IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019 2401

A Compact Wideband Slot-Loop DirectionalAntenna for Marine Communication Applications

Lidong Chi, Student Member, IEEE, Yihong Qi , Senior Member, IEEE, Zibin Weng , Member, IEEE,Wei Yu, Member, IEEE, and Weihua Zhuang , Fellow, IEEE

Abstract—A compact wideband high gain directional antenna isproposed in this paper for marine communications. The antennaconsists of a back plane (reflector), a slot-loop antenna (driven ele-ment), and a director of rod. Based on the current distribution, theslot-loop antenna is equivalent to an array of three linear currentdipoles. The dipole acts as an element of the director, which pro-vides a high efficient director for slot-loop antenna according totheoretical analysis. Both the impedance bandwidth and the 1 dBgain bandwidth are 45% covering 1.7–2.7 GHz in this design. Again of 11.2 to 12.2 dBi is obtained in the working band with onlyone set of directors. The directors have a dimension less than theantenna aperture in the H-plane, which can be seen as one directorin the end-fire direction. Compared with the classic Yagi antennawith the same gain level, the newly proposed antenna is only halfof the size in radiation direction, which is a breakthrough for theYagi antenna design. This size reduction can significantly mini-mize the interference between ship-borne antennas. The antennais fabricated using only metal of low cost, which is useful for theship-to-shore/ship communicating applications and desirable forthe large-scale deployment.

Index Terms—Directional antenna, ship-to-shore, ship-to-ship,slot-loop antenna, director.

I. INTRODUCTION

W ITH rapid development of wireless communication sys-tems, communication technologies such as internet of

things and vehicle-to-everything and etc. are changing the waypeople live [1]–[4]. Highway and high speed railway have al-ready been covered by the 4th generation (4G) cellular net-works and recently some aircrafts are equipped with Wi-Fisystems, which are welcomed by passengers. Still, fewer andfewer people travel by ships in consideration of not only time,

Manuscript received August 25, 2018; revised October 31, 2018; acceptedDecember 15, 2018. Date of publication January 28, 2019; date of currentversion March 14, 2019. The review of this paper was coordinated by Dr. Y.Gao. (Corresponding author: Yihong Qi.)

L. Chi is with the Hunan University, Changsha 410082, China, andalso with the General Test Systems Inc., Shenzhen 518000, China (e-mail:,565548254@qq.com).

Y. Qi is with the Hunan University, Changsha 410006, China, with the Gen-eral Test Systems, Shenzhen 518102, China, and also with the Missouri Uni-versity of Science and Technology, Rolla, MO 65409 USA (e-mail:, yihong.qi@dbjtech.com).

Z. Weng is with the Xidian University, Xi’an 710071, China (e-mail:,zibinweng@mail.xidian.edu.cn).

W. Yu is with the General Test Systems Inc., Shenzhen 518000, China (e-mail:,fred.yu@generaltest.com).

W. Zhuang is with the Department of Electrical and Computer Engi-neering, University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail:,wzhuang@uwaterloo.ca).

Digital Object Identifier 10.1109/TVT.2019.2892154

but also communication services. Because it is difficult to enjoybasic mobile communication services on ships, let alone highspeed 4G communication and video streaming. Sometimes, pa-tients on board need urgent video medical first aid [5]–[7]. Tomeet these demands, some researchers proposed 4G ship-to-ship/shore communications as shown in Fig. 1; and the U.S.Navy ships have been getting 4G LTE broadband service since2011 [8]–[10]. With fast research and deployments, 5G is be-lieved to be one of the main marine communication infrastruc-ture in the near future. A shipborne antenna with widebandhigh-gain property and mechanical scanning characteristic isdesirable for ship-to-shore communications. Compared to log-periodic antenna array, parabolic reflector antenna and base sta-tion antenna, the array composed of cost-effective Yagi antennasobtains a higher gain with low wind load [11]–[19]. However,the classic Yagi antenna has only 5% fractional bandwidth (theratio of bandwidth to center frequency), which is inadequatefor the 4G LTE applications. Besides, the large dimension ofthe high gain Yagi antenna arrays can increase the undesirableelectromagnetic interference between the ship-borne antennas.Thus, a compact size, wideband bandwidth and high gain im-provement should be considered for the classic Yagi antenna[20]–[21].

According to recent studies, the impedance bandwidth ofYagi antenna has been significantly improved. The concept ofQuasi Yagi antenna was first proposed by William R. A, and abandwidth of 48% and a gain of 3–5 dBi were obtained withonly one director [22]. Then Hui, using the self-complementarydriver, enlarged the bandwidth to 74% with a gain of 4–8 dBiby three directors [23]. Junho proposed a double-dipole quasi-Yagi with two directors, obtaining a bandwidth of 78.4% anda gain of 6.4–7.4 dBi [24]. The existing studies focus mainlyon the impedance bandwidth improvement of driven elements.However, the directors are nearly unchanged from the conven-tional Yagi antenna, and the driven elements are still traditionaldipoles. As a result, the gains obtained are less than 8 dBi inthe working bands. Wideband and high gain compact directionalantennas have not been realized according to the open literature.

