Control of Horizontal Radiation Pattern of Base StationAntenna for Cellular Mobile Communications by Performing

Approach Arrangement of Slender Metal Conductors

Yasuko Kimura and Yoshio Ebine

Wireless Link Development Department, NTT DoCoMo, Inc., Yokosuka, 239-8536 Japan

SUMMARY

In the IMT-2000 system, in which the subscribercapacity is increased by using the same frequency repeat-edly, it is known that the base station antenna becomes moreeffective if its beam width in the horizontal plane is nar-rower than the sector division angle. In order to narrow thebeam width of the wireless zone antenna designed with thebeam width in the horizontal plane identical to the sectordivision angle, the antenna elements must be replaced.Replacement of the antenna causes interruption of serviceand is economically unfavorable. In order to solve theseproblems, this paper proposes an antenna configuration forreducing the beam width by placing metal conductors nearthe antenna which is used generally. An analysis using themethod of moments is employed to find the relationship ofthe metal conductor to the beam width when metal conduc-tors are placed close to the antenna in the base station. Theeffect of the metal conductor on the impedance is alsoexplained by measurement. © 2004 Wiley Periodicals, Inc.Electron Comm Jpn Pt 1, 88(4): 20–31, 2005; Publishedonline in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20178

Key words: mobile communications; base stationantenna; semicircular cylindrical reflector; metal conduc-tor; nearby placement; narrower beam.

1. Introduction

In general, the service area for mobile communica-tions base station antennas is sectored in order to increasethe subscriber capacity. In the PDC system (personal digitalcellular system: second-generation system), wireless zonesconsisting of one cell with three or six sectors with differentfrequencies are used. In the case of a three-sector wirelesszone configuration, an antenna with a 120° beam is used forone sector. In the six-sector configuration, the antenna beamwidth is 60° [1]. The IMT-2000 system (International Mo-bile Telecommunications-2000 system: third-generationsystem) is characterized by one-cell frequency reuse. Inorder to increase the subscriber capacity, the number ofsector divisions must be increased for the wireless zonedesign [2]. Further, it is known that a beam width narrowerthan the sector angle used as a service area is effective forincreasing the subscriber capacity [3].

At present, when the antenna load capacity is smallor subscriber capacity is light, three-sector wireless zoneconfigurations are used since the wireless equipment re-quired is simple and economical. At the beginning of intro-duction of IMT-2000 systems, service areas identical tothose of the PDC system are assumed. Therefore, the sectordivision angle and the beam width in the horizontal planeare both 120°. In the areas where the subscriber capacity islarge, six-sector wireless zone configurations are used.Then, the sector division angle and the beam width in thehorizontal plane are both 60°. In order to reduce the beamwidth so as to increase the subscriber capacity, severalmethods of increasing the antenna aperture are conceivable,

© 2004 Wiley Periodicals, Inc.

Electronics and Communications in Japan, Part 1, Vol. 88, No. 4, 2005Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J87-B, No. 5, May 2004, pp. 673–684

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including the use of larger reflectors and the use of moreelements [4]. In any of these approaches, the wind pressureis increased so that problems of antenna mounting areencountered. In order to reduce mounting problems, thebeam width is chosen identical to the division angle at thesacrifice of subscriber capacity.

In the future, in order to attempt a further increase ofsubscriber capacity with the three-sector wireless zoneconfiguration, it is desirable to reduce the beam width inthe horizontal plane from the sector division angle. Basedon Ref. 3, it is concluded from the relationship between theplace of the entire wireless zone and the subscriber capacitythat the beam width in the horizontal plane should be about90° for the three-sector wireless zone. However, if the 120°beam antennas presently installed are replaced with 90°beam antennas, service interruptions may occur and aneconomic impact may arise. Further, there is a mountingproblem, since the antenna with a 90° beam has a largeraperture. As an antenna for three-sector wireless zones, it isproposed to attach slender metal conductors at locations of±90° to the main beam direction of the antenna. In thismethod, the antenna aperture is equivalently increased bythe currents induced in the metal conductors placed on bothsides of the antenna. The thickness of the metal conductorsis less than 10 mmφ and hence the diameter is effectivelysmaller than in the installation of a conventional antennawith a 90° beam width. Since the antennas installed on theexisting base stations are used, the beam width can bechanged without service interruption.

