines 2020, 11, 975 7 of 15 Micromachines 2020, 11, x 7 of 16

Micromachines 2020, 11, x 7 of 16

FigureFigure 3. 3.Parametric Parametric study study of of the the position position of the proposed directive directive element element based based on on reflection reﬂection

coecoefficienfficient andt and gain gain performance performance analysis. analysis. Figure 3. Parametric study of the position of the proposed directive element based on reflection Finally,Finally,coefficien the the performancet and performance gain performance of the of proposedthe analysis. proposed antenna antenna element element having having a wide a tightly wide tightlycoupled coupled directive element,directive is element, studied is and studied compared and compared with the with antenna the antenna without without wide directicetive wide directicetive element. element. The majorThe performancemajor performanceFinally, parameters the performanceparameters such as such of reflection the as proposedreflection coeffi antennacient,coefficient, realised element realised gain having gain and a inputand wide input impedance tightly impedance coupled of bothof directive element, is studied and compared with the antenna without wide directicetive element. The antennaboth antenna elements elements are calculted are calculted and plotted. and plotted. The simulatedThe simula resultsted results of reflection of reflection coffi cofficientcient (S (S)11 and) major performance parameters such as reflection coefficient, realised gain and input impedance11 of realisedand realised gain aregain shown are shown in Figure in Figure4a,b, 4a respectively.,b, respectively. The The antenna antenna without without widewide directive element element both antenna elements are calculted and plotted. The simulated results of reflection cofficient (S11) presents lower bandwidth as compared to the proposed design. The former antenna element exhbits presentsand realised lower bandwidth gain are shown as compared in Figure 4a to,b the, respectively. proposed The design. antenna The without former wide antenna directive element element exhbits reflection coefficient values (parameter S11) less than −10 dB in the frequency range 2.5–3.2 GHz, which reflectionpresents coe lowerfficient bandwidth values (parameteras compared Sto11) theless proposed than design.10 dB inThe the former frequency antenna range element 2.5–3.2 exhbits GHz, correspond to a 28% fractional bandwidth value. On the− other hand, the proposed antenna presents whichreflection correspond coefficient to a values 28% fractional (parameter bandwidth S11) less than value. −10 dB Onin the the frequency other hand, range the 2.5– proposed3.2 GHz, which antenna S11 less than −10 dB in the frequency range 2.6–4.3 GHz correspond to a 49.3% fractional bandwidth presentscorrespond S11 less to thana 28% fractional10 dB in bandwidth the frequency value. range On the 2.6–4.3 other GHzhand, correspondthe proposed to antenna a 49.3% presents fractional value. Similarly, the presence− of tightly coupled directive element enhances realized gain value by bandwidthS11 less than value. −10 dB Similarly, in the frequency the presence range of2.6– tightly4.3 GHz coupled correspond directive to a 49.3% element fractional enhances bandwidth realized more than 1.5 dB. The maximum realized gain values of the Yagi without and with wide directive gainvalue. value Similarly, by more the than presence 1.5 dB. of The tightly maximum coupled realizeddirective gainelement values enhances of the realized Yagi without gain value and by with element are found 7.1 dBi and 8.6 dBi, respectively. To illustrate the effect on input impedance of widem directiveore than 1.5 element dB. The are maximum found 7.1 realized dBi and gain 8.6 values dBi, of respectively. the Yagi without To illustrate and with the wide eff ectdirective on input planar Yagi antenna because of additional directive element, Z-parameter of both designs are impedanceelement of are planar found Yagi 7.1 antennadBi and 8.6 because dBi, respectively. of additional To directive illustrate element, the effect Z-parameter on input impedance of both designs of calculatedplanar Yagi and antennacompared because shown ofin additionalFigure 5. The directive calculated element, Z11 (real) Z-parameter and Z11 (imaginary) of both designs are shown are are calculated and compared shown in Figure5. The calculated Z 11 (real) and Z11 (imaginary) are in calculatedFigure 5a, b,and respectively. compared shown It can inbe Figureseen that 5. The for calculatedthe antenna Z11 with (real) wide and directiveZ11 (imaginary) element, are Zshown11 (real) shown in Figure5a,b, respectively. It can be seen that for the antenna with wide directive element, valuesin Figure are close5a,b, respectively.to 50 over a Itwider can be bandwidth seen that for as thecompared antenna towith the wide antenna directive without element, wide Z directive11 (real) Z11 (real) values are close to 50 over a wider bandwidth as compared to the antenna without wide element.values Similarly,are close to the 50 antenna over a wider with wide bandwidth directive as element,compared Z 11to (imaginary) the antenna valueswithout are wide close directive to “zero” directive element. Similarly, the antenna with wide directive element, Z (imaginary) values are close overelement. a wider Similarly, bandwidth the antenna as compared with wide to directive the antenna element, without Z11 (imaginary) wide 11 directive values are ele ment.close to It “zero” can be toconcluded “zero”over aover wider that a wider bandwidthpresence bandwidth of as additional compared as compared parasitic to the to antenna the directive antenna without element without wide in wide close directive directive proximity element. element. of It the can It active canbe be concludeddipolesconcluded does that that presence not presence increase of additional ofthe additional overall parasitic antenna parasitic directive dimensions directive element element of ain planar close in close proximity Yagi proximity antenna. of the of However, active the active dipoles it doessignificantlydipoles not increase does improves not the increase overall the bandwidth antennathe overall dimensions and antenna gain parameters dimensions of a planar simultaneously. of Yagi a planar antenna. Yagi However, antenna. However, it significantly it improvessignificantly the bandwidth improves andthe bandwidth gain parameters and gain simultaneously. parameters simultaneously.

