responsible for the shift of the lowest cutoff value. The idea ofnegative dispersion has been employed for dispersion-compensat-ing optical fibers. Moreover, Pendry et al. [25] has recently devel-oped the method of making a perfect lens by means of negativerefractive-index materials, which restores not only the phase ofpropagating waves but also the amplitude of evanescent states. Theabove study may be useful for the analysis of other related helicalwaveguides and optical devices. This can also provide a linkbetween optical-fiber technology and TWT, and it may have sometechnological use as well.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of AICTE, NewDelhi, vide project No. 2043. One of the authors, S. K. Srivastava,wishes to thank CSIR for providing a fellowship.

REFERENCES

1. N.S. Kapany, Fiber optics principles and applications, AcademicPress, New York, 1967.

2. A.K. Ghatak and K. Thyagarajan, Introduction to fiber optics, Cam-bridge University Press, Cambridge, 1999.

3. G. Kieser, Optical fiber communications, McGraw-Hill, New York,1991.

4. C. Yeh, Elliptical dielectric waveguide, J Appl Phys 33 (1962), 3235–3242.

5. K.S. Chiang, Effective index method for the analysis of opticalwaveguides couplers and arrays: an asymptotic theory, J LightwaveTechnol 9 (1991), 62–72.

6. S.P. Ojha, P.K. Choudhury, and P. Khastgir, Glass fibers of triangularcross-section with metal loading on one or more sides: a comparativestudy, Proc SPIE 1580 (1991), 278–287.

7. W.Y. Yoin, P. Liand, and W.B. Wang, Guided electromagnetic wavesin a parallel-plate gyroelectronic biaxial chirowaveguide, J Mod Opt(1994), 412.

8. M.P.S. Rao, V. Singh, B. Prasad, and S.P. Ojha, An analytical study ofthe dispersion curves of an annular waveguide made by liquid crystal,Photon Optoelectron 5 (1998), 73–78.

9. N. Kumar, S.K. Srivastava, and S.P. Ojha, A comparative study ofmodal dispersion characteristics of different types of concave lensshaped dielectric waveguides, Microwave Opt Technol Lett 35 (2002),337–342.

10. J.F. Bertone, P. Jiang, K.S. Hwang, and D.M. Mitllaman, Thicknessdependence of the optical properties of silica-air and air-polymerphotonic crystals, Phys Rev 83 (1999), 300–303.

11. K.J. Bunch and R.W. Grow, The helically wrapped circularwaveguide, IEEE Trans Electron Devices ED-34 (1987), 1873–1885.

12. E.M.T. Jones, A negative dispersion helix structure, Elec Res Lab,Stanford University Tech Rep 27 (1950).

13. S. Ahn and A.K. Ganguly, Analysis of helical waveguide, IEEE TransElectron Devices ED-33 (1986), 1348–1355.

14. U.N. Singh, O.N. Singh II, P. Khastgir, and K.K. Dey, Dispersioncharacteristics of helically cladded step-index fiber: an analyticalstudy, J Opt Soc Amer B12 (1995), 1273–1278.

15. V.K. Chaube, K.K. Dey, S.P. Ojha, and P. Khastgir, Modal charac-teristics of a doubly clad step-index fiber: a general analytical ap-proach, Can J Phys 61 (1998), 796.

16. S. D’Agostino, F. Emma, and C. Paoloni, Accurate analysis of thehelix slow wave structure, IEEE Trans Electron Devices 45 (1998),1605.

17. D. Kumar, Propagation characteristics of helically cladded ellipticalstep-index fiber, Ph.D. Thesis, Appl Phys, IT, BHU, India, 1999.

18. S.K. Srivastava, P.C. Pandey, U.N. Singh, and S.P. Ojha, Effect ofpitch angle on modal propagation characteristics of an annular circulardielectric waveguide having helical windings on the inner and outerboundaries as the claddings, Microwave Opt Technol Lett 33 (2002),338–344.

19. J.R. Pierce, Travelling wave tubes, Van Nostrand, New Jersey, 1950,pp 179–183.

20. D.A. Watkins, Topics in electromagnetic theory, Wiley, New York,1958, pp 39–62.

21. E. Yablonovitch, Inhibited spontaneous emission in solid-state physicsand electronics, Phys Rev Lett 58 (1987), 2059–2062.

22. J.P. Dowling and C.M. Bowden, Anomalous index of refraction inphotonic band gap materials, J Mod Opt 41 (1994), 345–351.

23. S.P. Ojha, P.K. Choudhury, P. Khastgir, and O.N. Singh, Operatingcharacteristics of an optical filter with a linearly periodic refractiveindex pattern in the filter material, Japan J Appl Phys 31 (1992),281–282.