The driven element largely restricts the bandwidth and gainof Yagi antenna. Slot-loop antenna, also called wideband highefficient electromagnetic structure (WHEMS), with its wide-band high-gain characteristic, is a good candidate for the excitedelement [25]–[28]. Liang, by using a slot-loop antenna as theexcited element along with three sets of small loop directors,reached a gain of 11–13 dBi, showing a great potential of the

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2402 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Fig. 1. Ship-to-shore and ship-to-ship communications.

Fig. 2. Illustration of the proposed antenna.

WHEMS in attaining wideband and high gain [27]. In [27], eachset of directors has the dimension less than the H-plane aperture.Therefore, in the end-fire direction, one set of directors has thesame contribution to the size increment with one director in theclassic Yagi antenna. However, the large size in the radiatingdirection is not desirable for a highly integrated communicationsystem. Liang chose the loop as the element of directors, sinceWHEMS is in a shape of slot-loop. Nonetheless, with the currentdistribution study of the slot-loop antenna, which is shown inthis study, the loop may not be the most efficient director forslot-loop antennas. Therefore, the size is possible to be reducedby using an efficient director without gain degradation, whichcan minimize the interference between ship-borne antennas.

Using a dipole array as the director of slot-loop antenna,based on the current distribution study of the driven element,is first proposed in this paper. The rod-array directors is theo-retically proven to be a highly efficient director which reachesthe theoretical limits. As shown in Fig. 2, the designed direc-tional antenna consists of a flat back plate, a slot-loop antennaand a set of dipole-array director. With this structure, the pro-posed antenna is compact in the radiating direction, where the

conventional Yagi antenna achieving the same gain is two timeslarger. The antenna is targeted to reach a wide impedance andhigh gain bandwidth, which is believed as a good candidate forship-to-ship/shore communication applications.

Three major contributions of this paper are as follows1) As for conventional Yagi antennas, the gain is inversely

proportional to the bandwidth, which is always compro-mised when achieving a high gain. Therefore, it is difficultfor Yagi antennas to obtain a high gain in wide bandwidth.The antenna proposed in this paper can achieve a widehigh gain bandwidth, which is a breakthrough for Yagiantennas;

2) The dipole is first-time used as the director of slot-loopantennas and is theoretically proved as a high efficientdirector;

3) Miniaturization of Yagi antennas can always attract atten-tion from researchers. For Yagi antennas, the size alongthe radiating direction is too large for nowadays appli-cations. The newly proposed antenna reduces the size ofYagi antennas by half while maintaining the same gainlevel, which can decrease the undesirable interference be-tween shipboard antennas.

The rest of this paper is organized as follows. In Section II,we first discuss some limitations of conventional Yagi anten-nas. Then according to current distribution studies, the dipolearray director is derived, showing a highly efficient directingcharacteristic for slot-loop antennas. The antenna configurationis introduced in Section III, and current studies of the slot-loop antenna along with the design of director are presented inSection IV. Subsequently, the simulated and measured resultsare discussed in Section V, validating the design concept ofthis paper. Finally, some conclusions of this study are drawn inSection VI.

II. THEORY

The narrow bandwidth (including both impedance bandwidthand gain bandwidth) and large size are two main limitations forthe classic Yagi antenna in various practical applications such asin ship-to-shore/ship communications. The quasi-Yagi antennacan increase the impedance bandwidth of classic Yagi antennafrom 5% to more than 78% by modifying the feeding structure.

CHI et al.: COMPACT WIDEBAND SLOT-LOOP DIRECTIONAL ANTENNA FOR MARINE COMMUNICATION APPLICATIONS 2403

TABLE ICOMPARISON WITH EXISTING WIDE BAND DIRECTIONAL

ANTENNAS WITH DIRECTORS

λc is the wavelength of the center frequency.BW is fractional bandwidth.

However, in the quasi-Yagi antenna design, the driven elementis half a wavelength at high band, which means a shorter diploeat low band. It is known that the radiated power significantlydecreases when the length of diploe antenna is less than halfwavelength. This is the reason that the gain of quasi-Yagi an-tenna is less than that of the classic Yagi antenna, as shown inTable I. Overall, it is challenging for the Yagi antenna to achievewideband high-gain performance in a compact size.

To overcome the narrow bandwidth and large physical sizelimitations, here a slot-loop antenna is proposed as the drivenelement. Compared with the wideband dipole of the quasi-Yagiantenna, it has the following advantages:

1) The slot-loop antenna has wide impedance bandwidth,which is a desired candidate as the driven element for awideband end-fire directional antenna with directors;

2) The large dimension in the radiating direction is alwaysa problem of the Yagi antenna. In comparison, slightlyincreasing the dimension in H-plane is acceptable. How-ever, both classic Yagi antenna and quasi-Yagi antenna donot make use of their H-plane dimension. As the slot-loopantenna corresponds to an array of three linear currentsin the H-plane, the gain performance is enhanced with alarger H-plane aperture. Indeed, it can achieve the samegain with a smaller director, leading to a more compactantenna with better gain performance;

3) As dipole is a resonant antenna, reducing its lengthwill decrease its radiating performance. The structureof the slot-loop antenna ensures the radiated power ofits three equivalent dipoles in wideband without sacrific-ing the resonant length. Therefore, the gain performanceof the new antenna can be consistent in a wide bandwidth.

