design of a base station antenna with 60° beam width in the horizontal plane for cellular mobile...

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Design of a Base Station Antenna with 60° Beam Width in the Horizontal Plane for Cellular Mobile Radios Yuki Yamaguchi and Yoshio Ebine Wireless Laboratories, NTT DoCoMo Inc., Yokosuka, 239-8536 Japan SUMMARY Since the base station antennas for mobile communi- cations are installed on the same steel towers as the antennas of several communication systems, antenna installation space is very congested. Therefore, these antennas are often installed on the roofs of buildings. Reduction of the instal- lation space and of the wind load is highly desirable. In order to reduce the wind load, it is necessary to make the area exposed to the wind (the antenna aperture width) smaller. In this paper, miniaturization of the base station antenna for a six-sector wireless zone configuration is discussed. The antenna consists of two-element dipole an- tennas with a planar reflector. The main reflector width is 0.5λ and the antenna aperture width is 0.62λ in the design. By making the size of the main reflector smaller, reduction of the antenna aperture width is attempted. By an appropri- ate configuration of the side reflector plate and the parasitic element, the increase of the beam width and the degradation of the front-to-back ratio are limited. Reduction of the main reflector width by 50% and of the antenna aperture width by 38% is found to be possible. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(11): 68–76, 2004; Pub- lished online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecja.10185 Key words: mobile communications; base station antenna; 60° beam width; miniaturization; IMT-2000. 1. Introduction In order to accommodate rapidly increasing numbers of subscribers, the sector wireless zone configuration is being employed for cellular mobile communications sys- tems [1]. In the sector zone configuration, interference from the directions other than the antenna radiation pattern can be made smaller [2] and frequency use efficiency is im- proved [3, 4]. In addition, IMT-2000 service has been initiated for increased subscriber capacity and high-quality multimedia mobile communications [5]. In the IMT-2000 system, the subscriber capacity is larger for a larger number of sectors [5]. However, as the number of sectors is in- creased, the number of antennas also increases. Therefore, taking into consideration the antenna installation space and the wind load, a six-sector wireless zone configuration is used. For one base station, 6 base station antennas are needed. Further, if space diversity is introduced for im- provement of communications quality, 12 base station an- tennas must be installed. Since the base station antennas for mobile communications are installed on the same steel towers as the antennas of several communication systems, reduction of the wind load and the installation space of the antenna is important. Since the wind load is a product of the horizontal cross section and the wind power coefficient [6], the antenna structure must be one with a smaller hori- zontal cross section and with a smaller wind power coeffi- cient in order to reduce the wind load [7]. In the IMT-2000 system (CDMA system), it is more advantageous from the viewpoint of subscriber capacity if the beam width is smaller than the value obtained by dividing 360° by the number of sectors [8]. However, if the beam width is narrower, there is a possibility that more © 2004 Wiley Periodicals, Inc. Electronics and Communications in Japan, Part 1, Vol. 87, No. 11, 2004 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J86-B, No. 6, June 2003, pp. 951958 68

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SUMMARY

Since the base station antennas for mobile communi-cations are installed on the same steel towers as the antennasof several communication systems, antenna installationspace is very congested. Therefore, these antennas are ofteninstalled on the roofs of buildings. Reduction of the instal-lation space and of the wind load is highly desirable. Inorder to reduce the wind load, it is necessary to make thearea exposed to the wind (the antenna aperture width)smaller. In this paper, miniaturization of the base stationantenna for a six-sector wireless zone configuration isdiscussed. The antenna consists of two-element dipole an-tennas with a planar reflector. The main reflector width is0.5λ and the antenna aperture width is 0.62λ in the design.By making the size of the main reflector smaller, reductionof the antenna aperture width is attempted. By an appropri-ate configuration of the side reflector plate and the parasiticelement, the increase of the beam width and the degradationof the front-to-back ratio are limited. Reduction of the mainreflector width by 50% and of the antenna aperture widthby 38% is found to be possible. © 2004 Wiley Periodicals,Inc. Electron Comm Jpn Pt 1, 87(11): 68–76, 2004; Pub-lished online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10185

