circular slot antennas for ultrawideband

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Study of Printed Elliptical/Circular Slot Antennas for. Ultrawideband Applications. Pengcheng Li, Jianxin Liang, Student Member, IEEE, and Xiaodong Chen, ...

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 6, JUNE 2006

Study of Printed Elliptical/Circular Slot Antennas for Ultrawideband Applications Pengcheng Li, Jianxin Liang, Student Member, IEEE, and Xiaodong Chen, Member, IEEE

Abstract—Two novel designs of planar elliptical slot antennas are presented. Printed on a dielectric substrate and fed by either microstrip line or coplanar waveguide with U-shaped tuning stub, the elliptical/circular slots have been demonstrated to exhibit an ultrawideband characteristic. The performances and characteristics of the proposed antennas are investigated both numerically and experimentally. Based on these analyses, an empirical formula is introduced to approximately determine the lower edge of the 10 dB operating bandwidth. It is also shown that these antennas are nearly omnidirectional over a majority fraction of the bandwidth. Index Terms—Coplanar waveguide (CPW), microstrip line, printed antenna, slot antenna, ultrawideband (UWB).

I. INTRODUCTION UE to the attractive merits, such as low profile, lightweight, ease of fabrication and wide frequency bandwidth, printed slot antennas are currently under consideration for use in ultrawideband (UWB) systems. This type of antenna has been realized by using either microstrip line [1]–[4] or coplanar waveguide (CPW) feeding structure [5]–[7]. In [1], a round corner rectangular slot antenna which is etched on a substrate with dimension of 68 mm 50 mm can achieve a 10 dB bandwidth of 6.17 GHz. In [2], a fork-like tuning stub is used to enhance the bandwidth of microstrip line fed wide-slot antenna. A bandwidth of 1.1 GHz (1.821–2.912 GHz) has been achieved with gain variation less than 1.5 dBi (3.5–5 dBi) over the entire operational band. In [6], a CPW fed square slot antenna with a widened tuning stub can yield a bandwidth of 60%. The antenna has a dimension of 72 mm 72 mm and its gain ranges from 3.75 to 4.88 dBi within the operational band. In [7], a CPW fed rectangular slot antenna with a substrate of 100 mm 100 mm can provide a bandwidth of 110% with gain varying from 1.9 to 5.1 dBi. It is shown that the achieved bandwidths of these antennas can not cover the whole FCC defined UWB frequency band (3.1–10.6 GHz). Besides, the size of the antenna is not very small. In this paper, two novel designs of printed elliptical/circular slot antennas are proposed for UWB applications. One is fed by microstrip line, and the other by CPW. In both designs, a U-shaped tuning stub is introduced to enhance the coupling

between the slot and the feed line so as to broaden the operating bandwidth of the antenna. Furthermore, an additional bandwidth enhancement can be achieved by tapering the feeding line. Good agreement is obtained between the simulation and experiment. Both of them have shown that the proposed antennas can exhibit UWB characteristic with nearly omnidirectional radiation patterns over the entire bandwidth. Furthermore, these antennas feature a small size compared with those published in [1]–[7]. All of the simulations are performed by using CST Microwave Studio package which utilizes the Finite Integration Technique for electromagnetic computation [8]. The rest of the paper is structured as the following. Section II describes antenna geometries. The antenna performances are evaluated in Section III. Section IV considers the most important design parameters. A conclusion is given in Section V.

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Manuscript received March 30, 2005; revised November 8, 2005. The work of P. Li was supported by the China Scholarship Council. The authors are with Department of Electronic Engineering, Queen Mary, University of London, London E1 4NS, U.K. (e-mail: [email protected] qmul.ac.uk). Digital Object Identifier 10.1109/TAP.2006.875499

