Nagli (Tuli) Herscovici AnTeg 52 Agnes Drive Framingham, MA 01901 USA Tel: + I (508) 786-5152 Fax: +1 (506) 7865226 E-mail: [email protected]

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Christos Christodoulou Department of Electrical and Computer Engineering University of New Mexico Albuquerque, NM 87131-1356 USA (505) I 277-6580 Tel: + Fax: + I (505) 277-1439 E-mail: [email protected]

Microstrip-Fed Printed Lotus Antenna For Wideband Wireless Communication Systems Abdelnasser A. Eldek, Afef 2. Elsherbeni, and Charles E. Smith Center of Applied Electromagnetic Systems Research (CAESR), Department of Electrical Engineering The University of Mississippi University, MS 38677, USA E-mail: [email protected],[email protected], [email protected]

Abstract A novel printed antenna, fed by a microstrip line, has been designed for wideband wireless communication systems. The new antenna shows a wide -10dB bandwidth of over 60%. In addition to being small in size, the antenna exhibits stable far-field radiation characteristics in the entire operating band with relatively high gain, low cross polarization, wide beamwidth, and good front-to-back ratio. Keywords: Microstrip antennas; land mobile radio equipment; antenna radiation patterns; wideband antenna; Lotus antenna; PCMCIA

1. Introduction

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rinted microstrip antennas aTe widely used in wireless com.' munication systems. They exhibit a very low profile, small size, lightweight, low cost, and high efficiency, and are easy to install. Furthermore, they are generally economical to produce, since they are readily adaptable to hybrid and monolithic megrated-circuit fabrication techniques at RF and microwave frequencies [I]. Since modem communication systems require smaller size antennas but with wideband characteristics, many researchers have 164

investigated techniques to improve the antenna's bandwidth, and reduce its size to fit in the new wireless tools, such as PDAs, cell phones, and computer PCMCIA cards [Z-61.A probe-fed E-shaped patch antenna achieved a bandwidth of more than 30% with high directivity, but the antenna itself was quite large in size, and it required a much larger ground plane to suppress the hack radiation and to provide the claimed bandwidth. Very good bandwidths, ranging from of 57% to 70%, were achieved in [3-61. However, all of these antennas produced bi-directional patterns, which decreased directivity and gain. Moreover, a large ground plane was

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Figure 5a. The computed three-dimensional pattern for the Gain* of Antenna 1 at 1.7 GHz.

Figure 5b. The computed three-dimensional pattern for the Gains of Antenna 1 at 1.7 GHz. The color values are as given in Figure Sa.

Figure 6a. The computed three-dimensional pattern for the Gain+ of Antenna 1 at 1.9 GHz.

Figure 6b. The computed three-dimensional pattern for the Gains of Antenna 1 at 1.9 GHz. The color values are as given in Figure 6a.

Figure 7a. The computed three-dimensional pattern for the GainC for Antenna 1at 2.4 GHz.

Figure 7b. The computed three-dimensional pattern for the Gains for Antenna 1 at 2.4 GHz. The color values are as given in Figure 7a.