Considering the economy of the antenna and thereduction of wind pressure, this paper demonstrates thepossibility of modifying the beam width within the hori-zontal plane by placing slender cylindrical metal conduc-tors near the base station antenna with a 120° beam widthin the horizontal plane [5]. The effects of the structuralparameters of the antenna on the radiation pattern in thehorizontal plane are calculated by using the method ofmoments applied to the mesh structures of wire grids [7].In Section 2, a 2-GHz base station antenna with a reducedantenna aperture width to minimize the wind pressure isdescribed. Next, the position and thickness of the cylindri-cal metal conductors attached to both sides of the antennaare studied with regard to their influence on the beam widthin the horizontal plane. In Section 3, applications to existingtriple-band antennas for 0.8/1.5/2 GHz [8, 9] are discussed.First, a method for controlling the beam width only at 2GHz, for which IMT-2000 is used, is investigated with thebeam width fixed to 120° at 0.8/1.5 GHz. Next, a methodfor controlling the beam widths in all three bands is ex-plained. Section 4 studies the effects of the metal conduc-tors on the radiation pattern in the horizontal plane and onthe impedance when the metal conductors are attached toan existing antenna. Results of both calculation and meas-

urement are presented for the radiation pattern but onlymeasured data are given for the impedance.

2. 2-GHz Base Station Antenna

For three-sector wireless zones, an antenna with abeam width of 120° in the horizontal plane (120° beamantenna) can be realized easily to arrange a dipole antennawith a reflector.

One of the installation issues for antennas is reductionof wind pressure. Since the wind pressure of a base stationantenna is proportional to the wind reception size of theantenna aperture, it is desirable that the antenna aperture beas small as possible maintaining the required beam widthby reduction of the size and diameter of the antenna [1].

In this section, first a 120° beam antenna with thesmallest antenna aperture is discussed. Next, the radiationpattern obtained by attaching metal conductors to the an-tenna structure selected is calculated.

2.1. A slender 120° beam antenna

In general, a 120° beam antenna has a plane reflectoror a corner reflector. In Ref. 5, a semicircular cylindricalreflector is used. Three types of 120° beam antenna struc-ture are shown in Fig. 1. Type (a) uses a plane reflector, type(b) a corner reflector, and type (c) a semicircular cylindricalreflector. When the antenna is covered with a cylindricalradome, the inner diameter (or the equivalent diameter ofthe antenna) of type (c) is 50% smaller than type (a) and91% smaller than type (b). Figure 2 shows the currentdistributions on the reflector in types (a) and (c). Thesereflectors are constructed of 19 wires that are arranged atequal intervals along the Z axis. Figure 2 shows the resultof calculating the amplitude and phase of current flowingon a wire. The amplitude is normalized to the value at thefeed point and the phase is the relative phase on the basis

Fig. 1. 120° beam antennas with reflector.

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of the feed point. In case (a), the induced current on thereflector is almost uniform. On the other hand, inducedcurrents at both edges of the reflector are three timesstronger than those on the rest of the reflector. Since thephase is almost constant with respect to the antenna, theedges of the reflector are considered to suppress spreadingof the diffracted wave. Figure 3 shows the radiation patternsin the horizontal plane for Figs. 1(a) and 1(c). The beamwidths are about 119° and about 114°. In addition, the backlobe level (FB ratio) becomes smaller. This fact also indi-cates that the spreading of the diffracted wave is reduced bymeans of a semicircular cylindrical reflector. Hence, we seethat the antenna can be slender and its FB back ratioimproved by about 3 dB while keeping the beam width

identical to that in the conventional antenna if a semicircu-lar cylindrical reflector is used. Therefore, this structure isselected as the 120° beam antenna with the smallest antennaaperture width. In the following, this antenna structure isanalyzed.

Figure 4 shows the effect of the change in length Lz

of the reflector in the Z direction on the radiation pattern inthe horizontal plane for Fig. 1(c). It is evident that thechange in length Lz does not significantly affect the beamwidth in the horizontal plane but does affect the FB ratio.In a real antenna, however, the FB ratio is not a problemsince the antenna aperture length is increased by means ofan array configuration in the vertical direction in order toobtain high gain. Therefore, in this paper, the beam widthin the horizontal plane is the subject of study. Figure 5reveals that impedance matching of the slender antenna in

Fig. 2. Current distribution on the element.