II. ANTENNA GEOMETRY The proposed printed elliptical/circular slot antennas with two different feeding structures are illustrated in Fig. 1(a) and (b), respectively. For the microstrip line fed elliptical/circular slot antenna, the slot and the feeding line are printed on different sides of the dielectric substrate; for the CPW fed one, they are printed on the same side of the substrate. In both designs, the elliptical/circular radiating slot has a long ) axis radius and a short axis radius (for circular slot, and is etched on a rectangular FR4 substrate with a thickness and a relative dielectric constant . The feed line is tapered with a slant angle for a to connect with the U-shaped tuning stub which is length all positioned within the elliptical/circular slot and symmetrical with respect to the short axis of the elliptical/circular slot. The U-shaped tuning stub consists of three sections: the semi-circle ring section with an outer radius and an inner radius , and two identical branch sections with equal heights and equal widths . represents the distance between the bottom of the tuning stub and the lower edge of the elliptical/circular slot. III. PERFORMANCES OF ELLIPTICAL/CIRCULAR SLOT ANTENNAS Four printed elliptical/circular slot antennas with the optimal designs were fabricated and tested in the Antennas Laboratory at Queen Mary, University of London (QMUL). Their respective dimensions are given in Table I. It is noticed that all of these four antennas feature a small size and even the largest one, i.e. microstrip line fed circular slot, is still 37% less than the antenna presented in [1].

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LI et al.: STUDY OF PRINTED ELLIPTICAL/CIRCULAR SLOT ANTENNAS FOR UWB APPLICATIONS

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TABLE I THE OPTIMAL DIMENSIONS OF THE PRINTED ELLIPTICAL/CIRCULAR SLOT ANTENNAS

TABLE II MEASURED AND SIMULATED BANDWIDTHS OF PRINTED ELLIPTICAL/CIRCULAR SLOT ANTENNAS

the antennas. It is demonstrated that all of the four antennas can achieve much wider bandwidth compared with those in [1]–[7] and hence meet the bandwidth requirement for UWB antenna. B. Radiation Patterns

Fig. 1. Geometry of printed elliptical/circular slot antennas. (a) Elliptical/circular slot antennas fed by microstrip line and (b) elliptical/circular slot antenna fed by CPW.

The radiation patterns of the antennas were also measured inside an anechoic chamber. As shown in Figs. 3 and 4, the measured patterns agree well with the simulated ones for both elliptical slot antennas. It is noticed that the printed elliptical/circular slot antennas with different feeding structures can provide similar radiation patterns. The -plane pattern is monopolelike, and the number of lobes rises with the increase of frequency which means the antenna gets more directional. The slight asymmetry on the -plane pattern is due to the imperfection of fabrication of the antennas. The -plane pattern is nearly omnidirectional at lower frequency, but becomes more asymmetrical to -axis at higher frequency. This is due to the tuning stub acting as a radiator itself and its effect becoming more prominent at high frequency. C. Antenna Gain

A. Return Loss The return losses of the four antennas were measured by using an HP8720ES vector network analyzer. The measured and simulated return loss curves are plotted in Fig. 2. Their respective 10 dB bandwidths are tabulated in Table II. As shown in Fig. 2 and Table II, good agreement has been achieved between the measurement and experiment for each of

The measured gains of the four antennas are presented in Fig. 5. It is seen that the measured gains fluctuate within the range from 2 to 7 dBi and reach the maximum values at 10 GHz for all of the four slot antennas. Generally speaking, the measured gains are similar to those presented in [1]–[7] over most parts of the bandwidth. However, due to the wider operational bandwidth compared with those in [1]–[7], the gains have more variations, as shown in Fig. 5.

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 6, JUNE 2006

Fig. 3. Simulated (dotted line) and measured (solid line) radiation patterns of microstrip line fed elliptical slot antenna. (a) E -plane at 3.1 GHz, (b) H -plane at 3.1 GHz, (c) E -plane at 10 GHz, and (d) H -plane at 10 GHz.

Fig. 4. Simulated (dotted line) and measured (solid line) radiation patterns of CPW fed elliptical slot antenna. (a) E -plane at 3.1 GHz, (b) H -plane at 3.1 GHz, (c) E -plane at 10.6 GHz, and (d) H -plane at 10.6 GHz.

Fig. 2. Measured and simulated return loss curves of printed elliptical/circular slot antennas. (a) Microstrip line fed elliptical slot antenna, (b) microstrip line fed circular slot antenna, (c) CPW fed elliptical slot antenna, and (d) CPW fed circular slot antenna.