As a result, using a slot-loop antenna as the driven elementprovides a potential solution to the narrow bandwidth and largedimension of classic Yagi antennas.

TABLE IIEXPLANATIONS OF THE VARIABLES

The slot-loop driven element requires a properly designeddirector. Both classic Yagi antenna and quasi-Yagi antenna usea rod as the narrowband director. The directing phase can betuned only with the rod length and the distance between thedirector and the driven element. In contrast, the proposed slot-loop antenna can be equivalent to a three-dipole array in widebandwidth in terms of the current distribution. A three-rod arraydirector is devised accordingly for the proposed antenna, whichnot only ensures the H-plane directing but also enhances thedirecting bandwidth by introducing more phase tuning factors.This, thus, will make the proposed antenna reach a reasonablehigh gain over a wide band.

In summary, the newly proposed antenna achieves widebandhigh gain performance in a compact size. As shown in Table I,the proposed antenna has the same gain with only half size ofthe classic Yagi antenna, achieving an impedance and 1 dB gainbandwidth of 45%. The theoretical analysis of the proposed an-tenna is presented next, where all the calculations are performedby using Matlab [29].

A. The Yagi Antenna

Fig. 3(a) shows a simple model of the classic Yagi antenna,which contains three basic elements, a driven element, a direc-tor and a reflector. The variables in Fig. 3(a) are explained inTable II: Zmutual depends on d, which is generally from 1/4λ

to 1/3λ for the optimum phase tuning; Length of the rod decidesZself—another variable in phase tuning for directing. For bothclassic Yagi antenna and quasi-Yagi antenna, the directors mustbe shorter than the driven elements; A director will be changedinto a reflector when the rod is longer than the driven elementdipole. The directional radiation performance depends on bothZmutual and Zself .

To better present the theoretical discussion of the proposedantenna, the driven element and director of the classic Yagiantenna are first discussed. Fig. 3(b) shows a simple model of adipole antenna and a rod director, which are the crucial elementsof a classic Yagi antenna. Elements “0” and “1” represent the

2404 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Fig. 3. (a) A simple model of classic Yagi antenna. (b) Model of a drivenelement with a director.

driven element and the director, respectively. The variables arelisted in Table II. The model in Fig. 3(b) can be described by

U 0 = Z00I0 + Z01I1

U 1 = Z10I0 + Z11I1. (1)

In (1), U 1 equals 0 on account that the middle of the “1”element is short-circuited. Then, the input impedance Zin canbe deducted from (1) as

Zin = Z00 − k2Z11 (2)

where k = |I1/I0|, which is in the order of 10−1 [31]. As aresult, we can ignore the term −k2Z11. The input impedancemainly depends on that of the driven element. Dipole antenna,the driven element of the classic Yagi antenna, has narrow frac-tional bandwidth, which is around 5%. Hence the fractionalimpedance of the classic Yagi antenna is around 5%, which isnarrow for the 4G LTE applications. An easy way to widenthe bandwidth of the classic Yagi antenna is using a widebanddriven element. Thus, by improving the feeding structure, thequasi-Yagi antenna expands the impedance to more than 78%

Fig. 4. The current distribution on the dipole antenna of different lengths.

as shown in Table I, but the gain in wide band is not enhancedsimultaneously.

From (1), the relationship between I0 and I1 is

I1/I0 = − ∣∣Z01

∣∣ /

∣∣Z11

∣∣ ∠ + α (3)

where α = π + θ01 − θ11 is the phase I1 lagging behind I0, θ01

is the phase of the mutual impedance between element “1” and“0”, and θ11 is the phase of the self-impedance of the director.Let γ = 2πd/λ denote the wave path-difference between “1”and “0”, where λ represents the wavelength. Note that α shouldbe equal to γ, to let the phase of the wave generated by “1” equalto the phase of the wave propagating to “1”, which will finallyform an end-fire radiation. Thus, the phase should satisfy thefollowing relation

θ01 − θ11 = γ − π. (4)

From (4), the end-fire directional radiation performancebrought by the director is adjusted by both the phase of θ01

and θ11. If the distance between “0” and “1” is determined, θ01

will be a constant, so there is only one value for θ11 to satisfy(4). The situation is likewise while θ11 is fixed. As a result,the limited choices of the two factors restrict the antenna fromrealizing a wideband directing performance. This is one of thereasons that both the Yagi antenna and quasi-Yagi antenna can-not achieve wideband high gain. The total radiated power of thelinear dipole antenna is given by

P =√μ

ε

β2L2I2av

12π(5)

where μ is permeability and ε is dielectric constant, and β ispropagation constant [30], L is the length of dipole antenna,and Iav is the amplitude of average current on dipole. Since thecurrent is sinusoidally distributed, the value of Iav is Imax/

√2,

where Imax is the peak amplitude of the current. From (5),the radiated power P is proportional to the square of L andIav . Fig. 4 shows the current distributions on three dipoles of

CHI et al.: COMPACT WIDEBAND SLOT-LOOP DIRECTIONAL ANTENNA FOR MARINE COMMUNICATION APPLICATIONS 2405

different lengths, which are less than a half wavelength, a halfwavelength, and a wavelength, respectively. From Fig. 4, whenthe length of dipole is less than a half wavelength, the antennahas smaller Imax , which leads to the loss of radiated power by(5). When the diploe length is equal to or greater than a halfwavelength, Imax reaches its maximum value, and the radiatedpower suffers no loss from the amplitude of the current by (5).When L is larger than a wavelength, sidelobes occur, which isnot desired [30]. Therefore, it is desirable for the dipole to havea length between a half wavelength and a wavelength.