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1. Introduction

In order to accommodate rapidly increasing numbersof subscribers, the sector wireless zone configuration isbeing employed for cellular mobile communications sys-tems [1]. In the sector zone configuration, interference fromthe directions other than the antenna radiation pattern canbe made smaller [2] and frequency use efficiency is im-proved [3, 4]. In addition, IMT-2000 service has beeninitiated for increased subscriber capacity and high-qualitymultimedia mobile communications [5]. In the IMT-2000system, the subscriber capacity is larger for a larger numberof sectors [5]. However, as the number of sectors is in-creased, the number of antennas also increases. Therefore,taking into consideration the antenna installation space andthe wind load, a six-sector wireless zone configuration isused. For one base station, 6 base station antennas areneeded. Further, if space diversity is introduced for im-provement of communications quality, 12 base station an-tennas must be installed. Since the base station antennas formobile communications are installed on the same steeltowers as the antennas of several communication systems,reduction of the wind load and the installation space of theantenna is important. Since the wind load is a product ofthe horizontal cross section and the wind power coefficient[6], the antenna structure must be one with a smaller hori-zontal cross section and with a smaller wind power coeffi-cient in order to reduce the wind load [7].

In the IMT-2000 system (CDMA system), it is moreadvantageous from the viewpoint of subscriber capacity ifthe beam width is smaller than the value obtained bydividing 360° by the number of sectors [8]. However, if thebeam width is narrower, there is a possibility that more

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areas will be difficult to reach by radio waves. Also, theantenna aperture area becomes larger. Hence, in this paper,the antennas for the six-sector wireless zone configurationare formed for a 60° beam in the horizontal plane.

One type of wire antenna with directivity is the cornerreflector. In order to form a 60° beam with a corner reflector,a reflector width of 1λ with a vertex angle of 90° can beused [9]. However, this antenna aperture width is about 1.4λand hence miniaturization of the antenna is desirable. Inorder to obtain a 60° beam with a Yagi–Uda antenna, six orseven elements are needed. Thus, the antenna length be-comes greater than 1.5λ. Another deficiency is that wide-band characteristics are not realized [9]. Although there areways to use microstrip antennas, the relative bandwidth isnarrow [10].

In contrast to the above antennas, there is an antennaconfiguration that can be used to obtain a 60° beam bycombining two dipole antennas, each of which forms a 120°beam, in front of a planar reflector in such a way that theemissions are combined in the same phase and amplitude[11]. In this configuration, the number of antenna feedelements increases. Nevertheless, a 60° beam can be ob-tained with an element spacing of less than 0.5λ and theantenna aperture width is about 1λ.

Given the above background, in this paper we designa base station antenna for the IMT-2000 by means of thetwo-element dipole antenna with a planar reflector. Byappropriate configuration of a side reflector and a parasiticelement, a small antenna can be realized.

In Section 2, the design target is presented. In Section3, we obtain the relationship between the antenna structuralparameters and the front-to-back ratio (F/B) in a two-ele-ment dipole antenna with a planar reflector. In Section 4,experimental investigations of the trial antenna are re-ported. Section 5 presents conclusions.

2. Antenna Design Target

The design target values of the base station antennafor 2-GHz mobile communication are listed in Table 1. Thepresent antenna is applied to the IMT-2000 system (CDMA

system); its operating frequency range is 1.92 to 2.17 GHzand the relative bandwidth is 12.2%.

In the IMT-2000 system, service is provided with thesame frequency in all wireless zones. Therefore, it is impor-tant to reduce the interference with other zones and it isdesirable to increase the F/B ratio. The target value of theF/B ratio is set to more than 20 dB. The worst F/B value iswithin the range of 180 ± 60° from the main beam direction.

If the return loss of the base station antenna is lessthan –15 dB, then the loss in the antenna system is less thanabout 0.3 dB and does not change much [12]. Hence, thetarget value of the return loss is less than –15 dB.

