AMONG planar metal-plate monopole antennas of various

1262 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 4, APRIL 2005 Ultrawide-Band Square Planar Metal-Plate Monopole Antenna With a Trident-Shaped Feeding Strip Kin-Lu Wong, Senior Member, IEEE, Chih-Hsien Wu, and Saou-Wen (Stephen) Su Abstract A square planar metal-plate monopole antenna fed by using a novel trident-shaped feeding strip is presented. With the use of the proposed feeding strip, the square planar monopole antenna studied shows a very wide impedance bandwidth of about 10 GHz (about 1.4 11.4 GHz, bandwidth ratio about 1:8.3), which is larger than three times the bandwidth obtained using a simple feeding strip (about 1.5 3.3 GHz, bandwidth ratio about 1:2.3). In addition, the proposed feeding strip can be integrated with the square planar monopole, that is, the feeding strip and the square planar monopole together can be easily fabricated using a single metal plate, making the proposed antenna easy to construct at a low cost. Details of the experimental and simulation results for the proposed planar monopole antenna are presented and analyzed. Index Terms Antennas, planar metal-plate monopole antenna, planar monopole antennas, ultrawide-band monopole antennas. I. INTRODUCTION AMONG planar metal-plate monopole antennas of various shapes [1] [11], square monopoles are the simplest in geometry and, in addition, their radiation patterns are usually less degraded within the impedance bandwidth [4], [11]. These favorable features of square planar monopole antennas attract many studies, mainly on the bandwidth enhancement, such as the antennas with an offset feeding point [6], a semi-circular base [6], a beveling technique [7], a shorting pin [9], a double feed [11], and so on. Among these studies, it is demonstrated that, with the use of a double feed, an intense vertical current distribution in the planar monopole can be achieved, with the horizontal current distribution greatly suppressed, which leads to improvements in the polarization properties and impedance bandwidth of the square planar monopole antennas [11]. However, in the proposed double-feed design [11], an additional feeding network under the ground plane of the planar monopole antenna is required to excite at two separate feeding positions for the planar monopole. This complicates the total antenna configuration and increases the fabrication cost of the antenna. In this paper we propose a novel trident-shaped feeding strip very promising for achieving bandwidth enhancement of a square planar monopole antenna. The planar monopole with the proposed feeding strip together can be easily fabricated using a single metal plate and, in addition, no external feeding network is required. The proposed planar monopole antenna in general retains a simple configuration and can be easily fed using a 50 SMA connector placed under the ground plane Manuscript received February 12, 2004; revised September 18, 2004. The authors are with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C. (e-mail: wongkl@mail.nsysu.edu.tw). Digital Object Identifier 10.1109/TAP.2005.844430 of the antenna, similar as the case of a conventional planar monopole antenna using a single feed. Moreover, since the proposed trident-shaped feeding strip has three feeding points symmetrically connected to the lower edge of the square planar monopole antenna, a more uniform current distribution in the planar monopole can be expected, compared to a dual-feed design and a single-feed design. This behavior can lead to a much improved impedance bandwidth for the square planar monopole antenna. Prototypes of the proposed square planar monopole antenna with a trident-shaped feeding strip were constructed and studied. Effects of various dimensions of the trident-shaped feeding strip were also analyzed. II. ANTENNA DESIGN Fig. 1(a) shows the geometry of the proposed square planar monopole antenna with a trident-shaped feeding strip mounted above a ground plane of size 150 150 mm. In this design the square planar monopole and the trident-shaped feeding strip are integrated together and fabricated from a single metal plate (a 0.2 mm thick brass sheet was used). The square planar monopole has a side length of and is excited at three feeding points A, B, and C, at which the planar monopole is connected to the trident-shaped (or three branch) feeding strip. Note also that point B is along the central line of the planar monopole, and points A and C are symmetrically located on two sides of point B. All the widths of the trident-shaped feeding strip are uniform and set to 2 mm in this study. The trident-shaped feeding strip comprises one central branch (length ) connected to point B and two side branches of an inverted- shape connected to points A and C, which are spaced with a distance of. Through a via-hole in the ground plane, the central branch is connected to a 50 SMA connector behind the ground plane for signal transmission. The two side branches are identical in shape (a horizontal length and a vertical length ) and have a height of above the ground plane. By adjusting the three parameters, and, a much enhanced impedance bandwidth for the proposed planar monopole antenna can be achieved. For the study with a 40 mm 40 mm square planar monopole antenna, the optimal values of and are 1.0 and 3.5 mm, respectively, while that for is 15 mm, which suggests that points A and C should be roughly located at positions with a distance of about one-third the side length to the left or right side edge of the square planar monopole. Detailed effects of the three parameters on the impedance matching of the proposed planar monopole antenna are explored with the aid of Figs. 11 13 in Section III. Note also that, for comparison, the cases of the corresponding planar monopole antenna with a two-branch feeding strip [see 0018-926X/$20.00 2005 IEEE