Quasi-Yagi antenna increases the impedance bandwidth,comparing with the classic Yagi antenna, by improving the feed-ing structure. However, the radiating structure of driven elementremains to be the classic dipole. In [32], the length of the quasi-Yagi driven element is 0.348λh , where λh is the wavelength ofthe highest frequency. As the length of the driven element isdevised less than half wavelength of the highest frequency, theantenna works as a far less than half wavelength dipole in mostof the frequency band, leading to a less gain from the quasiYagi antenna calculated by (5). This interprets the phenomenonshown in Table I that the quasi-Yagi antenna achieves less gainthan the classic Yagi antenna.

Yagi antenna is able to obtain a higher gain by increasingthe number of directors, which causes its large dimension inthe radiating direction. Compared to the dimension in the ra-diating direction, the flank space is not well utilized. Both thegain improvement and antenna compactness can be achieved byincreasing the flank dimension of the Yagi antenna. A dipoleantenna in the H-plane has an equivalent aperture of half wave-length. If the H-plane aperture increases to a wavelength, thegain will be improved by 3 dB in maximum, which is morecost efficient compared with that of the end-fire aperture [30].Meanwhile, an increase of the H-plane aperture improves theflexibility of the director design. It is possible to introduce morephase tuning factors such as Zmutual and Zself to widen thebandwidth of the director.

Therefore, for the Yagi antenna, there are four points inachieving a wideband and high-gain compact directional an-tenna:

1) improving the impedance bandwidth of the driven ele-ment;

2) making the antenna radiate more efficiently by ensuringthat the resonant length of the driven element is longerthan half wavelength;

3) well utilizing the H-plane aperture to enhance the gainperformance;

4) introducing more phase tuning factors to widen the band-width of the director.

A slot-loop antenna is proposed as the driven element in orderto solve 1), 2), and 3), while the rod array director is proposedas the director aiming to solve 4).

B. Current Distribution on Slot-Loop Antenna

Fig. 5 shows the current distributions on a simple model of theslot-loop antenna. The antenna has a symmetric slot loop shape,where the part indicated by shading represents the conductor.

Fig. 5. Current distributions on the simple model of slot-loop antenna.

Fig. 6. The equivalent current distributions model on slot-loop antenna.(a) The E-plane and H-plane decomposition of the currents on slot-loop an-tenna. (b) The equivalent H-plane currents source model of slot-loop antenna.

In Fig. 5, l is the length of the slot edge and c is the perimeterof half of the whole slot. Due to the symmetric structure, thecurrent distributions are the same in both sides. Thus, only halfof the current distributions are shown.

As the middle of the slot-loop antenna edge is short circuited,region A is the current crest point. Counting from A, regionB represents two current trough points after a half wavelength.Suppose working band is within the interval of [fl , fh ]. Shownin Fig. 5, i1 is the current flowing on the slot edge, i2 is cur-rent between i1 and region B, and i3 is the current betweenregion B and the feeding point. As l is devised smaller than ahalf wavelength of fh , region B is not on the slot edge over thewhole working band. As c is designed roughly a wavelength atthe center frequency and the fractional bandwidth of the pro-posed antenna is 45%, hence, c is larger than 1/2λ of fl andsmaller than 3/2λ of fh . Then, there is only one pair of currenttrough points (region B) on this half of the antenna. The currentschange flowing directions at the two points, which can be seenfrom i2 and i3. Therefore, the flowing directions of the currents,shown in Fig. 5, can maintain consistent over a 45% bandwidth,while the two trough points are constrained between the slotedge and the feeding point.

C. Equivalent Radiation Model of Slot-Loop Antenna

The ideal current distributions are shown in Fig. 6(a), wherei1, i2 and i3 are decomposed into components of the H-plane and

2406 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Fig. 7. The H-plane of theoretical model of proposed antenna.

the E-plane, respectively. Owing to i1 is in E-plane originally, ithas no H-plane components. The same is with i2, which only hasH-plane components. Also, i3 is decomposed into the E-planeand the H-plane components, which are in blue dash line andgreen dot line, respectively. The E-fields generated by i3 in theH-plane cancel each other out, which is the same for i2. However,the E-fields generated by i3 in the E-plane reinforce each other.According to the superposition principle, the H-plane radiationsource of WHEMS is equivalent to a three linear currents arrayshown in Fig. 6(b). The linear currents are located right at threecrest points of the slot-loop antenna, respectively, which are atthe middle and both edges of the antenna. As a standing waveantenna, the amplitude and phase of the three linear currents aresupposed to be the same in order to achieve the maximum gain.

