Wideband Printed Dipole Antenna for Multiple Wireless Services This invited paper presents numerical and experimental results for a design offering bandwidth results that cover a range of frequency bands By Jeong Il Kim, Electronics and Telecommunications Research Institute; Byung Moo Lee and Young Joong Yoon,Yonsei University This article discusses an antenna for multiple wireless service using a coax-fed printed fat dipole backed by a ground plate. The general principle, that thicker dipole provides wider bandwidth [1], is also applicable to a printed or metallic strip dipole. The question is whether the bandwidth can be increased continuously as the width of printed dipole increases. This article first examines the relation between bandwidth and width to length ratio of a printed dipole and then presents design parameters pertaining to wideband performance. To predict antenna performance, we used the electromagnetic simulator Microwave Studio, developed by Computer Simulation Technology (CST). This program is based on the time domain method and uses the finite integral technique (FIT), so results over wide frequency bands can be extracted effectively [2]. The numerical and experimental results show that the proposed antenna has wide bandwidth, which can cover the global system for mobile communications 1800 (GSM-1800), personal communication service (PCS), international mobile telecommunication 2000 (IMT- 2000) and industrial scientific and medical (ISM) bands. Printed dipole antenna Wireless communication has developed rapidly over the past few decades. Currently, many mobile and wireless services, such as a cellular phone and wireless local area network (LAN), operate in multiple frequency bands. Using only one antenna system to support these multiple Figure 1. Center-fed printed dipole. services reduces cost and occupied space. Moreover, wideband antennas play a key role in other systems, such as software defined radio (SDR) and mobile radio direction finding (RDF) systems. The printed dipole is a good candidate for wideband antennas. Various types of printed dipoles [3], including center-fed coplanar strips dipoles, double sided printed dipoles, bowtie antennas and folded printed dipoles, have been studied. Printed dipoles on thin dielectric substrate with a metallic ground plate have a number of advantages over the conventional microstrip patch antennas such as wider bandwidth, smaller surface wave excitation and less parasitic radiation of feeding lines [4, 5]. To improve the impedance bandwidth of printed dipoles, flaring the dipole arms [6], endloading the dipole arms with triangular shaped loads [7], and the use of parasitic elements [8] 70 APPLIED MICROWAVE & WIRELESS
Figure 2. Input impedance with W/L ratio (L = 28 mm, G = 0.1L, h = 0.6 mm, r = 4.6, substrate size = 4L 3L): input resistance; input reactance. have been suggested. In addition, the impedance bandwidth can be enhanced by increasing the distance between the ground plate and dipole or the width of printed dipole arm [4]. Increasing the width of printed dipole arm to obtain wider bandwidth mimics increasing the wire diameter of a dipole [1]. Parameter studies for wideband performance Design parameters pertaining to wideband performance were examined without a ground plate. Design parameters are the width to length (W/L) ratio of printed dipole arm, the gap (G) between printed dipole arms, height (h) and dielectric constant ( r ) of the substrate, as shown in Figure 1. The 50-ohm discrete port in Microwave Studio is used to obtain the driving point impedance against frequency at the center of dipole. Figure 3. Impedance match versus W in the vicinity of L = 28 mm (G = 2.5 mm, h = 0.6 mm, r = 4.6, substrate size = 4L 3L): return loss; Smith Chart plot. This port provides a balanced feed. The effect of the W/L ratio for a given length of dipole arm is shown in Figure 2. The input impedance at the feed terminals as a function of frequency was calculated using different W/L ratios. Up to a certain W/L ratio, the input impedance variations are flatter with frequency as the W/L ratio increases, providing wider bandwidth. However, at larger W/L ratio, the variations increase. The same behavior also occurs in different Gs and substrate parameters, which means there is an optimum W/L ratio for wideband characteristics. Evaluating several cases, we found that the proper W/L ratio is about 1 (0.9 ~ 1.1). This value is appropriate for voltage standing wave ratio (VSWR) 1.5 (see Figure 3). If the required VSWR 2, then other ratios may be better. The length of the fat dipole (2 L + G) is determined roughly by 0.5 c, where c is the wavelength of the center frequency of the bandwidth. While the variation of W/L affects the size and loca- 72 APPLIED MICROWAVE & WIRELESS