D. Current Distributions The simulated current distributions of CPW fed elliptical slot antenna at four frequencies are presented in Fig. 6, as a typical

example. On the ground plane, the current is mainly distributed along the edge of the slot for all of the four different frequencies. The current patterns indicate the existence of different resonant harmonics, i.e. the first harmonic at 3.3 GHz in Fig. 6(a), the second around 5 GHz in Fig. 6(b), the third around 7.5 GHz in Fig. 6(c) and the fourth harmonic around 10 GHz in Fig. 6(d). Normally, the bandwidth of an antenna comes from one resonant mode. However, Fig. 6 indicates that the elliptical/circular

LI et al.: STUDY OF PRINTED ELLIPTICAL/CIRCULAR SLOT ANTENNAS FOR UWB APPLICATIONS

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Fig. 5. Measured gains of the four antennas.

slot is capable of supporting four resonant modes, and the overlapping of all of these four resonant modes leads to the UWB characteristic [9], as shown in Fig. 7. IV. DESIGN CONSIDERATIONS Studies in the previous sections have indicated that the ultra wide bandwidth of the slot antenna results from the overlapping of the multiple resonances introduced by the combination of the elliptical slot and the feeding line with U-shaped tuning stub. Thus, the slot dimension, the distance and the slant angle are the most important design parameters which affect the antenna performance and need to be further investigated. A. Dimension of Elliptical Slot It is noticed that the dimension of the slot antenna is directly related to the lower edge of the impedance bandwidth. In the case of elliptical disc monopoles [10], an empirical formula for estimating the lower edge frequency of the 10 dB bandwidth is derived based on the equivalence of a planar configuration to a cylindrical wire, as shown

(1)

Fig. 6. Simulated current distributions of CPW fed elliptical slot antenna. (a) 3.3 GHz, (b) 5 GHz, (c) 7.5 GHz, and (d) 10 GHz.

where in GHz, and in centimeters. is the disc height, is equivalent radius of the cylinder given by . In this paper, the elliptical slot can be regarded as an equivalent magnetic surface. Equation (1) is modified empirically as

(2) where , in cm, and in GHz; , . is the element factor which equals to 0.32 for elliptical slot and 0.35 for circular slot, respectively. The comparison between the calculated and the measured one for different printed slot antennas are tabulated in Table III. It is shown that the measured matches the calculated one quite well.

Fig. 7. Overlapping of the multiple resonant modes.

B. Distance The simulated return loss curves of CPW fed elliptical slot ) with antenna for various (

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TABLE III THE CALCULATED AND MEASURED LOWER EDGE OF

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 6, JUNE 2006

010 dB BANDWIDTH

C. Slant Angle In Fig. 9, the return loss curves of CPW fed elliptical slot , antenna for different slant angles with and are plotted. It is observed that the lower edge of the 10 dB bandwidth is independent of , but the upper edge is very sensitive to the variation of . The optimal , with a bandwidth slant angle is found to be at of 8.4 GHz (from 3.0 to 11.4 GHz). V. CONCLUSION It has been shown that ultrawideband characteristic was achieved for printed elliptical/circular slot antenna using tapered microstrip or CPW feeding line with U-shaped tuning stub. The slot dimension, the distance and the slant angle are the most important design parameters that determine the antenna performance. Experimental results have also confirmed UWB characteristics of the proposed antennas as well as nearly omnidirectional radiation properties over a majority fraction of the bandwidth. These features and their small sizes make them attractive for future UWB applications. The time domain characteristics of these printed elliptical/circular slot antennas will be investigated in the future work. ACKNOWLEDGMENT

Fig. 8. Simulated return loss curves of CPW fed elliptical slot antenna for different S with A = 14:5 mm, B = 10 mm and  = 15 degrees.

The authors would like to acknowledge Computer Simulation Technology (CST), Germany, for the complimentary license of the Microwave Studio package. The authors would like to thank Mr. J. Dupuy of the Department of Electronic Engineering, QMUL for his help in the fabrication and measurement of the antenna. REFERENCES

Fig. 9. Simulated return loss curves of CPW fed elliptical slot antenna for different  with A = 14:5 mm, B = 10 mm and S = 0:4 mm.

, and are illustrated in Fig. 8. It is seen that the curves for different have similar shape and variation trend, but the optimal distance is because it can provide the widest 10 dB bandwidth.