The equivalent three linear currents array increases the H-plane aperture of the slot-loop antenna, which can achieve ahigher gain and in turn reduce the size in the end-fire direction.Since the structure of the slot-loop antenna maintains the reso-nant length of the currents mentioned above, the three equivalentdipoles are efficient.

D. Theoretical Model and Analysis

To ensure the uniformity of the E-field aperture, a three-rod array director is designed accordingly. Fig. 7 shows thetheoretical model of the proposed antenna in the H-plane. InFig. 7, the driven element — slot-loop antenna is presentedas three linear currents based on the equivalent model. Thethree rods are right above the three linear currents, respectively.Supposing the reflector as an infinite back plate, both the currentcomponents and the three-rod director have their virtual imagecurrent sources based on imaginary theory, which are shown inFig. 7 [30].

In Fig. 7, d1 is the distance between the driven element andthe back plate; d2 is the distance between the director and thedriven element; s is the spacing of each adjacent director andthe linear currents; φ is the radiation direction counting fromclockwise from the positive y axis. As is mentioned, the dis-tance between director and driven element is optimally designed

TABLE IIIRESULTS OF THE THEORETICAL CALCULATION

between 1/4λ to 1/3λ. Considering the performance of thewhole working band, d1 is chosen as a quarter wavelength ofthe lowest frequency (fl = 1.7 GHz), which is a third wave-length of the highest frequency (fh = 2.7 GHz). From arraytheory, the element spacing is chosen to be less than half wave-length to ensure low side lobes [30]. Therefore, s is devised lessthan half wavelength of the center frequency (fc = 2.2 GHz).The H-plane radiation equation of the theoretical model shownin Fig. 7 is described by

E = A1 × ej×(ψd 1) +A1 × ej×(ψd 1+ψs ) +A1 × ej×(ψd 1−ψs )

−A1 × e−j×(ψd 1) −A1 × e−j×(ψd 1+ψs) −A1 × e−j×(ψd 1−ψs)

+A2 × e−j (ψd 1+ψd 2−ψd i r ) +A2 × e−j (ψd 1+ψd 2−ψd i r +ψs )

+A2 × ej (ψd 1+ψd 2−ψd i r −ψs ) −A2 × e−j (ψd 1+ψd 2+ψd i r )

−A2 × e−j (ψd 1+ψd 2+ψd i r +ψs) −A2 × e−j (ψd 1+ψd 2+ψd i r −ψs)

(6)

where ψd1= 2×π×d1×cosφ/λ, ψd2= 2×π×d2×cosφ/λ,ψs= 2 × π × s× sinφ/λ, and ψdir= 2 × π × d2/λ. Thesephases are obtained based on both the position in the modeland image theory. In (6), A1 and A2 are the currents amplitudeof the driven element and the director, separately. From Fig. 7the proposed antenna is actually an array of three Yagi antennas.In [31], the current amplitude ratio of the first director and thedriven element in an optimized Yagi antenna is 0.3. Hence, thevalue of A2/A1 is chosen to be 0.3 to calculate the maximumgain. From (6), the beamwidth of the H-plane is calculated andshown in Table III. The minimum beamwidth of the E-planeis decided by the largest antenna aperture in this plane, whichis related to the size of the reflector. Generally, the length ofthe directional antenna‘s back plate is a wavelength. To achievebetter gain performance at fl , the length of the plate should be awavelength of the lowest frequency. According to the full-wavedipole, the E-plane beamwidth is obtained and shown in TableIII [30]. For the directional antenna, the gain can be derivedfrom

Gain =40000

φH × φE(7)

where φH and φE are the 3-dB H-plane and E-planebeamwidths, respectively [30]. By (6) and (7), the gain is

CHI et al.: COMPACT WIDEBAND SLOT-LOOP DIRECTIONAL ANTENNA FOR MARINE COMMUNICATION APPLICATIONS 2407

Fig. 8. The gain results of the theoretical calculation.

obtained as shown in Table III. The calculated gain curve isshown in Fig. 8. As can be seen, the theoretical model achieves a10.9 dBi to 13.2 dBi gain in a 45% bandwidth, signifying a 45%2.3 dB gain bandwidth. The result is attained based on the as-sumption that the director has an optimum directing phase overthe whole band. This is impractical in the actual antenna design,for the director is narrowband. It means that if we want to getbest gain performance at low band, close to the ideal theoreticalresult, the gain at high band may be sacrificed. Because whenthe director is designed optimized in low band, the length of thedirector may larger than a half of the wavelength in high band.Then the director may become the reflector in high band.

III. ANTENNA CONFIGURATION

The configuration of the proposed antenna is shown in Fig. 9.The antenna is composed of a flat back plate, a slot-loop antenna,and a rod-array director. The plate is devised with the dimensionof one wavelength at fl in consideration of the gain performanceat low frequency band. The height of the driven element from theplate is a quarter wavelength at fc . A strictly axisymmetric slot-loop antenna is designed with a polygonal shape along with apolygonal slot. The perimeter of half the slot is 143.8 mm whichis approximately a wavelength of fc . Parameter W3 is chosen asless than a wavelength at fh to make the sidelobe low. The drivenelement and the director are connected by the metal connector. Inorder to reach wide band directional performance, the rod-arraydirector is composed of three rods which are arranged in a row,paralleling to the slot-loop antenna. The length of each rod is lessthan a half wavelength at fh , and the rods are connected by themetal crossbeam. The geometrical parameters of the proposedantenna are given in Table IV. The electromagnetic simulationsare done using the High Frequency Structure Simulator [32].

