2. C.H. Tsao, Y.M. Hwang, F. Killburg, and F. Dietrich, Aperturecoupled patch antenna with wide bandwidth and dual polarizationcapabilities, IEEE Antennas Propagat Soc Symp Dig, New York,NY, 1988, pp. 936�939.

3. F.E. Gardiol and J.F. Zurcher, Broadband patch antenna�ASSFIP update, IEEE Antennas Propagat Soc Symp Dig, Balti-more, MD, 1996, pp. 2�5.

4. W.S.T. Rowe and R.B. Waterhouse, Broadband CPW fed stackedŽ .patch antenna, Electron Lett 35 1999 , 681�682.

5. Y.-W. Jang, H.-S. Shin, D.-J. Oh, Characteristic analysis of mi-crostrip slot antenna as a function of permittivity and height ofdielectrics, Proc ICT-CSCC ’2000, Vol. 1, pp. 183�186.

6. S.H. Al-Charchafchi, W.K. Wan Ali, M.R. Ibrahim, and A.R.Barnes, Design of a dual patch triangular microstrip antenna,

Ž .Appl Microwave & Wireless 1998 , 60�67.7. S.-T. Fang, K.-L. Wong, and T.-W. Chiou, Bandwidth enhance-

ment of inserted microstrip-line-fed equilateral-triangular mi-Ž .crostrip antenna, Electron Lett 34 1998 , 2184�2185.

8. C.L. Mark, K.M. Luk, and K.F. Lee, Wideband triangular patchŽ .antenna, IEE Proc Inst Elect Eng 146 1999 , 167�168.

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� 2001 John Wiley & Sons, Inc.

A LOW-PROFILE CIRCULARLYPOLARIZED CURL ANTENNAOVER AN ELECTROMAGNETIC

( )BANDGAP EBG SURFACEFan Yang1 and Yahya Rahmat-Samii11 Department of Electrical EngineeringUniversity of California at Los AngelesLos Angeles, California 90095-1594

Recei�ed 29 May 2001

( )ABSTRACT: An electromagnetic bandgap EBG surface is utilized asa ground plane for a curl antenna to achie�e a low-profile design as wellas a circularly polarized pattern. The antenna height is greatly reducedcompared to a normal curl antenna o�er a PEC ground plane. Thefrequency bandgap and ground plane size are discussed. Experimentalresults demonstrate the applicability of this concept. � 2001 John Wiley& Sons, Inc. Microwave Opt Technol Lett 31: 264�267, 2001.

Key words: curl antenna; EBG; low profile; CP

1. INTRODUCTION

Ž .The electromagnetic bandgap EBG surface, also referred toŽ .as a photonic bandgap PBG surface or a high-impedance

� �surface 1 , has demonstrated a good potential to build low-� �profile and high-efficiency antenna structures 2 . Its in-phase

reflection feature enables one to put wire antennas very closeto the surface without losing radiation efficiency. In addition,its high surface impedance helps to suppress the surfacewave. Therefore, an EBG surface is desirable to be used asan antenna ground plane. A curl antenna was proposed as a

� �simple radiator to generate a circular polarization pattern 3 .The design has been suggested for applications in wireless

� �communications such as GPS, LAN, and satellite links 4 .However, it cannot function well when it is placed close tothe finite PEC ground plane because of its reverse imagecurrent. In this paper, the performance of a square curlantenna over the EBG ground plane is investigated in depth.

Contract grant sponsor: U.S. Army Research OfficeContract grant number: DAAH04-96-1-0389

Compared to the conventional curl antenna, this design hasan attractive low-profile structure, as well as a circularly

Ž .polarized pattern. The finite-difference time-domain FDTD� �method is used for analysis 5 . A curl antenna with only

0.06� height has been simulated, and it provides a good axialratio of less than 3 dB. The frequency bandgap and EBGground plane size are also discussed. Finally, some experi-mental results for return loss and axial ratio are presented todemonstrate the applicability of this concept.

2. CHARACTERISTICS OF CURL ANTENNAS OVER ANEBG SURFACE

The configuration of the curl antenna over an EBG surface isdisplayed in Figure 1. It consists of two parts: an EBG groundplane and a square curl. The electromagnetic bandgap struc-ture is achieved by periodic patches connected to the ground

Ž .plane through vias mushroom-like surface . The frequencybandgap is determined by the patch size, the width of the gapbetween the patches, the height, and the dielectric constant

� �of the substrate 2 . The square curl antenna is investigated inthis paper because it is easily analyzed by the finite-difference

Ž .time-domain FDTD method. The parameters of the curl arecurl height h, curl radius R, and extended curl length L.