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required for the antenna to yield the aforementionedbandwidth; in addition, high cross polarization levels and a narrow beamwidth were produced by these antennas. Among the most widely used printed antennas in wireless communication systems are printed dipoles and quasi-Yagi antennas, fed by coplanar stripline (CPS), which are usually used to obtain an end-fire radiation pattern. In order to feed this type of antenna, a microstrip-to-coplanar-stripline transition, which includes a 180" phase shifter (balun), is usually used [7]. The balun consists of a T-junction, with one side of the microstrip line delayed by a half wavelength, to produce a predominantly odd mode for the coplanar stripline. An attractive quasi-Yagi antenna design that uses the transition in [7] was presented in [8, 91. It exhibited a wide bandwidth (48%) and good radiation characteristics. The antenna consisted of a half-wavelength dipole as a driver, and an approximately quarter-wavelength rectangular director to increase the gain and improve the front-to-back ratio. While the driver and director were placed on one side of the substrate, the ground plane was placed on the other side and truncated, to act as a reflector. Recently, the authors showed that by replacing the dipole and the director by a bowtie for X-band operation, improvements in the bandwidth, antenna size, and radiation characteristics were obtained [IO]. Further research by the authors resulted in novel coplanar waveguide-fed slot and microstrip-fed printed antennas, which are called slot and printed Lotus antennas [ll]. In this paper, two printed Lotus antennas, Antenna 1 and Antenna 2, are designed and presented for widehand wireless communication systems. Antenna I and Antenna 2 were designed to cover the two frequency ranges from 1.56 GHz to 2.91 GHz and from 3.8 GHz to 6.7 GHz, respectively. The r e m loss and far-field radiation characteristics of these antennas are introduced. The simulation and analysis for the presented antennas were performed using the commercial computer software package Ansoft HFSS, which is based on the Finite-Element Method. Verification for the computed return loss was performed using measurements and further computations using the commercial software Momentum of Advanced Design System (ADS) of Agilent Technology, which is based on the Method of Moments. A fabrication-tolerance analysis is also performed, to predict the antenna's performance sensitivity due to any small fabrication errors.

defined by two ellipses. The smaller ellipse is located completely in one half of the larger ellipse. The larger ellipse has Rhl and Rvl as the horizontal and vertical semi-axes, respectively. The smaller ellipse has Rh2 and RvZ as the horizontal and vertical semi-axes, respectively, and is rotated by an angle a . The vertical and horizontal distances between point P, shown in Figure 1, and the smaller ellipse's center point are LI and W I , respectively. The

Figure la. The geometry and parameters of the printed Lotus antenna.

I

c .

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.

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2. Printed Lotus Antenna Geometry and Dimensions The proposed antennas were printed on a Rogers RTiDuroid 6010/6010 LM substrate with a dielectric constant of 10.2, and a conductor loss ( t a n s ) of 0.0023. The substrate height was 100 mil (2.54 mm) for Antenna 1, and 50 mil (1.27 mm) for Antenna 2. The use of a high dielectric constant substrate material reduces radiation losses, because most of the electromagnetic field is concentrated in the dielectric between the conductive ship and the ground plane. Another benefit in having a high dielectric constant is that the antenna size decreases by the square root of the effective dielectric constant. To minimize conductor loss, the conductor thickness should be greater than 56 [l], where 6 is the skin depth, which is approximately 0.65 pm for copper. The conductor thickness used in these designs was 34 pm. The description of the printed Lotus antenna is introduced in the following section. The geometry, parameters, and prototype of the proposed printed Lotus antenna are shown in Figure 1. The antenna is 166

Figure lb. The prototype printed Lotus antenna.

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b

A -*

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Figure 8a. The computed three-dimensional pattern for the Gaind for Antenna 1at 2.9 GHz.

Figure 8b. The computed three-dimensional pattern for the Gains for Antenna 1 at 2.9 GHz. The color values are as given in Figure 8a.

Figure 9a. The computed two-dimensional gain pattern for Antenna 1at 1.7 GHz.

Figure 9b. The computed two-dimensional gain pattern for Antenna 1at 1.9 GHz. The lines are as identified in Figure 9a.

Figure 9c. The computed two-dimensional gain pattern for Antenna 1at 2.4 GHz. The lines are as identified in Figure 9a.

Figure 9d. The computed two-dimensional gain pattern for Antenna 1 at 2.9 GHz. The lines are as identified in Figure 9a.

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a4.75x4.75 @ 3.8x3.8 @ 1.9x1.9

0238x238

0451~4.51

-40

.......

___ ADS Freq.(GHz)

Figure 2a. The dimensions in mm for the balun of Antenna 1.

Figure 3b. The return loss.for the printed Lotus Antenna 2.

@2X2 e1.6x1.6

0 0.8xO.a

............................