IV. ANTENNA ANALYSIS AND DESIGN

A. The Simulated Current Distribution on Slot-Loop Antenna

Antenna directors can be divided into two types from itsstructure: similar director and non-similar director. A similardirector represents a director whose shape is similar to its excitedpart. Compared to the driven element, the similar director issmaller in size, following the Yagi rules [30]. On the contrary,the structure of non-similar director is different from its excitedpart. When devising a non-similar director, the induced current

Fig. 9. The configurations of the proposed antenna. (a) The configuration ofthe front view. (b) The configuration of the side view. (c) The configuration ofthe slot-loop antenna.

TABLE IVGEOMETRIC PARAMETERS OF THE PROPOSED ANTENNA (UNIT: mm)

distribution on the excited antenna should be carefully analyzedto obtain a high directing efficiency.

As previously mentioned, the slot-loop antenna is the excitedpart of the antenna; its computed current distributions are shownin Fig. 10. The simulated current distributions on the slot-loopantenna are nearly the same over the working band (45% frac-tional bandwidth). The differential mode currents in the middleof the antenna are on the E-plane, and are the main radiatingsource in this part of the antenna. The perimeter of half the slotis nearly a wavelength at fc . According to the property of time-harmonic guide wave, there are always two trough points in awavelength. The flowing directions of the currents are changedat trough points, as shown in Fig. 10. Meanwhile, the points aremoving away from the feed point when the working frequencyincreases. The two E-plane currents, existing on both sides ofthe antenna, have the same flowing direction as that in the mid-dle. To design an effective director, the three rods should be

2408 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Fig. 10. Simulated current distributions on slot-loop antenna on the E-plane:(a) at 1.7 GHz; (b) at 2.2 GHz; and (c) at 2.7 GHz.

devised right above the three sources separately, at both sidesand in the middle.

The simulated current distributions on the slot-loop antennaagree with the analysis in Section II. The H-plane simulated cur-rent results in Fig. 10 validate the theoretical equivalent modelshown in Fig. 6. These current distributions can be the samewithin the whole working band, giving rise to a wideband radi-ation at the designed frequency. Hence, a wideband accordantradiation characteristic can be realized.

B. Director Design

Based on the current study, the radiation characteristic of theslot-loop antenna is equivalent to an array of three equidistantlinear current distributions—the differential currents in the mid-dle and the two currents along the perimeter of the antenna slot.The rod is an efficient director to direct the wave generated bythe linear current source. Therefore, to direct the source of thethree linear currents, a three-rod array is proposed to be the di-rector of the slot-loop antenna. As shown in Fig. 9(b) and (c),the three rods are arranged in a row paralleling to the slot-loopantenna. They are distributed right above the three currents crestpoints, which are at the feed point and both edges of the slot,respectively. From Table IV, the length of rod in the middle islonger than that of the rods at both sides, and all of them are lessthan half wavelength at fh .

Fig. 11 shows the impedance variation brought by the direc-tor, which is proven as a secondary adjustment for the inputimpedance of the antenna, as mentioned in Section II. The di-rector introduces another resonant point in the working band,making the impedance matching easier without widening thebandwidth.

The three rods in the director bring more phase tuning fac-tors, improving the bandwidth of the director. For convenience,the three rods of the director are numbered #1, #2 and #3,

Fig. 11. (a) |S11 | curves of the antenna with and without the director.(b) Impedance curves of the antenna with and without the director.

Fig. 12. Gains of the antenna in different cases of director.

respectively, as shown in Fig. 12. The simulated gain resultsof various directors are also presented in Fig. 12. The gain en-hancement of the three cases in Fig. 12 are nearly the same inthe low frequency band. However, the performance in the highfrequency band varies greatly. When the director is composedonly of rod #2, the gain at fh is less than that of the antenna

CHI et al.: COMPACT WIDEBAND SLOT-LOOP DIRECTIONAL ANTENNA FOR MARINE COMMUNICATION APPLICATIONS 2409

Fig. 13. E-field distribution of the antenna in H-plane at 1.7 GHz, 2.2 GHz, and 2.7 GHz, (a) with rod #2, (b) with rods #1 and #2, (c) with all rods, and(d) without rods.

without the director. The best gain enhancement property isrealized when all three rods are included in the director.