(a()a ) (b(b) )

Figure 4. Comparison of (a) reflection coefficient (S11) and (b) realized gain parameters of the FigureFigure 4. Comparison 4. Comparison of ( a of) reﬂection (a) reflection coeﬃ coefficientcient (S ) and(S11) ( andb) realized (b) realized gain parameters gain parameters of the of proposed the proposed antenna element, without and with tightly11 coupled wide directive element. antennaproposed element, antenna without element, and without with tightly and with coupled tightly wide coupled directive wide directive element. element.

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(a(a)) ((b))

FigureFigureFigure 5. 5. ComparisonComparison Comparison of ofof ( (a(aa)) )input inputinput impedance impedanceimpedance (real)(real) andand (( (bb)) input input impedance impedance (imaginary)(imaginary) (imaginary) parametersparameters parameters ofofof the the the proposed proposed antenna antenna element, element, without without andand withwith tightly tightly coupled wide directive element. element.

5.5.5. Antenna Antenna Antenna Fabrication FabricationFabrication and andand Measurements MeasurementsMeasurements TheTheThe designed designed antenna antennaantenna was waswas fabricated fabricatedfabricated using using aa conveconv conventionalentionalntional chemical-basedchemical chemical-based-based fabricationfabrication fabrication process.process. process. AA moisture moisture moisture and andand dust dust green green-solder green-solder-solder mask mask mask waswas was usedused used toto protectprotect to protect the themetalized metalized layers layers fromfrom from oxidation.oxidation. oxidation. The The spacingThespacing spacing used used used between between between the the theplated plated-through plated-through-through holesholes holes ofof of11-mm-mm 1-mm ininin diameter diameter was was 1.6 1.6 mm.mm. mm. TheseThese These holesholes holes werewere were fabricatedfabricatedfabricated using using using a a a conventional conventional conventional drillingdrilling andand platingplating process. The The overall overall dimensions dimensions ofof of thethe the proposedproposed proposed 2 antenna structure are 60 140 mm 2. A photograph of the fabricated Yagi antenna is shown in Figure6. antennaantenna structure structure are are 60 60 ×× × 140 140 mm mm2. .A A photographphotograph ofof thethe fabricatedfabricated Yagi antenna isis shownshown inin Figure Figure The measured and simulated reflection coefficient results (parameter S ) are presented in Figure7. 6.6. The The measured measured and and simulated simulated reflection reflection coefficientcoefficient resultsresults (parameter11 S1111)) areare presentedpresented inin FigureFigure The7. The reflection reflection coe coefficientfficient values values (parameter (parameter S11 S)11 of) of the the proposed proposed antenna antenna areare lessless than −1010 dB dB in in the the 7. The reflection coefficient values (parameter S11) of the proposed antenna are less than −−10 dB in the frequencyfrequencyfrequency range range range 2.6 2.6–4.3 2.6–4.3–4.3 GHz, GHz, which which isis equivalentequivalent to to a a fractional bandwidth of of 49.3%. 