01x1 1Jx1.9

2.6 2.8 Figure 2b. The dimensions in mm for the balun of Antenna 2.

3

Figure 4a. The VSWR for the printed Lotus Antenna 1.

5

4

g 3

>

2

3 Figure 3a. The return loss for the printed Lotus Antenna 1. 168

Freq.(GHz)

Figure 4b. The V S W R for the printed Lotus Antenna 2. IEEE Antennasandpropagation Magazine, Vol. 46, No. 6, December 2004

4 -7.98830.6m

-1.BBaBe.edi -1.z58Bo+m1 -1.58881*1 -1.neaWE31 -2.eaarBa1 -2.Z588crBB1

-2.58880.ml -2.7988a.881

-3.8088e.edl -3.2588aBal -3.98831.0a1

Figure loa. The computed three-dimensional pattern for the Gain* for Antenna 2 at 4.0 GHz.

Figure 10h. The computed three-dimensional pattern for the Gains for Antenna 2 at 4.0 GHz. The color values are as given in Figure loa.

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Figure l l a . The computed three-dimensional pattern for the Gain4 for Antenna 2 at 5.2 GHz.

b

Figure llh. The computed three-dimensional pattern for the Gaine for Antenna 2 at 5.2 GHz. The color values are as given in Figure l l a .

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Figure 12a. The computed three-dimensional pattern for the Gain+ for Antenna 2 at 5.8 GHz.

Figure 12h. The computed three-dimensional pattern for the Gains for Antenna 2 at 5.8 GHz. The color values are as given in Figure 12a.

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~

parameter L2 defines the vertical dimension of the antenna, while L3 is the distance between the substrate's edge and the antenna in they direction, and L4 is the length of the coplanar stripline. The coplanar stripline is connected to a microstrip feed line through a balun to introduce the odd mode at the edge of the coplanar stripline. The truncated ground plane acts as a reflector, which helps in suppressing back radiation. The first design, Antenna 1, was designed for 1.8 GHz, 1.9 GHz, and 2.4 GHz. This antenna had Rhl, R v l , Rh2, Rv2, L I , W I , L2, L3, and L4 equal to 15.91 mm, 18 mm, 7.88 mm, 5.2Xmm, 19.35mm, 8.Xmm, 19.95mm, IO", and 21.6mm, respectively, with a = 44". The coplanar-stripline dimensional parameters w and s were 1.425 mm and 0.95 mm, respectively, for an approximate characteristic impedance of 100 0. The second design, Antenna2, was designed for a frequency range that included 5.2 GHz and 5.8 GHz. This antenna had Rhl, Rvl, Rh2, Rv2, L I , WI, L2, L3, and L4 equal to 6.8 mm, 7.2 mm, 3.14 mm, 2.12mm, 7.74mm, 3.52mm, 8.4mm, 11.6mm, and 9.1mm, respectively, with a = 41'. The coplanar-stripline dimensional parameters w and s were 0.6 mm and 0.4 mm, respectively, for an approximate characteristic impedance of 100 Q.. Figure 2 shows the dimensions in mm for the haluns of the two antennas. The legend on the left side of the figure indicates by the symbols the horizontal and vertical dimensions of the designated inclined lines.