3. Antenna Structure and EffectsParameters

Figure 1 presents the structure of the proposed two-element dipole antenna with a planar reflector. The radiat-ing elements are two dipole antennas, combined with thesame amplitude and phase so that a 60° beam can begenerated [11, 13]. The reason for choosing this method isthat the antenna aperture becomes smaller than that for anantenna generating a 60° beam with one element. Since theaperture width of the antenna is mainly determined by thewidth of the main reflector, a side reflector is attached onboth sides of the main reflector. Also, since the beam widthbecomes larger as the main reflector is made smaller, para-

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sitic elements are placed in front of the dipoles to adjust thebeam width.

The antenna structural parameters are listed in Table2. The principal parameters in this paper are W, the mainreflector width; T, the side reflector width; θ, the attachmentangle of the side reflectors; and G, the distance between thedipole and the parasitic element. In order to avoid compli-cation of the design, the other parameters are determined asfollows. The element spacing S is 0.5λ. The reason is asfollows. The beam width is about 60° in a two-elementdipole with an element spacing of 0.5λ. Impedance match-ing is difficult if the spacing is less than 0.5λ. The distanceD between the main reflector and the dipole is 0.25λ, so thatbroadband impedance matching is easily obtained. Thedipole length is 0.5λ and the height of the main reflectorand the side reflectors is 20λ.

The effect of the antenna structural parameters givenabove on the beam width and the F/B ratio is analyzed bythe method of moments. The diameter of the conductor is0.9 mm (0.006λ) and the element length is less than1/10λ.

3.1. Effect of main reflector width

First, the side reflectors and the parasitic elements areremoved in the antenna configuration in Fig. 1, confirmingthe characteristics of the structure with the dipoles and themain reflector. Figure 2 shows the beam width and the F/Bversus the main reflector width. The lower limit of the mainreflector width W is 0.4λ, somewhat smaller than the ele-ment spacing S (= 0.5λ), while the upper limit is 1.5λ. Thereason for using a value of 1.5λ is that this is the antennaaperture width for a 60° beam designed with other wireantennas, and that an antenna smaller than this value isdesirable. The figure shows that the beam width becomessmaller and the F/B becomes larger as the main reflectorwidth is increased, so that the reflector functions as in-

tended. A 60° beam can be obtained with W = 1.0λ. Thus,the F/B ratio satisfies the desired value. The correspondingantenna aperture width is 1λ.

3.2. Effect of side reflectors

When the main reflector width is made narrower, thebeam width becomes wider and the F/B becomes smaller.In order to resolve these problems, side reflectors are at-tached on both sides of the main reflector, perpendicular(θ = 0) to the latter. Let the side reflector width be T. Thevariations of the beam width and F/B versus T are computedin terms of the main reflector width W and the results areshown in Fig. 3. Panel (a) shows the beam width and panel(b) the F/B. The lower limit of the side reflector width T is0 and the upper limit is 0.25λ, the distance between the mainreflector and the dipole. In the case of W = 0.5λ, T is 0.2λbecause the side reflector touches the dipole if T = 0.25λ.From panel (a), it is found that there exists a minimum valueof the beam width as a function of T. The value of Tproviding the minimum value is estimated to be 0.1λ to0.15λ. In the case of W = 0.5λ, anomalous variations appearbecause the dipole and the side reflector are very closetogether. On the other hand, F/B increases with increasingT. In the case of W = 0.5λ, the functions of the reflector arenot sufficiently improved by side reflectors and the com-puted results are poor, that is, less than 20 dB as estimatedfrom Fig. 2.