49.3%. TheTheThe simulated simulatedsimulated and and measured measured measured E E-plane- E-planeplane (xy(xy-plane) (xy-plane)-plane) and and HH-Plane-Plane H-Plane (yz-plane)(yz- (yz-plane)plane) radiationradiation radiation patternspatterns patterns at at 3 3 GHz,atGHz, 3 GHz,3.5 3.5 GHz, GHz, 3.5 and GHz,and 4.0 4.0 and GHz GHz 4.0 are are GHz presented presented are presented inin FigureFigure in 8a8a–c, Figure–c, respectively.8a–c, respectively. Good agreementagreement Good is agreementis observedobserved betweenisbetween observed measuredmeasured between andand measured simulationsimulation and simulationresults.results. TheThe results. proposedproposed The antenna proposed exhibits antenna excellent excellent exhibits radiation radiation excellent radiationcharacteristics characteristics with the maximum with the maximum value of the value side-lobe of the level side-lobe (SLL) level being (SLL) less than being −10 less dB than compared10 dB characteristics with the maximum value of the side-lobe level (SLL) being less than −10 dB compared− tocomparedto the the main main to lobe thelobe. main .Other Other lobe. radiation radiation Other radiationparameters,parameters, parameters, suchsuch asas the such halfhalf-power as- thepower half-power beam widthwidth beam (HPBW),(HPBW), width (HPBW), cross-cross- polarization,cross-polarization,polarization, realized realized realized gain, gain, gain, andand andsideside-lobe side-lobe-lobe levelslevels levels forfor forthe the EE-plane-plane E-plane and and H-planeH H-plane-plane radiationradiation radiation patternspatterns patterns atat at differentdidifferentfferent frequencies frequencies are are summarized summarized inin TableTable3 3..3.

(a) (b) (a) (b) FigureFigure 6.6. PhotographPhotograph of the fabricated proposed antenna ( (aa)) front front view view ( (bb)) back back view. view. Figure 6. Photograph of the fabricated proposed antenna (a) front view (b) back view.

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Figure 7. Measured and simulated reflection reﬂection coecoefficientﬃcient response ofof thethe printedprinted YagiYagi antenna.antenna.

Table 3. Summary of the proposed antenna radiation characteristics. Table 3. Summary of the proposed antenna radiation characteristics. Frequency Gain HPBW HPBW SLL SLL Frequency Gain HPBW HPBW SLL SLL (GHz) (dBi) (E-plane) (H-plane) (E-plane) (H-plane) (GHz) (dBi) (E-plane) (H-plane) (E-plane) (H-plane) 2.9 6.5 63 88 −15 −13 2.9 6.5 63 88 15 13 − − 3.53.5 8.4 8.4 53 53 65 65 −1717 12−12 − − 3.8 8.2 45 53 14 10 3.8 8.2 45 53 −−14 − −10