The FDTD method is used to design the circularly polar-ized antenna at a GPS frequency of 1.57 GHz. A finiteground plane of 1�� 1� is kept for practical applications.Here, � is the free-space wavelength at 1.57 GHz. Theparameters of the EBG surface are carefully chosen so thatthe bandgap will cover the GPS frequency. The EBG patchsize is 0.12�� 0.12�, and the gap between the patches is0.02�. The substrate thickness is 0.04�, and its dielectricconstant is 2.20. The curl antennas are designed to obtaingood circularly polarized patterns over a PEC or EBG groundplane. For the PEC ground plane, the optimized curl parame-ters are

Ž .R � 0.13�, L � 0.11�, h � 0.23� 1

and the optimized parameters of the curl over the EBGground plane are

Ž .R � 0.14�, L � 0.08�, h � 0.14�. 2

Figure 2 compares the simulated axial ratio of these twoantennas. Both antennas have good axial ratios at boresight,

Figure 1 Configuration of a square curl antenna over an EBGsurface. The EBG surface consists of periodic patches connected to

Ž .the ground plane by vias mushroom-like surface

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 4, November 20 2001264

Ž . Ž .Figure 2 Axial ratios FDTD simulated of curl antennas over a aŽ . Ž .PEC ground plane h � 0.23� , and b an EBG ground plane

Ž .h � 0.14�

while the EBG case shows the attractive feature of a lowerprofile than the PEC case. It can also be observed that theEBG case has a similar axial ratio performance at differentcuts, whereas it changes noticeably for the PEC case. Toexplore the potential of the height reduction effect of theEBG ground plane, a curl antenna with decreased height isanalyzed. It is found that, when the height is reduced, theaxial ratio will increase. An even lower profile curl antennawith the following parameters

Ž .R � 0.14�, L � 0.13�, h � 0.06� 3

is simulated, and its results are shown in Figure 3. Asrevealed in the figure, this antenna still generates an accept-able axial ratio less than 3 dB.

Frequency bandgap is an important feature of an EBGsurface, and it affects the curl antenna’s performance. To thisend, a curl antenna over different EBG surfaces is studied.The curl remains the same size, while the EBG size changesin ratio of wavelengths so that different frequency bandgaps

Ž .will be obtained. Figure 4 a shows radiation patterns of the1.2 GHz EBG case whose bandgap ranges from 1.15 to 1.65GHz. The GPS frequency is located in the upper edge of thefrequency bandgap, and therefore the cross polarization inthe boresight is very low. However, the cross polarization is

Ž .Figure 3 Low-profile curl antenna h � 0.06� over an EBG groundŽ . Ž .plane. a Simulated axial ratio. b Simulated circular polarization

pattern

Ž .high at a tilted angle. Figure 4 b shows radiation patterns ofthe 1.8 GHz EBG case whose bandgap ranges from 1.73 to2.48 GHz. In this case, the GPS frequency is outside thebandgap. As shown in the figure, the left-hand circularlypolarized patterns have similar amplitudes to the right-handcircularly polarized patterns, indicating no CP performance.

Ž .These results are less desirable than Figure 3 b , which showsthe performance of the 1.57 GHz EBG case with a frequencybandgap of 1.5�2.1 GHz. From this comparison, it can beconcluded that the best CP performance can be obtained ifthe antenna frequency lies in the lower edge of the frequencybandgap of the EBG surface.

The ground-plane size is another important factor in prac-tical applications. In previous cases, 8 � 8 patches on a1� � 1� EBG ground plane were used. The size of the EBGsurface is reduced to test its effects, whereas the other

Ž .parameters remain as their previous values shown in Eq. 3 .ŽTwo cases, a 0.82�� 0.82� EBG surface including 6 � 6

. Žpatches and a 0.56�� 0.56� EBG surface including 4 � 4

.patches , are analyzed, and their results are displayed inFigure 5. It can be observed that the former one still hasgood CP performance, whereas the latter one shows poor CPpatterns. Thus, it can be summarized that a minimum EBGground-plane size of 0.82�� 0.82� is required to obtaingood CP patterns.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 4, November 20 2001 265

ŽFigure 4 Simulated radiation patterns of curl antennas f � 1.57.GHz over EBG ground planes with different frequency bandgaps.

Ž . Ž .a Bandgap at 1.15�1.65 GHz. b Bandgap at 1.73�2.48 GHz

3. EXPERIMENTAL RESULTS

Some experiments have been done to prove the applicabilityof this antenna structure. Since the EBG surface for 1.57GHz is large, antennas are scaled to 7 GHz for experiments.Figure 6 shows a photo of such an antenna. The EBG surface

Ž .is built on 2-mm-thick RT�Duroid 5880 � � 2.20 . TherEBG patch is 6 mm � 6 mm, and the gaps between patchesare 1 mm wide. The patches are connected to the groundplane by vias in the center of the patches. The overall size of

Žthe EBG structure is 52 mm � 52 mm about 1.20� � 1.20�.at 7 GHz . The parameters of the curl are

Ž .R � 5.5 mm, L � 5 mm, h � 3 mm. 4

Figure 7 compares the return loss of the curl antenna overthe EBG and PEC surfaces. As revealed in the figure, thecurl over the PEC ground plane is not matched well, whereasthe curl over the EBG ground plane shows a good match inthe frequency range from 6 to 8.5 GHz due to the in-phasereflection feature inside the bandgap. Figure 8 shows themeasured antenna axial ratio both versus frequency andversus angle. It is demonstrated in this figure that the curlantenna over the EBG surface achieves a good axial ratio of0.9 dB at 7.18 GHz with a low profile.