From the above investigations, the following effectsare confirmed by attachment of the side reflectors perpen-dicular to the main reflector:

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3.3. Effect of parasitic elements

In general, the beam becomes sharper and F/B be-comes larger with higher gain as parasitic elements areplaced in front of the radiating elements [14, 15]. Also, by

including the parasitic elements, the bandwidth may beenlarged [16, 17]. In order to adjust the increase of the beamwidth when the main reflector becomes smaller, parasiticelements are placed in front of the dipoles. First, in order toconfirm the effect of the parasitic elements, a parasiticelement is placed in front of the dipole in the structureconsisting of a main reflector and a dipole. The parasiticelement length is Lp and the beam width and F/B arecalculated with the main reflector width W and the distanceG between the dipole and the parasitic element as theparameters. Figure 4 shows the results. Panel (a) shows thebeam width and panel (b) the F/B. The case without aparasitic element is Lp = 0. Since the director of a Yagi–Uda

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antenna whose length is about the radiating element length× 0.92 (0.46λ in this paper) provides good antenna effi-ciency [15], the value of Lp used for computation is up to0.47λ, a larger value than the above. The figure shows thatthe beam width decreases rapidly as the parasitic elementlength Lp is increased. This result can be explained asfollows. As Lp is increased, the phase of the parasiticelement approaches that of the wave from the dipole. There-fore, the structure works like a Yagi–Uda antenna and thebeam becomes sharper. F/B is found to have a maximumvalue. Also, the variations of F/B increase as G becomessmaller.

From the above observations, it is confirmed that a60° beam can be obtained even if the main reflector widthis smaller than 1λ, by including a parasitic element. Thebeam width is expected to become narrower as the parasiticelement becomes longer. If the length is too great, F/Bdeteriorates. Hence, in the following analysis, it is set to0.4λ. In the following, the effects due to the inclusion of theparasitic element and the variations of the parasitic elementlength are summarized.

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Next, let us consider the attachment angle θ of theside reflector. Based on the discussion up to the previoussection, the parameters are determined as follows. First,with regard to the side reflector width T, there exists no Tthat minimizes the beam width in the case of W = 0.5λ fromFig. 3. For other values of W, the minimum exists for 0.1λto 0.15λ. Hence, the calculations are carried out with twovalues, 0.1λ and 0.15λ. Let W = 0.5λ because a 60° beamis obtained with W = 0.5λ from Fig. 4(a). Although it isdescribed in more detail later, the distance G between thedipole and the parasitic element is 0.1λ for which F/B isfound to be large in Fig. 4(b). In Fig. 5, the variations of thebeam width and F/B versus the attachment angle θ areshown. It is found that the beam width does not depend onT and θ and is almost constant. Although there exists amaximum value as a function of θ, the variations for allvalues of θ are within 5 dB. Note that T is a factor affectingthe antenna aperture width and should be as small as pos-sible. However, since F/B can be made larger, a constant

0.15λ is used henceforth. In the case of θ = 90°, parameterssimilar to those with W = 0.7λ, Lp = 0.4λ, and G = 0.1λ areused. In the following, the effects due to the attachmentangle θ are summarized:

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As shown in Fig. 6, the antenna aperture width de-pends on the attachment angle θ of the side reflector andthe distance G between the dipole and the parasitic element

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when the main reflector width W and the side reflectorwidth T are determined. Figure 7 presents the numericallycalculated beam width and F/B versus the distance G be-tween the dipole and the parasitic element for θ = 20° andθ = 40°. The results for the case without parasitic elementsare identified as G = 0. As G is increased, the beam widthis decreased and F/B is increased. The beam width becomes60° when G = 0.08λ for θ = 20°, and G = 0.06λ for θ = 40°.The antenna aperture width is 0.62λ and 0.7λ, respectively.Thus, F/B is more than 20 dB.

The above investigation confirms the followingpoints with regard to the antenna structural parameters, thebeam width, and the F/B. The findings are summarized inTable 3, and the dimensions of the antenna structure for a60° beam are presented in Table 4.

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4. Measurement of Trial Antenna

Performance evaluation of a test structure with thefive parameters listed in Table 3 was performed. The hori-zontal cross section of the trial antenna is shown in Fig. 8.The radiating elements and the parasitic elements wereformed on printed circuit boards. Measurements were per-formed by inserting the trial antenna in a cylindrical radomewith the smallest wind power coefficient. The inner diame-ter of the radome was larger than the antenna aperture widthin Table 3, so that the current distribution was not disturbedby contact of the parasitic elements and the side reflector

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with the radome. The inner diameter was 0.67λ (about 100mm) and the outer diameter was 0.72λ (about 108 mm).