Fig. 3. Radiation characteristics of 120° beam antennain horizontal plane.

Fig. 4. HPBW versus reflector length Lz.

Fig. 5. Impedance characteristics.

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Fig. 1(c) is possible by attaching a parasitic element nearthe feed element [11, 12]. As shown in Fig. 6, there is littledifference in the beam width of the antenna with andwithout a parasitic element for impedance matching.Hence, the subsequent discussions deal with the case inwhich metal conductors are attached to a dipole antennawith a semicircular cylindrical reflector without a parasiticelement.

2.2. Effect of metal conductors

Figure 7 shows the analysis model which attachedmetal conductors to the 120° beam antenna. The origin ofthe coordinate axes is the feed point. The metal conductorsare placed to right-and-left symmetry from the main beamdirection. Figure 8 shows the beam width versus the lengthL of the metal conductors. It is seen that the effect of themetal conductors is almost unchanged if L is more than0.6λ. Note that the base station antenna has an array con-figuration in the vertical direction in order to obtain highgain. Thus, the reflector is elongated along the verticaldirection instead of using an array of reflectors. In this case,the metal conductors are also elongated in the same direc-tion. In the calculation, the length L of the metal conductorsis chosen the same as the length of the reflector, even forthe one-element case.

The beam width is controlled by changing threeparameters: the distance S between the metal conductor andthe antenna, the diameter D of the metal conductors, andthe location Hy in the direction of the Y axis. The variationsof the beam width are shown in Figs. 9 to 12. Figure 9 showsthe variations of the beam width versus changing the dis-tance S between the antenna and the metal conductor.Figure 10 shows the variations of the beam width versuschanging the diameter D of the metal conductors, Fig. 11shows the variations of the beam width versus the location

Hy in the direction of the Y axis of the metal conductors,and Fig. 12 shows the variations of the beam width versuschanging the angle θ of the metal conductor while thedistance S between the antenna and the metal conductor isfixed. When the diameter D of the metal conductor becomes

Fig. 6. HPBW versus frequency.

Fig. 7. 120° beam antenna with metal conductors.

Fig. 8. HPBW versus conductor length L (D = 0.027λφ,Hy = 0).

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0.013λφ, it is calculated as a wire, and if it is larger, it iscalculated as a cylinder. From these figures, it is found thatthe beam can be narrowed if the distance S is about 0.5λ,the diameter D is larger than 0.1λ, and the position Hy andthe angle θ are toward the main beam direction from Hy =0 (θ = 0°). The beam width variation for each parameter ismore than 25° with regard to the distance S and the positionHy. It is more than 30° for the angle θ and more than 15°for the diameter D of the metal conductors.

Figure 13 shows a contour map of the distance S andthe diameter D obtained from the above calculations. Thelocation in the Y direction is Hy = 0. This figure proves thatthe beam width can be varied in the range of about 100° to55° by selecting an appropriate diameter D and distance S.

In the above, the effect of the metal conductors on thebeam width was obtained. Next, the current induced by the

Fig. 10. HPBW versus diameter D (L = 1λ, Hy = 0).

Fig. 11. HPBW versus position of metal conductors Hy

(L = 1λ, D = 0.067λφ).

Fig. 12. HPBW versus angle θ (L = 1λ, D = 0.067λφ).

Fig. 13. Space S versus diameter D (L = 1λ, Hy = 0).

Fig. 9. HPBW versus space S (L = 1λ, Hy = 0).