TheThe 3D-radiation3D-radiation pattern graphs at 2.75 GHz and 3.5 GHz are presented in Figure 9 9a,b,a,b, respectively.respectively. At At higher higher frequencies, frequencies, the the antenna antenna beams beams become become more more directive directive and and concentrates concentrates towardtoward thethe end-fireend-fire direction.direction. AA snapshotsnapshot photographphotograph presentingpresenting thethe electricelectric fieldfield distributiondistribution overover thethe toptop planeplane ofof thethe antenna’santenna’s surfacessurfaces withwith andand withoutwithout plating via-holesvia-holes is shown in Figure 10a,b,a,b, respectively.respectively. It can be observed that via-holes via-holes concentrate concentrate maximum energy toward the the end-fire end-fire direction.direction. On the other hand, in the antennaantenna without via-holes,via-holes, a strong electricelectric fieldfield nearnear thethe edgesedges isis observed. observed. The The antenna antenna gain gain was was measured using standard gain horn-antennahorn-antenna measurements. Initially,Initially, thethe free-spacefree-space transmissiontransmission coecoefficientfficient (parameter(parameter SS2121)) waswas measuredmeasured usingusing (i)(i) aa broadbandbroadband hornhorn antennaantenna (LB-10180)(LB-10180) fromfrom thethe A-infoA-info companycompany andand (ii)(ii) a standard-gainstandard-gain hornhorn (SGH)(SGH) antenna from thethe NSINSI company.company. Next, the standard-gainstandard-gain horn antenna was replaced by thethe antennaantenna underunder testtest (printed(printed Yagi), Yagi), and and the the S21 measurements S21 measurements were were recorded. recorded. The gain The values gain were values calculated were calculated by consulting by theconsulting SGH data-sheet the SGH and data comparing-sheet and both comparing readings. both A comparison readings. A between comparison the simulated between andthe simulated measured gainand measured performance gain is performance shown in Figure is shown 11a. in Figure 11a. Both simulated and measured results are in good agreement with a difference not exceeding 0.8 dB. Commonly, such difference may occur due to connector’s insertion loss, fabrication tolerances, ± radiation pattern measurements performed in non-anechoic chamber environment, difference in substrate material definition used in simulation and real fabrication especially the dissipation factor (tanδ) value. From the parametric analysis, it can be discovered that antenna’s performance parameters especially the reflection coefficient and the gain values are significantly affected by the varying structural parameters in the order of few millimeters. The simulated antenna’s directivity and gain at 3.5 GHz are found to be 9.93 dBi and 8.31 dBi, respectively. This difference might be due to the fabrication tolerances of the antenna manufacturing process. Secondly, we considered that the substrate used in fabrication may have exhibited more dissipation loss than that one used in our simulation setup.

Micromachines 2020, 11, 975 10 of 15 Micromachines 2020, 11, x 10 of 16

H-Plane (Phi = 90) E-Plane (Theta = 90)

(a)

(b)

90 5 f= 4.0 GHz 120 60 0 Simulated -5 Measured -10 150 30 -15 -20 -25

180 0

210 330

240 300 270 (c)

FigureFigure 8. 8. SimulatedSimulated and and measured measured normalized normalized radiatio radiationn patterns patterns in in E-Plane E-Plane and and H-plane H-plane of of the the proposedproposed antenna, antenna, ( (aa)) ff == 33 GHz, GHz, (b (b) )f f= =3.53.5 GHz GHz and and (c ()c f) =f 4.0= 4.0 GHz. GHz.

Micromachines 2020, 11, x 11 of 16 MicromachinesMicromachines 20202020,, 1111,, 975x 1111 ofof 1516

f = 2.75 GHz f = 3.5 GHz f = 2.75 GHz f = 3.5 GHz

(a) (b) (a) (b) Figure 9. Simulated 3D radiation pattern patterns of the proposed antenna (a) f = 2.75 GHz and (b) f = Figure3.5Figure GHz. 9. SimulatedSimulated 3D 3D radiation radiation pattern pattern patterns patterns of the of theproposed proposed antenna antenna (a) f (=a )2.75f = GHz2.75 and GHz (b and) f = (3.5b) fGHz.= 3.5 GHz.