24. P.K. Chaudhury, P. Khastgir, S.P. Ojha, D.K. Mahapatra, and O.N.Singh, Design of an optical filter as monochromatic selector fromatomic emissions, J Opt Soc Amer A9 (1992), 1007.

25. J.B. Pendry, Negative refraction makes a perfect lens, Phys Rev Lett18 (2000), 3966–3969.

© 2003 Wiley Periodicals, Inc.

DESIGN OF A DOUBLE-LOOPEDMONOPOLE ARRAY ANTENNA FORA DSRC SYSTEM ROADSIDEBASE STATION

Yongjin Kim,1 Choongho Song,1 Inmo Koo,2 Hyungjin Choi,1

and Sangseol Lee1

1 Division of Electronic and Computer EngineeringHanyang UniversitySeoul, 133-791, Korea2 Division of Electronic Communication EngineeringAnsan CollegeAnsan, Kyonggido, 425-701, Korea

Received 11 September 2002

ABSTRACT: The antenna system for a dedicated short-range commu-nication (DSRC) system must have pattern characteristics that satisfythe system’s specifications. Also, the antenna system needs to have goodspace efficiency for mounting on a roadside base station. The proposedantenna has the traveling-wave structure of a double-looped monopolearray. The Wilkinson power divider is used for the in-phase feeding ofthe array. The designed antenna system’s performance is suitable forthe DSRC system. © 2003 Wiley Periodicals, Inc. Microwave OptTechnol Lett 37: 74–77, 2003; Published online in Wiley InterScience(www.interscience.wiley.com). DOI 10.1002/mop.10829

Key words: antenna; monopole; loop; DSRC

1. INTRODUCTION

The intelligent transportation system (ITS) has been actively stud-ied all over the world. It has contributed to improving safety,reducing congestion, enhancing mobility, minimizing environmen-tal impacts, saving energy, and promoting economic productivity[1]. The dedicated short-range communication (DSRC) system forITS makes communication between roadside base stations andon-board mobile terminals possible at the short range. In the DSRCsystem, the line of sight (LOS) path must be guaranteed betweena base station and a mobile terminal. A wide-frequency band isrequired to support multiple access to one base station for severalmobile terminals. The antenna system of the base station is sup-posed to be small enough to be mounted on a roadside object, suchas the strut of a streetlight [2].

The patch and linear wire antennas are currently popular forvarious mobile communication systems. The monopole type is a

Contact grant sponsor: Korea Research Foundation GrantContact grant number: KRF-2001-041-E00228

74 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003

more typical kind of wire antenna than the patch type, because ofits ease of manufacture, inexpensiveness, and wider band charac-teristics.

The traveling-wave electric current is distributed on a linearantenna by inserting a resistor �/4 away from its end. This broad-band antenna has much weaker mutual coupling characteristicsthan a conventional linear antenna [3]. The main disadvantage ofthe resistive-loaded traveling-wave antenna is its low efficiency,since the inserted resistor absorbs the input power by the 50%. Toavoid this resistance loss, the inserted resistor can be replaced witha resonant antenna with radiation resistance approximately equalto the resistor’s resistance [4]. The inserted resonant antennaincreases the radiation power of the linear antenna.

In this paper, the antenna system is designed with four mono-pole antennas, and each has two loops. This array antenna systemsatisfies all the requirements for a roadside base-station antennausing the 5.8-GHz DSRC system. The designed antenna systemshows more substantial dimension reduction than the patch arraywith the same gain. All the measured values of the designedantenna are compared with the simulated results using Zeland,Inc.’s IE3D tool.

2. DESIGN OF THE ANTENNA SYSTEM

A wire antenna loaded with a loop at �/4 distance from its end hasthe horizontal element from the loop and the vertical element fromthe monopole, which are radiating perpendicularly to each other[4]. The monopole element has a donut-shaped radiation pattern inthe horizontal plane and the loop element has maximum directivity

in the zenith direction [5]. Thus, this antenna has a hemisphericradiation pattern.

One more loop is added in the monopole to sharpen the beam.And a linear array scheme is used to increase the directive gain.The design specifications of the DSRC system base station antennaare listed in Table 1.

TABLE 1 Design Specifications of the DSRC Base StationAntenna

Item Specification

Center Frequency 5.8 GHzBandwidth 10 MHzVSWR � 1.5Gain 10 dBi

Figure 1 A double-looped monopole antenna

Figure 2 Structure and design parameters of the antenna array

Figure 3 Photographs of the designed antenna: (a) front view of thearray antenna; (b) Wilkinson power divider on the back of the arrayantenna. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003 75

The important design data are the dimensions of the loops andthe monopole, the spacing between the two loops, and the spacingbetween the monopoles. For the simulation, we took the early dataof the design from [4]; the optimum design data were obtained byusing Zeland, Inc.’s IE3D.