WONG et al.: ULTRAWIDE-BAND SQUARE PLANAR MONOPOLE ANTENNA 1263 Fig. 2. Measured and simulated return loss for the proposed antenna shown in Fig. 1(a) with L =40mm, t =15mm, h =3:5mm, d =1:0mm. Fig. 3. (a) Measured return loss for the planar monopole antennas with a trident-shaped feeding strip (t = 15mm, h = 3:5 mm, d = 1:0 mm), a two-branch feeding strip (t = 15mm, h = 3:5 mm, d = 1:0 mm), and a simple feeding strip (d =2:5mm). Other parameters are the same as in Fig. 2. (b) Simulated surface current distributions for the three antennas studied in (a); f =2:5 GHz. Fig. 1. (a) Geometry of the proposed square planar monopole antenna with a trident-shaped (three-branch) feeding strip. (b) Geometry of a corresponding planar monopole antenna with a two-branch feeding strip. (c) Geometry of a corresponding conventional planar monopole antenna with a simple feeding strip. Fig. 1(b)] and a simple feeding strip [see Fig. 1(c)] were constructed and studied. As shown in Fig. 1(b), the two-branch feeding strip comprises only two side branches, which are connected to points A and C (also spaced with a distance of ) for exciting the square planar monopole. Other dimensions are the same as given for the proposed trident-shaped feeding strip in Fig. 1(a). This two-branch feeding arrangement is easier to fabricate than the two-feed design studied in [11], because of no external feeding network required here. As for the simple feeding strip shown in Fig. 1(c), it has a uniform width of 2 mm and a length of, and is connected to point A (center of the lower edge of the square planar monopole) for exciting the planar monopole. For comparison, optimal parameters of the three different feedings strips [,, in Fig. 1(a) and (b), and in Fig. 1(c)] are selected in this study to achieve maximum impedance bandwidths for the square planar monopole antenna. III. EXPERIMENTAL RESULTS AND DISCUSSION Prototypes of the proposed square planar monopole antenna with a trident-shaped feeding strip, a two-branch feeding strip, and a simple feeding strip were fabricated and studied. The measured and simulated results of the return loss for the case with the proposed trident-shaped feeding strip are shown in Fig. 2. The size of the square planar monopole was chosen to be 40 40 mm, which easily makes the obtained impedance bandwidth (10 db return loss) have a lower edge frequency less than 2 GHz. In addition, by selecting the parameters

