Ž .Microwave & Commun. Technologies Conf. M & RF’97 , Wemb-ley Conference Centre, U.K., 1997, pp. 34]39.

8. T. Edwards, Foundations for Microstrip Circuit Design, 2nd ed.John Wiley & Sons, New York, 1992.

Q 1998 John Wiley & Sons, Inc.CCC 0895-2477r98

A COMPACT FLAT REFLECTORANTENNA FOR POTENTIAL BASESTATION APPLICATIONSM. Ali,1 S. S. Stuchly,1 and K. Caputa11 Department of Electrical and Computer EngineeringUniversity of VictoriaVictoria, B.C. V8W 3P6, Canada

Recei ed 22 January 1998

ABSTRACT: A compact flat reflector antenna that utilizes a meander-( )line bow-tie MLBT monopole as its primary radiator is described. The

( )antenna radiates predominantly ¨ertically polarized electromagnetic EMwa¨es, and is suitable for application in a base station. The new antennahas a resonant input resistance of 50 V . The bandwidth of the antennais 5.3% within a return loss of less than y9.5 dB. The directi ity of theantenna is 8.4 dBi, and the half-power beamwidth in the horizontal-planepattern is 948. Since the reflector acts as a ground plane for the primaryradiator, a balun is not required. Q 1998 John Wiley & Sons, Inc.Microwave Opt Technol Lett 18: 319]320, 1998.

Key words: flat reflector antennas; bow-tie antennas; base station;wireless communications

1. INTRODUCTION

In mobile telephone base stations, antennas with 7]15 dBdw xgain are usually required 2 . The majority of these antennas

utilize vertical polarization. Usually, sector beam antennasare employed in order to increase the capabilities of thesystem. These antennas can have beamwidths of 60, 90, and1208 in the horizontal plane.

A corner reflector antenna can be a suitable candidate forapplication in a base station. Such antennas can be arrangedto form a collinear array so that the gain requirement issatisfied. A flat reflector can also be used. The primaryradiator for either the corner or the flat reflector is usually avertical half-wave dipole. The dipole is placed at a distance Sfrom the reflector. The spacing S between the reflector andthe dipole is crucial since a small S makes the antenna morenarrowband, whereas a large S introduces lobes in the radia-

w xtion pattern 3 . The feeding transmission line is either atwo-wire line or a coaxial line. For the coaxial line feed,a balun is required.

In this paper, a compact flat reflector antenna is intro-w xduced that utilizes a meander-line bow-tie monopole 1 as

the primary radiator. The new antenna is shown in Figure 1.ŽUnlike the conventional configuration corner reflector or

.flat reflector with half-wave dipole as the primary radiator ,the new antenna is not required to be positioned at a dis-tance S from the reflector since the primary radiator is amonopole. The antenna radiates and receives predominantlyvertically polarized EM waves, and has a resonant inputresistance of 50 V. Also, since the antenna is fed by a 50 Vcoaxial line from behind the reflector, a balun is not required.

Figure 1 Compact flat reflector antenna. Wire length s 54.9 cm,wire diameter s 1.25 mm, h s 17.3 cm, a s 1018, l s 5.6 cm, b s 0.5cm, and e s 2.8 cm; reflector dimensions 122]120 cm2

2. DESIGN

The new antenna is designed using the Numerical Electro-Ž . w xmagnetic Code NEC 4 . To verify the results of computa-

tion, the antenna is manufactured and measured using anautomatic vector network analyzer. A description of the mea-

w xsurement method can be found in 5 . The parameters of theantenna are given in Figure 1. The initial analysis using NECwas conducted with two objectives in mind: to obtain verticalpolarization, and approximately 50 V input resonant resis-

Žtance for proper impedance matching with the 50 V coaxial.transmission line . A large number of simulations were per-

formed to obtain the geometry shown in Figure 1. Note thatan ordinary monopole lying parallel to the y-axis in Figure 1would radiate horizontally polarized EM waves.