(a) (b) (a) (b) Figure 10. SnapshotSnapshot photograph photograph presenting presenting th thee electric electric field field distribution distribution (a) (witha) with via-holes via-holes and and (b) Figure 10. (withoutb) without via-holes.Snapshot via-holes. photograph presenting the electric field distribution (a) with via-holes and (b) without via-holes. ItBoth is important simulated to and calculate measured the antenna’s results are total in good efficiency agreement as following with a [difference39]: not exceeding ± 0.8 dB.Both Commonly, simulated such and difference measured may results occur are due in goodto connector’s agreement insertion with a loss,difference fabrication not exceeding tolerances, ± 0.8 dB. Commonly, such difference may occurT due= Rto connector’s insertion loss, fabrication tolerances,(2) radiation pattern measurements performed∈ in ∈non-anechoic× ∈lm chamber environment, difference in substrateradiation materialpattern measurementsdefinition used performedin simulation in andnon-anechoic real fabrication chamber especially environment, the dissipation difference factor in where,substrate materialR and definitionlm, correspond used in simulation to radiation and efficiencyreal fabrication and especially mismatch the loss, dissipationrespectively. factor (tanδ) value.∈ From∈ the parametric analysis, it can be discovered that antenna’s performance Congruently, total efficiency is defined as the ratio of the total power radiated by the antenna parameters(tanδ) value. especially From the the parametricreflection coefficient analysis, anit dcan the begain discovered values are that significantly antenna’s affected performance by the to the total power input to the antenna [40] varyingparameters structural especially parameters the reflection in the coefficient order of few and millimeters. the gain values The aresimulated significantly antenna’s affected directivity by the andvarying gain structural at 3.5 GHz parameters are found toin bethe 9.93 order dBi ofand few 8. 31millimeters. dBi, respectively. The simulated This difference antenna’s might directivity be due T= Prad/Pinput (3) toand the gain fabrication at 3.5 GHz tolerances are found of theto be antenna 9.93 dBi∈ manufa and 8.cturing31 dBi, respectively.process. Secondly, This difference we considered might that be due the to the fabrication tolerances of the antenna manufacturing process. Secondly, we considered that the however,substrate theused radiation in fabrication efficiency may comprises have exhibited the conduction more dissipation and dielectric loss losses than and that it canone beused calculated in our substrate used in fabrication may have exhibited more dissipation loss than that one used in our bysimulation the total setup. power radiated by the antenna to the total power accepted by the antenna [40], simulationIt is important setup. to calculate the antenna’s total efficiency as following [39]: It is important to calculate the antenna’s total efficiency as following [39]: R= Prad/Paccept (4) ∈ ∈= ∈×∈ (2) ∈= ∈×∈ (2)

Micromachines 2020, 11, 975 12 of 15 likewise, mismatch loss is [40] = 1 Г 2 (5) ∈lm − | | where, Г correspond to voltage reflection coefficient at antenna’s input terminal. If there is no mismatch Micromachinesat the input 20 terminal20, 11, x of the antenna the radiation efficiency will correspond to the total efficiency.13 of 16

(a) (b)

(c)

Figure 11.11. ProposedProposed Antenna’s Antenna’s (a ()a Gain) Gain vs. vs. frequency frequency (b) %(b e)ﬃ %ciency efficiency vs. frequency vs. frequency and (c )and time-domain (c) time- domainexcitation excitation response response received received by the x-oriented by the x-oriented probe. probe.

6. ConclusionsAntenna’s radiation efficiency is very important figure of merit for its characterization. Many measurements methods such as pattern integration, directivity/gain, wheeler cap, sliding wall In this paper, the concept of bandwidth enhancement of a conventional printed Yagi bi-layer cavity and Q-factor has been developed to find antenna’s radiation efficiency. A comprehensive antenna operating at the S-band was presented. Both metalized layers at the top and bottom antenna discussion on different efficiency measurement methods is found in [40]. Among several state of planes comprise ground plane, microstrip feedline, driven elements, and directive elements. The the art techniques, efficiency calculation based on directivity and gain values is found to be simple, bottom plane directive elements are placed with some offset toward the direction of propagation to accurate and fast [40]. We know antenna’s radiation efficiency is related to gain (G) and directivity (D) enhance the overall effective width. The aim was to simultaneously maximize gain and bandwidth. as following [34]: To achieve broad bandwidth matched to 50 Ω input impedance, elliptically-shaped directive η = G/D (6) elements were introduced in close proximity of the active dipole elements. Full-wave analysis of the tightland fory-coupled a directional directive antenna elements the directivity was performed is closely linked by conducting to the horizontal multipleθHP and electromagnetic vertical ϕHP simulationsbeam-widths using as following CST Microwave [41]: Studio. The performance of proposed antenna element having 4π wide directive element is compared with theD typical design without the wide directive element.(7) Compared to the traditional Yagi antenna, ≈ theθ HP proposedϕHP antenna outperformed by presenting simultaneously, larger bandwidth and higher gain values. The designed antenna exhibited a 49.3% fractional bandwidth operating in the frequency range 2.6−4.3 GHz. Plated via-holes were introduced near the left and right boundaries of the PCB board to concentrate the electromagnetic energy toward the main direction of the electric-field flow. The proposed antenna exhibits excellent radiation performance with an average gain of more than 7 dBi and maximum side-lobe levels of less than −10 dB. Due to the high-fidelity factor value, the proposed antenna can be used in broadband RF applications.