3. Results and Applications of the Printed Lotus Antennas The two printed Lotus antennas were simulated using HFSS and ADS Momentum. Antenna2 was fabricated and measured using an HP 8510 vector network analyzer. Figures 3 and 4 show comparisons between the resulting retum loss and VSWR for the presented printed Lotus antennas, from HFSS, ADS Momentum, and measurements. Acceptable agreement among these results was obtained in spite of the differences between the two simulators, and the effects of the SMA coaxial connector on the measurements. In HFSS, the exact geometry of the antenna was simulated with a finite substrate of size 53 mm x 101.2 mm for Antenna 1, and 30 nun x 50 mm for Antenna 2. Finite ground planes with sizes of 53 mm x 54.15 mm for Antenna 1 and 30 mm x 20 nun for Antenna 2 were used. In ADS Momentum, the program can only simulate an infinite substrate, which bad some effect on the retum loss. The same ground-plane size was used in the ADS simulation. As shown in Figures 3 and 4, Antenna 1 operated from 1.56 GHz to 2.91 GHz with a bandwidth of 60.4%, and Antenna 2 operated from 3.8 GHz to 6.7 GHz with a bandwidth of 55.2%. The three-dimensional and two-dimensional radiation patterns of the printed Lotus antennas were computed using HFSS at selective frequencies within the operating band. The radiation patterns of Antenna 1 were computed at 1.7 GHz, 1.9 GHz, 2.4 GHz, and 2.9 GHz, and are shown in Figures 5 to 9. The radiation patterns of Antenna 2 were computed at 4.0 GHz, 5.2 GHz, 5.8 GHz, and 6.4 GHz, and are presented in Figures 10 to 14. As shown in the three-dimensional pattems, the two antennas had almost stable radiation pattems over their entire operating bands. The antennas radiated mainly in the half space above the x-z plane, with low back radiation. Antenna 1 had an average gain of 4 dB, a maximum cross polarization level of -17dB, and a minimum front-toback ratio of 4.5 dB. In addition, the radiation fields exhibited wide beam characteristics, as the pattems showed 3 dB beamwidths that exceeded 95' and 120' in the E and H planes, respec170

tively. The second design, Antenna 2, had an average gain of 5 dB and a maximum cross polarization level of -14dB, its front-toback ratio exceeded IO dB, and the corresponding 3 dB beamwidths exceeded 100' and 130" in the E and H planes, respectively. In order to predict the effect of fabrication accuracy on the antenna's performance, the dimensions of Antenna 2 were changed as shown in Figure 15, where LI and L2 represent the error in mm in the Lotus fabrication, "CPS" represents the new dimensions of the coplanar stripline due to the fabrication errors, and "Balun" represents the error in mm in the balun fabrication. The selected values were chosen for an approximate tolerance of 10% of the exact size. Figure 16 shows the effects of these fabrication errors. It is interesting to note that there was no significant effect on the antenna's retun-loss level and on the expected opeiating bandwidth. The aforementioned wideband radiation characteristics - in addition to the wide bandwidth of these antennas - make them very good candidates for modem wireless communicarion applications. It is worth mentioning that the total size (width x length) of Antennas 1 and 2, including the ground plane, were 53 mm x 101.2 mm and 30 mm x 50 mm, which can easily be adapted for PCMCIA cards of size 54 mm x 1IO mm. Using these antennas with their wideband characteristics in PCMCIA cards would certainly enhance their performance. In addition, Antenna I allows for simultaneous operation in the frequency hands of the GSM 1800 and GSM 1900, and the industrial, scientific, and medical ISM band around 2.4 GHz. Similarly, Antenna 2 could sewe WLAN (wireless local-area network) applications at 5.2 GHz. (5.15 GHz5.35 GHz) and 5.8 GHz (5.725 GHz-5.875 GHz).

L1

_._.-

Figure 15. The parameters for evaluating the effects of fabrication errors on the printed Lotus antenna's performance.

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Figure 13a. The computed three-dimensional pattern for the Gainm for Antenna 2 at 6.4 GHz.

EO -E Plane (xy)

- H Plane (yz)

Figure 13b. The computed three-dimensional pattern for the Gains for Antenna 2 at 6.4 GHz. The color values are as given in Figure 13a.

+ +

Figure 14a. The computed two-dimensional gain patterns for Antenna 2 at 4 GHz.

Figure 14b. The computed two-dimensional gain patterns for Antenna 2 at 5.2 GHz. The lines are as identified in Figure 14a.

Figure 14c. The computed two-dimensional gain patterns for Antenna 2 at 5.8 GHz. The lines are as identified in Figure 14a.