1264 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 4, APRIL 2005 TABLE I MEASURED RESULTS OF THE THREE PLANAR MONOPOLE ANTENNAS STUDIED IN FIG. 3. f AND f ARE, RESPECTIVELY, THE LOWER AND UPPER EDGE FREQUENCIES OF THE 10 DB RETURN-LOSS IMPEDANCE BANDWIDTH OBTAINED Fig. 4. Measured radiation patterns at 2 GHz for the proposed antenna studied in Fig. 2.,, and of the feeding strip to be 15, 3.5, and 1.0 mm, respectively, the upper edge frequency of the impedance bandwidth obtained is found to be larger than 11 GHz. A parametric study of the dimensions of the trident-shaped feeding strip on the impedance bandwidth of the proposed antenna will be analyzed in detail in the latter half of this section. From the results shown in Fig. 2, good agreement between the measurement and simulation is seen. The simulated results are obtained using the Ansoft simulation software High Frequency Structure Simulator (HFSS). From the measured results, a very wide impedance bandwidth of about 10 GHz (1.376 11.448 GHz) has been achieved. This wide impedance bandwidth makes the proposed antenna very promising for application in the new broadband wireless metropolitan area network system using the IEEE 802.16a (2 11 GHz) standard [12]. Fig. 3(a) shows a comparison of the measured return loss for the square planar monopole antennas with a trident-shaped feeding strip mm, mm, mm,a two-branch feeding strip mm, mm, mm, and a simple feeding strip mm. Note that, for the other two cases of using a two-branch feeding strip and a simple feeding strip, the parameters of the feeding strips were also optimized for achieving a maximum impedance bandwidth. In this study, the optimal parameters of, and of the twobranch feeding strip are the same as those of the trident-shaped feeding strip studied in Fig. 2. For the case of using a simple feeding strip, the optimal value of was determined to be 2.5 mm, which is about the same as the existing study [10]. By adjusting, the coupling between the ground plane and the lower edge of the planar monopole is varied, which effectively introduces a variation in the input reactance of the antenna. Thus impedance matching of the antenna can be fine-tuned, and optimized impedance bandwidth can be obtained for the antenna. The corresponding measured data of the three different feeding strips used are also listed in Table I for comparison. It is seen that the case with the proposed trident-shaped feeding strip shows a frequency ratio of 8.32 for the impedance bandwidth obtained, which is larger than that (7.52) of the case with a two-branch feeding strip and much larger than that (2.26) of the case with a simple feeding strip. This behavior is largely because a much more uniform current distribution in the square planar monopole is achieved [see Fig. 3(b)], similar as the two-feed design studied in [11]. Radiation characteristics of the proposed planar monopole antenna were also analyzed. Figs. 4 6 plot the measured radia-

WONG et al.: ULTRAWIDE-BAND SQUARE PLANAR MONOPOLE ANTENNA 1265 Fig. 5. Measured radiation patterns at 6 GHz for the proposed antenna studied in Fig. 2. Fig. 6. Measured radiation patterns at 10 GHz for the proposed antenna studied in Fig. 2. tion patterns at 2, 6, and 10 GHz, respectively. Simulated radiation patterns are also shown in Figs. 7 9 for comparison. Note that the radiation patterns shown in three principal planes are normalized with respect to the peak gain of the antenna, and good agreement between the measurement and simulation is obtained. It is also seen that, mainly due to the wide width of the square planar monopole, the gain variations in the azimuthal plane ( - plane) are greatly dependent on the operating frequency. Fig. 10 shows the measured and simulated antenna gain for frequencies across the impedance bandwidth obtained. The agreement is good between the measured and simulated results. For frequencies up to about 6 GHz, it is seen that the antenna gain monotonically increases from about 4.0 to 7.0 dbi. For the higher frequency portion of the impedance bandwidth, however, the antenna gain varies relatively slightly in the range of 6.5 7.0 dbi. Finally, experiments for analyzing the effects of the parameters,, of the proposed trident-shaped feeding strip on the impedance bandwidth obtained were performed. Fig. 11 presents the measured return loss of the proposed antenna shown in Fig. 1(a) as a function of. The value of, which is the gap between the feeding strip and the ground plane, varies from 0.5 to 3.0 mm in this study. The corresponding measured data are listed in Table II for comparison. The impedance matching is seen to be greatly dependent on the value of, and there exists an optimal (1.0 mm in this study) for achieving a very wide impedance bandwidth for the proposed antenna.