To explain the results, the E-field distributions of the threecases and the case without rods are presented respectively inFig. 13. The three sub figures in Fig. 13(a) are the antenna withonly rod #2 at 1.7, 2.2 and 2.7 GHz, respectively; It is similarto those in the other three subfigures, where rod #1 and #3 areapplied in Fig. 13(b), all rods are used in Fig. 13(c), and no rodsare in Fig. 13(d). Following the array principle, the maximumdirectivity is achieved by a uniformity in E-field distributions[30]. As shown in Fig. 13, these distributions are nearly the samein all of the first three cases at 1.7 GHz, which explain the gainenhancement at fl . At 2.7 GHz, the obvious inhomogeneousdistributions, seen in Fig. 13(a)–(b), lead to a gain depressionin both cases. In comparison, a favorable uniformity is shownin the E-field distribution in Fig. 13(c), revealing a 3 dB gainenhancement at 2.7 GHz. Besides the end-fire aperture increase,we observe from Fig. 13(c) that the E-field distribution is smoothacross the whole working band with three rods as compared withFig. 13(d), which is accordant with the simulation results inFig. 12. In this way, the proposed rod-array director is efficientin directing the radiation in a wide frequency band.

C. Parametric Study

The parametric studies of Ld1 and Ld2 are shown inFig. 14(a)–(b), respectively. As is shown in Fig. 9(a), Ld1 isthe length of director #2, which is in the middle; and Ld2 is thelength of director #1 and #3, which are on both sides.

In Fig. 14(a), when Ld1 is 30 mm and 40 mm, the gainperformance are nearly the same. This is because the directoris too short to be efficient in directing. The gain in most ofthe band is higher than that without the director, which is dueto the contribution of the other two directors. When Ld1 is 50mm, the gain performance becomes even in the operating band,and the antenna has a wide 1-dB gain bandwidth. when Ld1 is60 and 70 mm, the gain performance decreases greatly in themiddle of the band. However, the gain at high band remains still,which is caused by the directing of the other two directors.

Fig. 14. (a) Parameter study of Ld1 . (b) Parameter study of Ld2 .

In Fig. 14(b), when Ld2 is 30 mm the director is too short tobe efficient in directing. However, the gain in low band has a0.6 dB enhancement caused by the director in the middle. WhenLd2 is 40 mm, a 2.5 dB gain increase in the high band is broughtand the gain performance is even in the whole working band.when Ld2 increases from 50 mm to 70 mm, the gain decreasesmore at high band. However, the gain increases in low bandwhen Ld2 is larger. If we want to get best gain performance atlow band, close to the ideal theoretical result, the gain at highband may be sacrificed. Therefore, the final value ofLd1 andLd2

are chosen in consideration of the wide gain bandwidth and theeven gain performance in the whole working band.

2410 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Fig. 15. Photograph of the prototype.

Fig. 16. Simulated and measured |S11 |.

From Fig. 13(d), the E-filed in the middle, generated byWHEMS, is leading before that on both sides. Therefore, thecurrent induced in rod #2 is leading before that induced in rods#1 and #3. As mentioned, the maximum directivity is achievedby a uniformity in E-field distributions. To obtain a uniformE-filed distribution, shown in Fig. 13(c), the final E-filed gener-ated by rods #1, #2 and #3 should be in phase with each other.Thus, Ld1 should be larger than Ld2 to make the self-phase ofrod #2 lagging behind that of rods #1 and #3. This is the reasonthat Ld1 is larger than Ld2 in the final optimized parameters.

V. SIMULATED AND MEASURED RESULTS

Fig. 15 shows a prototype of the antenna. The antenna is fedby a coaxial transmission line which has electric connectionwith the back plate. The slot-loop antenna is connected withthe plate by 8 polyester rods. The middle of both edges of thedriven element is the minimum voltage point, which is the samein the middle of the three rods. The driven element and therods are connected at the voltage trough points, which do notaffect the performance of the antenna in terms of the Yagirules [30]. The proposed antenna was measured by a KeysightE5071C vector network analyzer and a far-field measure system.

A. |S11 |

Simulated and measured |S11 | results are shown in Fig. 16.The trend of the simulated results is quite accordant with themeasured results. As is discussed in Section IV, an extra res-

Fig. 17. Simulated and measured gains.

Fig. 18. Simulated and measured patterns.

onant point is introduced by the director, which improves thematching performance over the working band. The simulated|S11 | is below −10 dB from 1.69 GHz to 2.78 GHz, and themeasured |S11 | is below −10 dB from 1.77 GHz to 2.79 GHz.

B. Gains

Fig. 17 shows simulated and measured gain results. The sim-ulated gain is between 11.2 dBi to 12.2 dBi, and 1-dB gainfractional bandwidth is 45.45%. Generally, an 11.5 dBi gainis obtained by an optimized classic Yagi antenna with 7 ele-ments, which includes the reflector and the excited part [21].The size of the classic Yagi antenna is two times larger thanthe proposed antenna in the radiation direction. The measured

CHI et al.: COMPACT WIDEBAND SLOT-LOOP DIRECTIONAL ANTENNA FOR MARINE COMMUNICATION APPLICATIONS 2411

results are 10.6 dBi to 12.7 dBi, which is quite accordant withthe simulated results.

C. Radiation Patterns

Fig. 18 shows simulated and measured pattern of the proposedantenna at 1.7 GHz, 2.2 GHz and 2.7 GHz, respectively. A fineagreement is obtained between the simulation and measurementresults. The beamwidth is 36 degree to 61 degree on the E-planeand 35 degree to 47 degree on the H-plane. The results show anexcellent directivity property.