Figure 14d. The computed two-dimensional gain patterns for Antenna 2 at 6.4 GHz. The lines are as identified in Figure 14a.

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~

Return Loss (dB)

Return Loss (dB) 1

4

5

I

fi

7

4

f(GHz) Figure 16a. The effects of the LI fabrication errors on the return loss of Antenna 2.

Return Loss (dB)

5 f(GHz)

6

J 7

Figure 16d. The effects of the Balun fabrication errors on the return loss of Antenna 2.

4. Conclusion

0,

Two printed Lotus antennas were designed and presented for wireless communication applications at 1.8 GHz, 1.9 GHz, and 2.4 GHz, and at 5.2 GHz and 5.8 GHz, respectively. The two antennas were characterized by wide bandwidths of 60.4% and 55.2%, stable radiation patterns, low cross-polarization levels, wide heamwidths, high front-to-pack ratios, and relatively high gains. The presented designs were not sensitive to fabrication errors up to 10%. The small profile of these antennas makes them suitable for a variety of modem wireless communication devices.

5. References Figure 16h. The effects of the L2 fabrication errors on the return loss of Antenna 2.

I . L. G. Maloratsky, ”Reviewing the Basics of Microstrip Lines,” Microwaves & RF,March 2000, pp. 79-88. 2. F. Yang, X. Zhang, X. Ye, and Yahya Rahmat-Saniii, “WideBand E-Shaped Patch Antennas for Wireless Communications,” IEEE Transactions on Antennas and Propagation, AP-49, 7, July 2001, pp. 1094-1100.

Return Loss (dB)

3. X-C Lin and L-T Wang, “A Broadband CPW-Fed Loop Slot Antenna with Harmonic Control,” IEEE Antennas and Wireless Propagation Letters, 2,2003, pp. 323-325. 4. J-Y Chiou, J-Y Sze, and K-L Wong, “A Broadband CPW-Fed Strip-Loaded Square Slot Antenna,” IEEE Transactions on Antennas and Propagation, AP-S1,4, April 2003, pp, 719-721. 5 . H-D Chen, “Broadband CPW-Fed Square Slot Antennas with a Widened Tuning Stub,” IEEE Transactions on Antennas and Propagation, AP-51, 8, August 2003, pp. 1982-1986.

6. N. Behdad and K. Sarahandi, “A Multiresonant Single-Element Widehand Slot Antenna,” IEEE Antennas and Wireless Propagation Letters, 3,2004, pp. 5-8. Figure 16c. The effects of the CPS fabrication errors on the return loss of Antenna 2. 172

IEEE

7. N. Kaneda, Y . Qian, and T. Itoh, “A Broad-Band Microstrip-toWaveguide Transition Using Quasi-Yagi Antenna,” IEEE TransAntennasandPropagation Magazine, Vol. 46, No. 6. December 2004

actions on Microwave Theory and Techniques, MTT-47, 12, December 1999, pp. 2562-2567.

8. W. Deal, N. Kaneda, J. Sor, Y. Qian, and T. Itoh, “A New Quasi-Yagi Antenna for Planar Active Antenna Arrays,” IEEE Transactions on Microwave Theory and Techniques, MTT-48, 6, June 2000, pp. 910-918. 9. N. Kaneda, W. Deal, Y. Qian, R. Waterhouse, and T. Itoh, “A Broad-Band Planar Quasi-Yagi Antenna,” IEEE Transactions on Antennas and Propagation, AP-50, 8 , August 2002, pp. 11581160. 10. A. A. Eldek, A. 2. Elsherbeni, and C. E. Smith, “Cbaracteristics of Microstrip Fed Printed Bow-Tie Antenna,” Microwave and Optical Technology Letters, 43,2, October 2004, pp. 123-126.

11. A. Z. Elsherbeni, A. A. Eldek, and C. E. Smith, “Wideband Slot and Printed Antennas,” in K. Change (ed.), Encyclopedia of RF and Microwave Engineering, New York, John Wiley, 2005.