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placement of metal conductors is discussed. Figure 14shows the current distribution induced on the edges andmetal conductors. The amplitude is shown in panel (a) andthe phase in panel (b). Here, D = 0.067λφ and Hy = 0. Fromthe calculation results obtained by the method of moments,the current on the cylindrical metal conductors is predomi-nantly along the wires on the main beam direction side.Therefore, the amplitude in Fig. 14(a) is the sum of thecurrents at the calculation points for which Y ≥ 0 and Z = 0.The normalization is to the current at the feed point. Figure14(b) shows the phase of the wire induced by the strongestcurrent in the Z direction on the main beam side and thephase induced at the feed element and the edges. From Fig.14(a), it is found that a large current is induced in the metalconductors if the distance S is smaller than 0.6λ. It is alsofound from Fig. 14(b) that the phase of the current induced

at each wire is constant, with no significant changes even ifS changes. Figure 15 shows the electric field distribution inthe horizontal plane (Z = 0) for distances S of (a) 0.27λ, (b)0.53λ, and (c) 0.67λ. It is seen that the electric field intensityin the main radiation direction is stronger and the FB ratiois larger if the metal conductors are placed near the antenna.Also, when S is far from the antenna, the currents inducedon the metal conductors are reduced. From Figs. 15, 9, and14(a), it is found that the current is induced in the metalconductors if S is smaller, and that the beam width does notdecrease since the aperture width is small. Also, from Fig.15(c), it is found that no current is induced if S is large. Inorder to reduce the beam width, it is necessary to choose adistance of about 0.6λ, for which currents are induced onthe metal conductors and the aperture width is not too large.From these calculation results, it is found that the beam

Fig. 14. Current distribution on the reflector edge and the conductor (L = 1λ, D = 0.067λφ, Hy = 0).

Fig. 15. The electric field in the horizontal plane (D = 0.067λφ, Hy = 0, Z = 0).

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width can be changed freely up to about 55° if metalconductors are used for the slender 120° beam antenna andthe width in the wind exposure size is about 0.4λ (when thediameter of the metal conductors is 0.067λφ). With a slightincrease in the wind exposure size, the antenna aperture iseffectively increased.

3. Effect of Metal Conductors onTriple-Band Antenna

At base stations where the antenna loading capacityis small, triple-band 120° beam antennas covering 0.8GHz/1.5 GHz/2 GHz are employed [1, 8, 9].

In order to study the adaptability of the metal conduc-tors to the triple-band antennas, metal conductors are sym-metrically placed on the antenna element to the main beamdirection as shown in the analytical model in Fig. 16. Thebeam width is calculated while varying various parameters.The length of the reflector is set to 300 mm. Since the metalconductors are placed near the edges of the reflector, theinitial value of the position Hy in the Y direction is chosenas –20 mm. The distance from the antenna element to theedge in the Y direction is 25 mm. Figure 17 shows the beam

width versus the length L of the metal conductors. It is seenthat the beam width becomes the narrowest if L is slightlyshorter than the half wavelength (0.5λ0.8 GHz, 0.5λ1.5 GHz,0.5λ2 GHz) at each frequency (0.8, 1.5, 2 GHz). Also, thebeam width for a length L of the metal conductor longerthan the reflector or 300 mm changes very little at 0.8 and1.5 GHz, although there are slight variations at 2 GHz.Hence, a constant effect is obtained for the triple-bandantenna with conductor lengths beyond 300 mm. When thedependence of the length L of the metal conductors for thetriple-band antenna in Fig. 17 is compared with that for asingle-frequency antenna in Fig. 8, it is confirmed that theclose placement of the metal conductors has an influence

Fig. 16. Triple-band antenna with metal conductors.

Fig. 17. HPBW versus conductor length L (D = 4 mm φ, Hy = –20 mm).

Fig. 18. HPBW versus number of elements (S = 65 mm, D = 4 mm φ, Hy = –20 mm).

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on the convergence of the beam width. Considering an arrayimplementation of this antenna in the vertical direction atthe base station, Fig. 18 shows the effects of the elementsabove or below on the beam width. For an array implemen-tation, a single-piece reflector elongated in the verticaldirection is used. The spacing between the feed points is200 mm and the distance from the upper and lower edgesof the reflector to the edges of the element is 80 mm in thecalculation. Then, the two types of metal conductors areused. In one case, those with L = 70 mm are used at eachelement. In the other case, the lengths of the conductors areidentical to that of the reflector. The figure reveals that thereis little influence of the elements above and below on theradiation pattern in the horizontal plane. Hence, the analyti-cal model with one element shown in Fig. 16 is used in thesubsequent calculations for the triple-band antenna withmetal conductors.