[1] H. L. Lee, H. J. Lee, J. G. Yook, and H. K. Park, “Broadband planar antenna having round corner rectangular wide slot,” in Proc. IEEE Antennas and Propagation Society Int. Symp., Jun. 16–21, 2002, vol. 2, pp. 460–463. [2] J.-Y. Sze and K.-L. Wong, “Bandwidth enhancement of a microstripline-fed printed wide-slot antenna,” IEEE Trans. Antennas Propag., vol. 49, no. 7, pp. 1020–1024, Jul. 2001. [3] Y. W. Jang, “Broadband cross-shaped microstrip-fed slot antenna,” Electronics Letters, vol. 36, no. 25, pp. 2056–2057, Dec. 7, 2000. [4] M. K. Kim, K. Kim, Y. H. Suh, and I. Park, “A T-shaped microstripline-fed wide slot antenna,” in Proc. IEEE Int. Symp. APS, Jul. 2000, pp. 1500–1503. [5] J.-Y. Chiou, J.-Y. Sze, and K.-L. Wong, “A broad-band CPW-fed striploaded square slot antenna,” IEEE Trans. Antennas Propag., vol. 51, no. 4, pp. 719–721, Apr. 2003. [6] H.-D. Chen, “Broadband CPW-fed square slot antennas with a widened tuning stub,” IEEE Trans. Antennas Propag., vol. 51, no. 8, pp. 1982–1986, Aug. 2003. [7] R. Chair, A. A. Kishk, and K. F. Lee, “Ultrawide-band coplanar waveguide-fed rectangular slot antenna,” IEEE Antennas Wireless Propag. Lett., vol. 3, no. 12, pp. 227–229, 2004. [8] “CST-Microwave Studio, User’s Manual,” 2003. [9] J. Liang, L. Guo, C. C. Chiau, X. Chen, and C. G. Parini, “Study of CPW-fed circular disc monopole antenna,” Proc. Inst. Elect. Eng. Microwaves, Antennas and Propagation, 2005, Accepted for publication. [10] N. P. Agrawall, G. Kumar, and K. P. Ray, “Wide-band planar monopole antennas,” IEEE Trans. Antennas Propag., vol. 46, no. 2, pp. 294–295, Feb. 1998.

LI et al.: STUDY OF PRINTED ELLIPTICAL/CIRCULAR SLOT ANTENNAS FOR UWB APPLICATIONS

Pengcheng Li received the B.Sc. degree from Xiangtan University, China, in 1985, and the M.Sc. degree from Beijing Research Institute of Telemetry Technology, Beijing, China, in 1991. He is working toward the Ph.D. degree at the Electronic Engineering College ,Beijing University of Aeronautics and Astronautics, Beijing. He is currently working at Beijing Research Institute of Telemetry Technology, China. His current research interests focus on UWB antenna design and analysis and microwave circuit. Mr. Li was the recipient of a scholarship from the Chinese Scholarship Fund to pursue research as an Academic Visitor in the Communications Research Group, Department of Electronic Engineering, Queen Mary, University of London, U.K., from September 2004 to March 2006.

Jianxin Liang (S’04) received the B.Sc. and the M.Sc. degrees from Nankai University, China, in 1995 and 1998, respectively. Currently, he is working towards the Ph.D. degree in the Communications Research Group, Department of Electronic Engineering, Queen Mary, University of London, U.K. His current research interests focus on UWB antenna design and analysis.

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Xiaodong Chen (M’96) received the B.Sc. degree from the University of Zhejiang, Hangzhou, China, in 1983 and the Ph.D. degree from the University of Electronic Science and Technology of China, Chengdu, in 1988. In September 1988, he joined the Department of Electronic Engineering at King’s College, University of London, as a Postdoctoral Visiting Fellow. In September 1990, he was employed by the King’s College London as a Research Associate. In March 1996, he was appointed to an EEV Lectureship at King’s College London. In September 1999, he joined the Department of Electronic Engineering at Queen Mary and Westfield College, University of London as a College Lecturer. In October 2003, he was promoted to a Readership at the same institution. His research interests are in microwave devices and antennas, bio-electromagnetic and nonlinear dynamics and chaos. He has authored and coauthored over 160 publications (book chapters, journal papers and refereed conference presentations). Dr. Chen has been involved in the organization of many international conferences: he has served as Chairman of Institute of Electrical Engineers (IEE) International Workshop on Ultra Wide Band Technologies and Systems (2004), co-chairman of the Institute of Physics (IoP)/IEE International Workshop on RF Interaction with Humans (2003), Executive Chairman of The International Conference on Telecommunications (ICT), 2004. He is currently a member of the U.K. EPSRC Review College and Technical Panel of the Institution of Electrical Engineers (IEE) Antennas and Propagation Professional Network.

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