Figure 9 shows the radiation pattern in the horizontalplane at 2.0 GHz. The measured beam width is 58.9° andthe numerical result is 59.9°, which is somewhat larger butagrees well. The measured F/B ratio is 19.1 dB, in compari-son with the computed value of 23.1 dB. The difference isabout 4 dB but the value is close to the target. The cause ofthe difference in the backside pattern is disturbance of thecurrent distribution on the backside due to the radome, feedsystem, antenna attachment mechanism, and cable, whichare not considered in the calculations. The cause of theasymmetry between the left and right sides is the presenceof the cable.

Figure 10 shows the return loss. In the operatingbandwidth (1.92 to 2.17 GHz), an excellent result of lessthan –16 dB is obtained.

Figure 11 shows an example array configuration forhigher gain using the same antenna structural parameters asfor the trial antenna. The measured results for this exampleare 58.5° for the beam width and 22.0 dB for F/B. It isconfirmed that the variations of the beam width and F/B dueto the array configuration are small.

5. Conclusions

Size reduction of a base station with 60° beam widthto be used for the IMT-2000 system is discussed in thispaper. In an antenna structure consisting of side reflectorsattached on both sides of the main reflector and parasiticelements in front of the dipoles, an attempt was made todetermine which parameters have the dominant influenceon the beam width and F/B.

In order to obtain a 60° beam width with only themain reflector and the dipoles, the width of the main reflec-tion is about 1λ and the antenna aperture width is likewiseabout 1λ. In order to reduce the antenna aperture width, themain reflector width must be reduced. Increase of the beamwidth and degradation of F/B due to the reduction of themain reflector width are limited by using appropriate con-figurations of the side reflectors and the parasitic elements.It is further found that F/B is improved by attachment of theside reflectors and that there is a minimum value of thebeam width as a function of the side reflector width. Also,the beam width varies significantly as a result of attachmentof the parasitic elements. There exists a maximum value ofF/B as a function of the parasitic element length. It is foundthat the attachment angle θ of the side reflectors does not

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significantly affect the beam width and F/B. In order toreduce the antenna aperture width, the relationship of thedistance G between the dipole and the parasitic element, theattachment angle θ of the side reflectors, and the antennaaperture width is studied. The structural parameters for theminimum antenna width providing the desired beam widthand F/B are obtained. In the antenna structure minimizingthe beam width, the aperture width is about 0.62λ, which isabout 38% smaller than the structure with only a mainreflector.

Experimental investigations of the trial antenna werecarried out. The antenna structure minimizing the size inthe calculations was fabricated and installed in a radomewith an inner diameter of about 0.67λ. The computed andmeasured beam widths were very close and were about 60°,while the F/B was somewhat smaller. The difference inshape of the backside pattern appears to be caused by thefeed system, radome, cable, and attachment mechanism.The return loss is less than –16 dB and is excellent in theoperating frequency range.

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AUTHORS

Yuki Yamaguchi (member) graduated from the Department of Electrical and Information Engineering, Nagoya Instituteof Technology, in 1994 and joined NTT Mobile Communications Networks, Ltd. After working on intellectual property rights,she has been working on the development of mobile communications base station antennas since 1997. She is now affiliatedwith the Wireless Laboratories, NTT DoCoMo Research and Development Center.

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Yoshio Ebine (member) graduated from the Department of Electronic Engineering, Adachi Technical High School, in1968 and joined the Electrical Communications Laboratory of Nippon Telegraph and Telephone (now NTT). Since then, he hasbeen engaged in the development of mobile communications antennas and diplexers. In 1993 he was transferred to NTT MobileCommunications Networks, Ltd. He is now a chief researcher at the Wireless Laboratories, NTT DoCoMo Research andDevelopment Center. He is in charge of antennas and propagation technologies. He holds a D.Eng. degree.

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