3.1. Beam width control at only one frequency

The 120° beam width, identical to the sector divisionangle, is maintained in the 0.8 GHz/1.5 GHz antenna forusual service. For the antenna at 2 GHz, for which asubstantial increase in the subscriber capacity is expectedin the future, the beam width must be narrower than thesector division angle. It is known from Fig. 17 that it iseffective to make the length L of the metal conductorsslightly shorter than 0.5λ2 GHz if the beam width only at 2GHz is made narrower. Hence, in the calculations, 70 mm(0.47λ2 GHz) is chosen. Figure 19 shows the variations ofthe beam width with the location Hy of the metal conductorsin the Y direction. Figure 20 shows the variations of thedistance S between the antenna and the metal conductors.

Figure 21 shows the variations of the diameter D. Theseresults reveal that the beam width of the 2-GHz antenna canbe varied in the range of about 55° to 120° by varying thelength L, distance S, diameter D, and position Hy. However,if the beam width at 2 GHz is narrowed, the beam width at1.5 GHz tends to expand due to the close proximity of thefrequencies. In order to limit the expansion of the beamwidth at 1.5 GHz, the results indicate that the distance Sshould be made close to the antenna, the diameter D shouldbe small, and the position Hy should be toward the reflector.Hence, beam shaping at 2 GHz can be achieved by changing

Fig. 19. HPBW versus position of metal conductors Hy

(L = 70 mm, S = 65 mm, D = 4 mm φ).

Fig. 20. HPBW versus space S (L = 70 mm, D = 4 mm φ).

Fig. 21. HPBW versus diameter D (L = 70 mm, S = 65 mm).

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the parameters other than L so that L is effective at 2 GHzand limits deterioration at other frequencies. Note that theinfluence of the metal conductors is negligible at 0.8 GHz,since this frequency is far from 2 GHz. Figure 22 shows theradiation pattern in the horizontal plane when the metalconductors are placed so that only the beam at 2 GHz ischanged to 90° in the antenna structure in Fig. 16.

3.2. Triple-band simultaneous beam widthcontrol

Currently the 0.8- and 1.5-GHz bands use the PDCsystem. In the future, however, it is conceivable that allsystems may be transitioned to IMT-2000, with a high-speed and large-capacity network at all frequency bands. Insuch cases, the beam widths of the triple-band 120° beamantenna must be narrowed at all frequencies. Therefore, wewill discuss an antenna configuration in which a 90° beamantenna is realized in all three bands by attaching metalconductors to the existing triple-band 120° beam antenna.In order to derive the common parameters of the metalconductors so that the beam width is 90° at all three bands,metal conductors are attached on both sides of the antennaas shown in Fig. 16. Thus, the length L of the metalconductors is 300 mm. Figure 23 shows the variations ofthe beam width versus the distance S. From this figure, it isfound that the beam width can be adjusted from about 80°to 120° at 0.8 and 1.5 GHz by changing the distance S ofthe metal conductor to have the same length as the reflector.However, the beam width at 2 GHz is larger than 120°. Thisresult is identical to that in Fig. 9 when S is larger than 0.7λ.The beam width tends to be larger for 0.7λ to 1λ. Hence, in

order to control the radiation patterns in three bands, metalconductors 1 to control the patterns at 0.8 and 1.5 GHz areinstalled and in addition metal conductors 2 with a lengthL2 slightly shorter than 0.5λ2 GHz are placed between theantenna and metal conductors 1 as shown in Fig. 17. Thevariations of the beam width as a function of variousparameters are studied. The antenna structure is identical tothat of the triple-band antenna in Fig. 16. Figures 25 and 26show the beam width versus the metal conductor length L2

Fig. 22. Radiation characteristics of triple-band antennain horizontal plane (L = 70 mm, S = 65 mm,

D = 4 mm φ, Hy = –20 mm).

Fig. 23. HPBW versus space S (L = 300 mm, Hy = –20 mm).

Fig. 24. 90° beam triple-band antenna.