tion of a loop in the input impedance locus on the Smith Chart, the variation of G affects mainly the location of the loop (see Figures 3 and 4). The size and location of the impedance loop determines the impedance bandwidth. Similar impedance movements were found using the different antenna parameters. Evaluating several cases, we found that about 0.1L is appropriate for G. The substrate parameters (substrate height and dielectric constant) have relatively little effect on the impedance bandwidth compared to the W/L ratio and G. These substrate parameters affect the location of the resonance determining the higher frequency region of the response. It was found that h r / c = 0.03 is a good criterion for selecting substrate parameters. Generally, a thin FR4 substrate can be used in fabrication for low cost and easy machining. Figure 4. Variations of G (L = W = 28 mm, h = 0.6 mm, r = 4.6, substrate size = 4L 3L): return loss; Smith Chart plot. (c) The simulated radiation patterns and surface current of the printed fat dipole without a ground plate are depicted in Figures 5 and 6, respectively. The 3D radiation pattern shows the donut-shaped beam associated with a dipole. The beam has a little directivity in the z- axis direction due to the wide width of dipole arms. The Figure 5. Simulated radiation patterns: 3D pattern (f = 2.5 GHz); E-plane (xz-plane) pattern; (c) H-plane (yz-plane) pattern. Figure 6. Simulated surface current on printed dipole (f = 2.5 GHz): x-component; y-component. SEPTEMBER 2002 73
Figure 7. Geometry of coax-fed printed fat dipole backed by ground plate. Figure 8. Simulated return losses of the coax-fed printed dipole backed by ground plate. radiation patterns versus frequency are nearly the same except that the beamwidths become narrower as the frequency increases. The simulated antenna gains on boresight (z-axis) increase with increased frequency from 1.86 dbi at 1.5 GHz to 3.53 dbi at 3.5 GHz, due to the narrower beamwidths. These narrower beamwidths are attributed to larger electrical size of the printed dipole at higher frequency. The cross polarization levels are very low because the y-component of surface current has opposite direction symmetrically along the y-axis, as shown in Figure 6. As a result, the fields from the y- component of the surface current cancel each other. Printed dipole antenna backed by ground plate A unidirectional beam pattern is created by placing a ground plate at the back of the printed fat dipole. A semi-rigid 50-ohm coaxial cable connects to the dipole arms and spans the distance between the dipole and the ground plane, as shown in Figure 7. Next we determine the distance between the dipole and the ground plate. Normally, the optimum distance is /4, since the impedance looking into the ground plate at the printed dipole plane is an open-circuit condition [9]. In addition, the direct field from the dipole and the reflected field from the ground plate are added in-phase. Equation (4-116) and Figure 4.23 in reference [10] explain the effect of the distance on the radiation pattern. The array factor from Equation (4-116) of reference [10] is 2jsin(kdcosq) where k is the wave number, d is the distance between dipole and ground plane and q is the elevation angle from the z-axis. The printed fat dipole without the ground plate has a Figure 9. Simulated surface current on coax-fed printed dipole backed by ground plate (f = 2.45 GHz): x-component; y-component. 74 APPLIED MICROWAVE & WIRELESS
Figure 10. Simulated co-pol (solid line) and cross-pol (dotted line) radiation patterns (d = 42 mm): E-plane (xz-plane) pattern; H-plane (yz-plane) pattern. wide bandwidth of 57 percent (VSWR 1.5 for L = W = 28 mm, G = 2.5 mm, d = 0.6 mm and r = 4.6). Over this wide frequency range, the electrical distance d varies appreciably. As a result, /4 becomes /2 at high frequency and pattern null on boresight appears. The distance (d) also affects the impedance bandwidth [4]. Therefore, we investigated the antenna performances against d. The presence of the ground plate lowers the operating frequency. Thus, the length of the dipole with the ground plate should be shortened. It was found that increasing d provides wider impedance bandwidth [4] but deteriorates the radiation pattern and antenna gain in the operating frequency range. The frequency where d = /2 is shifted down as d increases, which causes a dip in the radiation pattern on the boresight and lowers the antenna gain within the operating frequency range. The circles in Figure 8 indicate resonances originating from current on the outside surface of the coax. This antenna does not have a balun (balance to unbalanced transformer) to feed a balanced current at the printed dipole. Therefore, the surface currents I 1 and I 2 on the each dipole arm are not equal in magnitude, as shown in Figure 9. Thus differential currents flow on the outside of the coax and induce the additional resonances. The squint radiation patterns in Figure 10 stem from unbalanced currents on the dipole and radiation from the outside surface current on coax. A depression in the pattern appears at high frequency and deepens into a pattern null on boresight at the frequency where d = /2. These variations result in decreasing antenna gain on boresight, as shown in Figure 11. Experimental results The printed fat dipole antenna with dimensions of W = L = 28 mm, G = 2.5 mm, Ws = 90 mm, Ls = 140 mm, Wr = 120 mm, Lr = 180 mm and d = 42 mm was fabricated and measured in an anechoic chamber, as shown in Figure 12. W, L and G were calculated using the above design guidelines. The distance d is determined from considerations of impedance matching, radiation pattern and antenna gain over the operating frequency Figure 11. Simulated antenna gains on the boresight (zaxis). Figure 12. Fat dipole antenna backed by ground plate. SEPTEMBER 2002 75