Figure 1 shows a single element of the array. The height of theelement is chosen to be 0.75�, since the reactance of the 1.5�linear dipole is very small. The radius of the loop R is slightlylonger than �/8 and the spacing L2 between two loops is fixed to0.25�. The phase difference of A and B in Figure 1 is 180°. Theradiation pattern characteristics are simulated for variations of L1and L3 to find the optimum dimension of the antenna. The isola-tion gap h in Figure 1 is chosen to be 1 mm. When the L1 is 0.5�,a radiation null appears in the direction of the z axis, and for 0.25�,the maximum radiation occurs in the same direction. The iterativesimulation gives the optimal values of the antenna characteristicsat about 0.9� for the loop circumference, and at 0.25� for L1, L2,and L3. The half-power beamwidth of this antenna is about 90° inXZ and YZ planes, respectively. The calculated directive gain isabout 4.7 dBi when the infinite ground plane is assumed.

Figure 2 represents a 4-element linear array. The array spacingis 0.5�. Four elements of the array are fed in-phase by the Wilkin-son power divider, which has broadband matching characteristics.

The calculated directive gain of the antenna array system is about10.5 dBi.

3. PERFORMANCE OF THE DESIGNED ANTENNA

Figure 3 shows the realized array antenna system and the Wilkin-son power divider. Each element of the array antenna is con-structed using copper wire whose diameter is 1.0 mm. The size ofthe ground plane is 90 cm by 90 cm. The 0.25� coaxial impedancetransformer is used for impedance matching and as the antennasupport.

The VSWR of the feeding system is measured by using anHP8719ES network analyzer and the radiation pattern is measuredby ORBIT/FR, Inc.’s far-field measurement system. An AL-PRB-187 horn antenna is used for the calibration. Figure 4 represents themeasured VSWR, which shows broadband characteristics over the5.4–6.0 GHz frequency range. Since the VSWR at the 5.8-GHzdesign frequency is 1.16, the matching state of the antenna systemis good.

The measured radiation pattern in the XZ plane is comparedwith the calculated one in Figure 5. The measured 3-dB beamwidthis about 25°. The side lobes of about �19 dB appear differently inthe measured radiation pattern from the simulated results. This isconsidered to be due to the simulation tool’s inaccuracy becausethe antenna system is geometrically complex. However, the sidelobes are not as critical to our antenna system, as shown inFigure 5.

Figure 4 Measured VSWR [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Figure 5 XZ-plane pattern

Figure 6 YZ-plane pattern

Figure 7 The antenna with the reduced ground plate [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com.]

76 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003

Figure 6 shows the measured radiation pattern in the YZ plane,compared with the calculated one. The measured 3-dB beamwidthis about 85°, while the calculated value is about 94°. The directivegain of the antenna system measures approximately 10.3 dBi at5.75 GHz, 5.8 GHz, and 5.85 GHz, which satisfies the designspecification.

For the practical application of this antenna system, the groundplane is reduced to the 19 cm � 19 cm plane, as shown in Figure7. Figure 8 shows the measured radiation patterns for the XZ andYZ planes of this antenna system. The 3-dB beamwidth is about25° for the XZ plane and 80° for the YZ plane, respectively, thusthis antenna system also satisfies the design specifications.

4. CONCLUSION

The main advantage of the proposed antenna system is that spaceefficiency is improved, compared with the microstrip patch array.This is a very important feature in the DSRC system. Also, theVSWR of the designed antenna system shows enough broadbandcharacteristics for the base station antenna. Therefore, the designedantenna may be useful in a DSRC system as the antenna mountedon the strut of a streetlight, serving as a roadside base station.

REFERENCES

1. D. Nelson (Ed.), Intelligent transportation primer, ITE, 2000.2. K. Fujitomo, and J.R. James, Mobile antenna system handbook. Artech

House, Norwood, MA, 2001.3. E.E. Altshuler, The traveling-wave linear antenna, IRE Trans Antennas

Propagat 9 (1961), 324–329.4. E.E. Altshuler, A Monopole Loaded with a Loop Antenna, IEEE Trans

Antennas Propagat 44 (1996), 787–791.5. C. Balanis, Antenna theory (analysis and design), Wiley, New York, 1982.

© 2003 Wiley Periodicals, Inc.

Figure 8 Radiation patterns for the antenna with the reduced groundplate

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 37, No. 1, April 5 2003 77