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and the distance S2. These figures reveal that, like theantenna in Section 3.1, the beam width at 0.8 GHz is notsignificantly affected but that at 1.5 GHz is. In order to limitthe expansion of the beam width at 1.5 GHz, the length L2

is chosen less than 70 mm (0.47λ2 GHz) in accordance withFigs. 25 and 26 and the distance S2 is made close to theantenna. In this way, it is confirmed that 90° beams can beobtained in all three bands. The radiation patterns in thehorizontal plane are shown in Fig. 27. By using metalconductors that affect the characteristics at various frequen-

cies, the antenna with a triple-band 120° beam can bemodified to one with a triple-band 90° beam by using metalconductors affecting various frequencies.

4. Effects of Metal Conductors onRadiation Patterns and Impedance

Up to this point, the effect of metal conductors placednear the antenna has been studied by calculations. Meas-

Fig. 26. HPBW versus space S2 (L1 = 300 mm, S1 = 120 mm, D1 = 8 mm φ, L2 = 70 mm,

D2 = 4 mm φ, Hy = –20 mm).

Fig. 25. HPBW versus conductors length L2 (L1 = 300mm, S1 = 120 mm, D1 = 8 mm φ, S2 = 75 mm,

D2 = 4 mm φ, Hy = –20 mm).

Fig. 27. Radiation characteristics of 90° and 120° beam antennas in horizontal plane (L1 = 300 mm, S1 = 120 mm, D1 = 8mm φ, L2 = 70 mm, S2 = 55 mm, D2 = 4 mm φ, Hy = –20 mm).

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urements were also used to confirm the beam width and theimpedance matching, which is important for antenna de-sign. For the slender antenna of Section 2.1 and the sameantenna with metal conductors placed closeby, the radiationpatterns in the horizontal plane and the return loss weremeasured in an anechoic chamber. Figure 28 shows thecalculated and measured radiation patterns and Fig. 29shows the return loss. From Fig. 28 it is found that themeasured and calculated beam widths are almost identical.Thus, it is demonstrated that the beam width can be con-trolled by installing metal conductors. The measured backlobes are different from the calculated ones due to the

presence of the feed lines, which was not considered in thecalculations. From Fig. 29, it is confirmed that the returnloss has the same tendency with and without conductors andthat almost the same values are obtained. Hence, it isdemonstrated that the effect of the metal conductors on theimpedance is small. The broken lines shown in the returnloss plot represent the specified VSWR value of 1.5 in thefrequency range of the 2-GHz band for IMT-2000. Themeasured results are found to satisfy these specifications.

5. Conclusions

In this paper, as a way to reduce the beam width of amobile communications base station antenna in the hori-zontal plane, a simply realizable method of placing metalconductors near the antenna sideward is proposed. Theeffectiveness of the approach is clarified by calculations andmeasurements. When metal conductors are applied to a120° beam antenna at a single frequency, the beam widthcan be varied up to about 60°. In the triple-band 120° beamantenna, calculations indicate that the beam width of onlythe highest frequency is controlled and that the beam widthsat all three frequencies can be brought to the desired 90°.Metal conductors are placed on a 120° beam antenna in anexperimental study. The measured beam widths coincidedwith the calculated values. It was found that the effect ofthe metal conductors on the impedance is negligible andcauses no problems in practice. The antenna with a semi-circular cylindrical reflector used as a reference antenna inthis research can achieve a narrow profile for the entireantenna in contrast to the conventional 120° beam antenna.

Fig. 28. Radiation characteristics for 2-GHz beam antenna in horizontal plane (S = 0.43λ, D = 0.033λφ, Hy = 0).

Fig. 29. Return loss for 2-GHz beam antenna.

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AUTHORS (from left to right)

Yasuko Kimura (member) graduated from the Department of Information and Communication Engineering, TokaiUniversity, in 1998, completed the M.S. program at the University of Tsukuba in 2000, and joined NTT DoCoMo. Since then,she has been engaged in the development of base station antennas for mobile communications. She is now affiliated with theWireless Link Development Department, NTT DoCoMo Research and Development Center.

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. Since then, he has been engagedin the development of base station antennas and common equipment for mobile communications. In 2003, he was transferredto NTT DoCoMo. He is now a manager in the Antenna Propagation Technology Section, Wireless Link DevelopmentDepartment, NTT DoCoMo Research and Development Center. He holds a D.Eng. degree.

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