Micromachines 2020, 11, 975 13 of 15 due to signiﬁcant advancement in electromagnetic simulations tools, it is easy to determine antenna’s directivity using numerical simulations. Therefore, antenna’s radiation eﬃciency is calculated using measured gain and simulated directivity values as following [34]:

(D G) [ − ] % R= 10− 10 100 (8) ∈ × finally, having simulated and measured reflection coefficients values, the simulated and measured total efficiency is calculated by using (8) and (5). The graph presenting simulated and measured total efficiency is shown in Figure 11b. Due to higher dissipation factor of FR4, proposed antenna’s efficiency is worse at higher frequencies. The antenna performance for a wide-band signal transmission can be characterized by calculating the following fidelity factor [42]:

R ∞ x (t) x (t + t )dt 1 2 d FF = max q −∞ ∗ q (9) R 2 R 2 ∞ x1(t) dt ∞ x2(t) dt −∞ −∞ where, x1(t) and x2(t) are transmitted and received signals, respectively. The antenna’s response for a wideband excitation signal was recorded in the CST simulation program using an x-oriented Electric Far-ﬁeld probe, which was placed at 500 cm toward the radiation direction (along y-axis). Using (1), the pulse ﬁdelity factor was calculated to be 0.97. It can be concluded that the proposed printed Yagi antenna is capable of radiating broadband signals with low dispersion.

6. Conclusions In this paper, the concept of bandwidth enhancement of a conventional printed Yagi bi-layer antenna operating at the S-band was presented. Both metalized layers at the top and bottom antenna planes comprise ground plane, microstrip feedline, driven elements, and directive elements. The bottom plane directive elements are placed with some offset toward the direction of propagation to enhance the overall effective width. The aim was to simultaneously maximize gain and bandwidth. To achieve broad bandwidth matched to 50 Ω input impedance, elliptically-shaped directive elements were introduced in close proximity of the active dipole elements. Full-wave analysis of the tightly-coupled directive elements was performed by conducting multiple electromagnetic simulations using CST Microwave Studio. The performance of proposed antenna element having wide directive element is compared with the typical design without the wide directive element. Compared to the traditional Yagi antenna, the proposed antenna outperformed by presenting simultaneously, larger bandwidth and higher gain values. The designed antenna exhibited a 49.3% fractional bandwidth operating in the frequency range 2.6 4.3 GHz. Plated via-holes were introduced near the left and right boundaries of − the PCB board to concentrate the electromagnetic energy toward the main direction of the electric-field flow. The proposed antenna exhibits excellent radiation performance with an average gain of more than 7 dBi and maximum side-lobe levels of less than 10 dB. Due to the high-fidelity factor value, the − proposed antenna can be used in broadband RF applications.

Author Contributions: Main idea, design and simulation results production by M.A.A. (Muhammad Ahmad Ashraf). Original draft preparation and rivision by A.T. Advisory comments for paper improvement by K.J. Measurements and graphs generation by M.A.A. (Mohammed A. Alzabidi). Supervisory role for the entire project by A.R.S. All authors have read and agreed to the published version of the manuscript. Funding: This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (12-ELE2462-02). Conﬂicts of Interest: The authors declare no conﬂict of interest. Micromachines 2020, 11, 975 14 of 15

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