VI. CONCLUSION

A compact wide band directional antenna is proposed witha rod-array director. The theoretical equivalent model of theantenna is derived. Through the theoretical analysis of the pro-posed antenna, the dipoles array is proven to be a high effi-cient director for WHEMS. Followed this method, a 45.45%1-dB gain bandwidth is obtained by the proposed antenna alongwith an 11.2–12.2 dBi gain. The fine pattern characteristic andimpedance matching performance are obtained within a 45.45%bandwidth. The measured results show a good agreement withthe simulated ones. Compared with the classic Yagi antenna inthe same gain level, the dimension in the end-fire direction of theproposed antenna has been reduced by half. This reduction insize can minimize the interference among ship-borne antennas.The antenna is an excellent candidate for marine communicationapplications.

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Lidong Chi (S’17) received the B.S. and M.S. de-grees in electronic information and technology fromthe Hunan University, Changsha, China, in 2014 and2017. He is currently working toward the Ph.D. de-gree in electronics from the same university. His re-search interests include end-fire directional antennas,millimeter-wave antennas, and antenna arrays.

2412 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 68, NO. 3, MARCH 2019

Yihong Qi (M’92–SM’11) received the B.S. degreein electronics from the National University of De-fense Technologies, Changsha, China in 1982, theM.S. degree in electronics from the Chinese Academyof Space Technology, Beijing, China in 1985, and thePh.D. degree in electronics from Xidian University,Xi’an, China, in 1989 respectively.

From 1989 to 1993, he was a Postdoctoral Fellowand then an Associate Professor with the SoutheastUniversity, Nanjing, China. From 1993 to 1995, hewas a Postdoctoral Researcher at McMaster Univer-

sity, Hamilton, ON, Canada. From 1995 to 2010, he was with Research inMotion (Blackberry), Waterloo, ON, Canada, where he was the Director ofAdvanced Electromagnetic Research. He is currently the President and ChiefScientist with General Test Systems, Inc., Shenzhen, China; he founded DBJayin 2011, and he is the CTO of ENICE. He is also an Adjunct Professor inthe EMC Laboratory, Missouri University of Science and Technology, Rolla,MO and an Adjunct Professor in Hunan University, Changsha, China. He is aninventor of more than 280 published and pending patents. The patent that ofmulti-resonance antenna has been used by more than 1.4 billion smart phonesannually. The O-ring connector invention is shipping more than 4 billion piecesper year. He is contributor of 3GPP and CTIA standards. Dr. Qi is a Fellow ofCanadian Academy of Engineering. He was a Distinguished Lecturer of IEEEEMC Society for 2014 and 2015, and serves as the Chairman of the IEEE EMCTC-12 and he is a member of advisory board and an Associate Editor for IEEETRANSACTION ON ELECTROMAGNETIC COMPATIBILITY. He received the 2017technology achievement award from IEEE EMC society.

Zibin Weng (M’17) received the B.S. degree inelectronic engineering and the Ph.D. degree in elec-tromagnetic field and microwave technology fromXidian University, Xi’an, China, in 2004 and 2009,respectively. He is currently an Associate Professorwith Xidian University. His current research inter-ests include circularly polarized antennas, millimeter-wave antennas, and antenna arrays.

Wei Yu (M’13) received the B.S. degree in electricalengineering from Xi’an Jiaotong University, Xi’an,China, in 1991, the M.S. degree in electrical engineer-ing from the China Academy of Space Technology(CAST), Beijing, China, in 1994, and the Ph.D. de-gree in electrical engineering from Xidian University,Xi’an, China, in 2000. From 2001 to 2003, he was aPostdoctoral Fellow with the University of Waterloo,Canada. He was a CTO with Sunway Communica-tions Ltd., from 2008 to 2012. He founded Antenova-tion Electronics Inc. in 2004 and cofounded General

Test Systems Inc., Shenzhen, China, in 2012. He is now with DBJ Technologiesas COO. His current research interests include signal processing and mobiledevice test system. He is an inventor of 91 published and pending patents.

Weihua Zhuang (M’93–SM’01–F’08) has been withthe Department of Electrical and Computer Engi-neering, University of Waterloo, Canada, since 1993,where she is a Professor and a Tier I Canada Re-search Chair in Wireless Communication Networks.She received the 2017 Technical Recognition Awardfrom IEEE Communications Society Ad Hoc & Sen-sor Networks Technical Committee, one of 2017 tenN2Women (Stars in Computer Networking and Com-munications), and several Best Paper awards fromIEEE conferences. Dr. Zhuang was the Editor-in-

Chief for IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY (2007–2013),Technical Program Chair/Co-Chair of IEEE VTC Fall 2017 and Fall 2016, andthe Technical Program Symposia Chair of the IEEE Globecom 2011. She is aFellow of the Royal Society of Canada, the Canadian Academy of Engineer-ing, and the Engineering Institute of Canada. She is an elected member in theBoard of Governors and VP Publications of the IEEE Vehicular TechnologySociety. She was an IEEE Communications Society Distinguished Lecturer(2008–2011).