The variations of the radiation pattern and the antenna gain also limit the usable bandwidth. Considering a radiation pattern with little depression and the antenna gain within 3 db of the maximum value, the bandwidth is limited to about 2.7 GHz. The measured antenna gains are 5.71 ~ 6.46 dbi and the measured radiation patterns are similar over 1.71 ~ 2.55 GHz, as shown in Figures 14 and 15, respectively. Figure 13. Measured and simulated return loss. Figure 14. Measured antenna gain on the boresight (zaxis). range. This printed dipole provides 75 percent bandwidth (1.64 GHz ~ 3.6 GHz) for VSWR 1.5 and 82 percent bandwidth (1.52 GHz ~ 3.64 GHz) for VSWR 2. Therefore, this antenna covers the frequency range of 1.71 ~ 2.55 GHz continuously and is useful for GSM1800, PCS, IMT-2000, and ISM band. However, the resonance at 3.6 GHz (see Figure 13) limits usefullness at the upper frequencies. Conclusion A wideband coax-fed printed dipole antenna has been discussed. This antenna shows good VSWR, gain and useful radiation patterns over the GSM1800, PCS, IMT- 2000 and ISM frequency bands. The antenna parameters for wide bandwidth have been investigated, and the effects of each parameter have been discussed. The rough design guides are as follows: The proper W/L ratio is about 1 (0.9 ~ 1.1) The gap G is about 0.1L The length 2 L + G is about 0.5 c, where c is the wavelength of the center frequency of the bandwidth. h r / c = 0.03 is a good criterion for selecting the substrate parameters The distance d between the dipole and the ground plate should be determined carefully considering the impedance bandwidth, radiation patterns, and antenna gain. Since the proposed antenna has no balun, an additional resonance appears and the radiation patterns are deteriorated. Folded baluns [11] or wideband tapered baluns [1] can be used with this antenna configuration. Although the results are not presented in this article, we found by using Microwave Studio that a folded or tapered microstrip balun can eliminate these resonances. This simple antenna can be used in multiple wireless services and various wideband applications. References 1. W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, Second Edition, New York: John Wiley and Sons, 1998. 2. Microwave Studio 3.4, Computer Simulation Technology, Germany. 3. P. Garg, et al, Microstrip Antenna Design Handbook, Boston: Artech House, 2001. 4. E. Levine, S. Shtrikman and D. Treves, Doublesided Printed Arrays with Large Bandwidth, IEE 76 APPLIED MICROWAVE & WIRELESS
Proceedings Part H, Vol. 135, No. 1, February 1988. 5. A. Nesic, S. Jovanoic and V. Brankovic, Design of Printed Dipoles Near the Third Resonance, IEEE Antennas Propagation Society International Symposium Digest, Vol. 2, 1998. 6. S. Dey, et al, Bandwidth Enhancement by Flared Microstrip Dipole Antenna, IEEE Antennas Propagation Society International Symposium Digest, 1991. 7. S. Dey, et al, WideBand Printed Dipole Antenna, Microwave & Optical Technology Letters, 1991. 8. G. A. Evtioushkine, J. W. Kim and K. S. Han, Very Wideband Printed Dipole Antenna Array, Electronic Letters, Vol. 34, November 1998. 9. T. Iwasaki, A. P. Freundorfer and K. Iizuka, A Unidirectional Semi-Circle Spiral Antenna for Subsurface Radars, IEEE Transactions EMC., Vol. 36, No. 1, February 1994. 10. C. A. Balanis, Antenna Theory Analysis and Design, Second Edition, New York: John Wiley and Sons, 1997. 11. M. C. Bailey, Broadband Half-Wave Dipole, IEEE Transactions Antennas Propagation, Vol. 32, No. 4, April 1984. Author information Jeong Il Kim received a bachelor of science degree in radio communications engineering and a master of science degree in electrical engineering from Yonsei University, Seoul, Korea in 1999 and 2001, respectively. He works as an engineer at the Electronics and Telecommunications Research Institute (ETRI), Daejeon, Korea. His research interests include antennas and radio frequency (RF) circuits. He can be reached via E-mail: llong@etri.re.kr. Byung Moo Lee received a bachelor of science degree in information communication engineering from Soonchunhyang University, Asan, Korea, in 1998, and a master of science degree in electrical engineering from Yonsei University, Seoul, Korea, in 2000. He is currently working toward a Ph.D. in electrical engineering at Yonsei University. His research interests include RF active circuits and antennas. He can be reached via E- mail: binny@mwant.yonsei.ac.kr. Dr. Young Joong Yoon received both a bachelor of science degree and a master of science degree in electronic engineering from Yonsei University, Seoul, Korea, in 1981 and 1986, respectively, and a Ph.D. in electrical engineering from the Georgia Institute of Technology, Figure 15. Measured co-pol (blue line) and cross-pol (red line) radiation patterns: E-plane (xz-plane) pattern; H-plane (yz-plane) pattern. Atlanta, GA, in 1991. From 1992 to 1993, he worked at the ETRI, Daejon, Korea, as the senior member of the research staff. In 1993, he joined the faculty of Yonsei University, where he is now an associate professor. His research interests are antennas, radio propagations and RF circuits. He can be reached via E-mail: yjyoon@ yonsei.ac.kr. SEPTEMBER 2002 77