Introducing the Authors Abdelnasser Eldek received a honor BSc in Electronics and Communications Engineering from Zagazig University, Zagazig, Egypt, in 1993, an MS in Electrical Engineering from Eindhoven University of Technology, Eindhoven, The Netherlands, in 1999, and a PhD in Electrical Engineering from the University of Missis,sippi in 2004. He was a research assistant with the Electronic Research Institute, in Cairo, Egypt, from 1995 to 1997. From 1997 to 1999 he was a masters student at Eindhoven University of Technology, with the cooperation of the Pbilips Center for Technology and Fontys University for professional education, Eindhoyen. From 1999 to 2000, he was an assistant teacher in the Industrial Education College, Beni Suif, Egypt. He has presented 26 conference presentations and co-authored 15 refereed journal articles and book chapters.

tems. His recent research has been on the application of numerical techniques to microstrip and planar transmission lines, antenna measurements, and antenna design for radar and personal communication systems. He has published 70 technical journal articles and 12 book chapters on applied electromagnetics, antenna design, and microwave subjects, and contributed to 215 professional presentations. He is the coauthor of the hook entitled MATLAE Simulationsfor Radar @stems Design (CRC Press, 2003), and the main author of the chapters “Handheld Antennas” and “The Finite Difference Time Domain Technique for Microstrip Antennas” in Handbook of Antennas in Wireless Commnnications (CRC Press, 2001). Charles E. Smith was horn in Clayton, AL, on June 8, 1934. He received the BEE, MS, and PhD degrees from Auburn University, Anbum, AL, in 1959, 1963, and 1968, respectively. His main areas of interest are related to the application of electromagnetic theory to microwave circuits, antennas, measurements, RF and wireless systems, radar, digital and analog electronics, and computer-aided design. His recent research has been on the application of numerical techniques to microship transmission lines, antenna measurements in lossy media, the measurement of electrical properties of materials, CAD in microwave circuits, radar design, and data acquisition using network analyzers. Dr. Smith bas published widely in these areas and has over 200 total publications, including journal papers, technical reports, book chapters, and paper presentations. He has advised or co-advised 46 MS theses and PhD dissertations, and has received six awards for outstanding teaching and scholarship at the University of Mississippi. @

Editor‘s Comments Conrinuedfrompage 173 Atef Z. Elsherbeni received a honor BSc in Electronics and Communications, a BSc degree in Applied Physics, and an MEng degree in Electrical Engineering, all from Cairo University, Cairo, Egypt, in 1976, 1979, and 1982, respectively. He received the PhD in Electrical Engineering from Manitoba University, Winnipeg, Manitoba, Canada, in 1987. Dr. Elsherheni bas conducted research in several areas, such as scattering and dimaction by dielectric and metal objects, inverse scattering, Finite-Difference Time-Domain analysis of passive and active microwave devices, field visualization and software development for EM education, dielectric resonators, interactions of electromagnetic waves with the human body, and development of sensors for soil moisture and for monitoring of airport noise levels, reflector antennas and antenna arrays, and analysis and design of printed antennas for wireless communications and for radars and personal communication sys-

magnitude affects all of us, wherever we are. I hope you will join me with your prayers and thoughts supporting those who have been directly affected, and the disaster relief efforts now getting underway. I also hope you will give to and do what you can to support these efforts. A list of global organizations accepting donations and through which you can help can be found by clicking on “Ways to help with tsunami relief” immediately below the search box at http:/lw.google.com. Several United Nations organizations accepting donations, along with information on UN efforts, can be found at h~:llwww.un.orglapps/newsl infocusRel.asp?infocusID=102&Body=tsunami&Body1=. A list of relief organizations in the US, provided by the US Department of State, can be found at hnp:llwww.usaid.govllocationsl asia-near-eastitsunamilngolist.htm1.

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