Figure 5 Simulated radiation patterns of curl antennas over EBGŽ .ground planes with different ground-plane sizes. a 0.84 � 0.84�

Ž . Ž . Ž .6 � 6 patches ; b 0.56 � 0.56� 4 � 4 patches

Figure 6 Photo of a curl antenna over an EBG ground plane. Theoverall size of the EBG ground plane is 52 mm � 52 mm, about1.2� � 1.2� at 7 GHz

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 4, November 20 2001266

Figure 7 Comparison of measured return loss of curl antennasover an EBG ground plane and a PEC ground plane

Figure 8 Measured axial ratio of a curl antenna over an EBGŽ . Ž .ground plane. a Axial ratio versus frequency at boresight. b Axial

ratio versus angle at 7.18 GHz

4. CONCLUSION

Ž .A curl antenna over an electromagnetic bandgap EBGsurface is proposed in this paper. It has the attractive fea-tures of low profile as well as circularly polarized patterns.This concept is demonstrated by both FDTD simulation and

experiments. The design is suitable for GPS, LAN, satellitelinks, and other wireless communication applications.

REFERENCES

1. D. Sievenpiper, L. Zhang, R.F.J. Broas, N.G. Alexopolus, andE. Yablonovitch, High-impedance electromagnetic surfaces with aforbidden frequency band, IEEE Trans Microwave Theory Tech

Ž .47 1999 , 2059�2074.2. Z. Li and Y. Rahmat-Samii, PBG, PMC and PEC surface for

antenna applications: A comparative study, 2000 IEEE AP-S Dig,July 2000, pp. 674�677.

3. H. Nakano, S. Okuzawa, K. Ohishi, H. Mimaki, and J. Yamauchi,Ž .A curl antenna, IEEE Trans Antennas Propagat 41 1993 ,

1570�1575.4. J.S. Colburn and Y. Rahmat-Samii, Quadrifilar-curl antenna for

the ‘‘big-LEO’’ mobile satellite service system, 1996 IEEE AP-SDig, July 1996, pp. 1088�1091.

5. M.A. Jensen and Y. Rahmat-Samii, Performance analysis of an-tennas for hand-held transceivers using FDTD, IEEE Trans An-

Ž .tennas Propagat 42 1994 , 1106�1113.

� 2001 John Wiley & Sons, Inc.

MATCHED LOAD TRUNCATION FORFEEDLINE OF PATCH ANTENNAS INTHE FDTD METHODHany E. Abd El-Raouf,1 Wenhua Yu,1 Nader Farahat,1 andRaj Mittra11 Electromagnetic Communication LaboratoryThe Pennsylvania State UniversityUniversity Park, Pennsylvania 16802

Recei�ed 8 June 2001

ABSTRACT: In this paper, a matched load termination of a finite-lengthline is used to represent an infinite feedline structure in FDTD simula-tions. A generator with an internal resistor�which is matched to theinput port�is employed to excite the feedline, with a �iew to achie�inga low reflection at the truncation point. A patch antenna, whose feedlineis truncated in this manner, is used as an example to illustrate theusefulness of the proposed technique. � 2001 John Wiley & Sons, Inc.Microwave Opt Technol Lett 31: 267�269, 2001.

Key words: FDTD; matched loads; patch antenna

I. INTRODUCTION

Ž .The finite-difference time-domain FDTD method findswidespread use as a field simulator of microwave circuits,patch antennas, and a variety of other electromagnetic sys-

� �tems 1�3 . In applying the FDTD method, it is often conve-nient to use lumped elements to truncate the microstrip linefeeding an antenna, in a manner such that it introduces littleor no reflections. However, lumped-circuit parameters, suchas resistors, capacitors, inductors, and diodes, are not directlytaken into account in the conventional FDTD update equa-

� �tions. In the past, a number of techniques 4, 5 have beenproposed to truncate an infinite microstrip line by usinglumped elements. However, these techniques only deal with auniform microstrip line, and not with lines containing discon-tinuities or with those that serve as feeds for patch antennas.In many applications involving microwave circuits and anten-nas, we frequently require that the feedline, the dielectricsubstrate, and the PEC ground plane all be truncated withoutreflection, so that we can simulate a long feedline withoutusing a Huygen’s box that cuts through the line as well as

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 4, November 20 2001 267