3. RESULTS

The input impedance of the new antenna as computed usingNEC as a function of frequency is shown in Figure 2.Although the antenna has several resonances, the one at

.940 MHz is the most important one since: 1 the impedance-frequency response in this region is smoother than from any

Figure 2 Computed input impedance of the antenna of Figure 1 asa function of frequency

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 18, No. 5, August 5 1998 319

Figure 3 Measured return loss of the antenna of Figure 1 as afunction of frequency

Ž .Figure 4 Horizontal-plane xy-plane pattern of the antenna ofFigure 1. Solid lines}computed, crosses}measured

.other resonance region, 2 the real part of the input.impedance at 940 MHz is close to 50 V, and 3 the antenna

Žhas the least amount of cross polarization horizontal polar-.ization at this resonance compared to the other two reso-

nances, at 720 and 805 MHz, respectively.The measured return loss of the antenna is shown in

Figure 3 as a function of frequency. The antenna has aŽ .bandwidth of 5.3% with RL F y9.5 dB VSWR F 2 . The

computed and measured radiation patterns for this antennaare shown in Figure 4. The antenna has a directivity of8.4 dBi and a half-power beamwidth of 948. The cross polar-ization is suppressed below y14 dB.

4. CONCLUSIONS

A novel compact flat reflector antenna is described whichconsists of a flat metallic reflector and a meander-line bow-tiemonopole as a primary radiator. A 50 V coaxial line is usedto feed the MLBT monopole. Since the primary radiatoris a monopole, a balun is not required. The antenna has a

Ž .bandwidth of 5.3% within RL F y9.5 dB VSWR F 2 . Itsdirectivity and half-power beamwidth are 8.4 dBi and 948,respectively.

REFERENCES

1. M. Ali, and S. S. Stuchly, ‘‘A Meander-Line Bow-Tie Antenna,’’1996 IEEE Antennas Propagat. Soc. Int. Symp. Dig., Vol. 3, pp.1566]1569.

2. K. Fujimoto and J. R. James, Eds., Mobile Antenna Systems Hand-book, Artech House, Norwood, MA, 1994.

3. J. D. Kraus, Antennas, 2nd ed., McGraw-Hill, New York, 1988.4. G. J. Burke and A. J. Poggio, ‘‘Numerical Electromagnetic Code

Ž .NEC }Method of Moments,’’ Part I and III, Tech. Document116, Naval Ocean Systems Center, CA, 1980.

5. M. Ali, S. S. Stuchly, and K. Caputa, ‘‘Characteristics of Bent WireAntennas,’’ J. Electromagn. Wa¨es Appl., Vol. 9, No. 9, 1995, pp.1149]1162.

Q 1998 John Wiley & Sons, Inc.CCC 0895-2477r98

A MEANDER-LINE POLARIZERCOVERING THE FULL

( .E-BAND 60 – 90 GHzJ.-F. Zurcher1¨1 Laboratoire d’Electromagnetisme et d’Acoustique´Ecole Polytechnique Federale de Lausanne´ ´CH-1015 Lausanne, Switzerland

Recei ed 21 January 1998

ABSTRACT: At millimeter-wa e frequencies, the generation of a circu-lar polarization is not always an easy task. One way to obtain such apolarization is to use a polarization transformer to con¨ert a linear to acircular polarization. Such a structure, using printed meander lines, isdescribed here. It performs well through the full E-band, with an axialratio -3 dB. Q 1998 John Wiley & Sons, Inc. Microwave OptTechnol Lett 18: 320]323, 1998.

Key words: polarizer; circular polarization; millimeter wa¨es

1. INTRODUCTION

There are many methods to generate a circularly polarizedwave. One is to use an antenna whose shape naturally gener-

Ž .ates such a wave a helical or spiral antenna, for instance .Another one would be to combine with the proper phaseŽ .908 the signals emerging from two orthogonally positioned

w xlinearly polarized antennas 1, 2 . At millimeter-wave fre-quencies, however, some problems arise with this method dueto tolerances, especially with printed structures, where a verysmall inaccuracy or unbalance could severely degrade thepolarization quality.

Another way to obtain a circular polarization is to use aspecial structure, acting as a polarization transformer, placedin front of a linearly polarized antenna. This structure, madeof several stacked printed-circuit sheets comporting meanderlines, works as follows: the incident linearly polarized wave isresolved into two equal components at "458. These compo-nents are in phase when incident on the polarizer. A differ-ential phase shift occurs when the wave passes throughthe polarizer, the two components being phase shifted by908 at the output of the polarizer, thus producing a circularpolarization.

Such a polarizer was first conceived in 1969, and has beenw xdescribed in 3 . An analysis method has been recently pre-

w xsented 4 . However, the present paper describes a polarizerworking at millimeter waves, where the realization is muchmore critical due to the very small dimensions involved.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 18, No. 5, August 5 1998320