1266 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 4, APRIL 2005 Fig. 7. Simulated (Ansoft HFSS) radiation patterns at 2 GHz for the proposed antenna studied in Fig. 2. Fig. 8. Simulated (Ansoft HFSS) radiation patterns at 6 GHz for the proposed antenna studied in Fig. 2. Fig. 12 shows the measured return loss of the proposed antenna shown in Fig. 1(a) as a function of, and Table III lists the measured data for varying from 11 to 21 mm. Results show that, for and 17 mm, good impedance matching (return loss ) for frequencies across a very wide bandwidth is obtained, especially for the case of mm. This indicates that, for achieving a maximum impedance bandwidth, points A and C of the trident-shaped feeding strip should be roughly located at positions with a distance of about one-third the side length to the left or right side edge of the square planar monopole. The effects of on the impedance bandwidth are studied in Fig. 13 and Table IV. Again, the dependence of the impedance matching on is also seen. The optimal value of is 3.5 mm in this study, which indicates that the optimal gap between the horizontal strips (width 2 mm) of the two side branches of the trident-shaped feeding strip and the lower edge of the planar monopole is 1.5 mm. IV. CONCLUSION A square planar metal-plate monopole antenna fed by using a novel trident-shaped feeding strip for achieving a very wide

WONG et al.: ULTRAWIDE-BAND SQUARE PLANAR MONOPOLE ANTENNA 1267 Fig. 9. Simulated (Ansoft HFSS) radiation patterns at 10 GHz for the proposed antenna studied in Fig. 2. TABLE II MEASURED RESULTS OF THE PROPOSED ANTENNA SHOWN IN FIG. 1(a) AS A FUNCTION OF d. ANTENNA PARAMETERS ARE THE SAME AS IN FIG. 11 Fig. 11. Measured return loss of the proposed antenna shown in Fig. 1(a) as a function of d; L =40mm, t =15mm, h =3:5 mm. Fig. 10. Measured and simulated antenna gain for the proposed antenna studied in Fig. 2. impedance bandwidth has been proposed. The proposed antenna can be easily fabricated using a single metal plate, thus making it easy to construct at a low cost. A very wide impedance bandwidth of about 10 GHz (about 1.4 11.4 GHz) has been achieved Fig. 12. Measured return loss of the proposed antenna shown in Fig. 1(a) as a function of t; L =40mm, h =3:5 mm, d =1:0 mm. for the proposed antenna, which makes it very promising for application in the new broadband wireless metropolitan area network system using the IEEE 802.16a (2 11 GHz) standard [12].

1268 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 4, APRIL 2005 TABLE III MEASURED RESULTS OF THE PROPOSED ANTENNA SHOWN IN FIG. 1(A) AS A FUNCTION OF t. ANTENNA PARAMETERS ARE THE SAME AS IN FIG. 12 [2] M. Hammoud, P. Poey, and F. Colombel, Matching the input impedance of a broadband disc monopole, Electron. Lett., vol. 29, pp. 406 407, Feb. 1993. [3] N. P. Agrawall, G. Kumar, and K. P. Ray, Wide-band planar monopole antennas, IEEE Trans. Antennas Propag., vol. 46, pp. 294 295, Feb. 1998. [4] M. J. Ammann, Square planar monopole antenna, in Proc. Inst. Elect. Eng. Nat. Conf. Antennas Propag., U.K., 1999, pp. 37 40. [5] Z. N. Chen and M. Y. W. Chia, Impedance characteristics of trapezoidal planar monopole antennas, Microwave Opt. Technol. Lett., vol. 27, pp. 120 122, Oct. 2000. [6] P. V. Anob, K. P. Ray, and G. Kumar, Wideband orthogonal square monopole antennas with semi-circular base, in IEEE Antennas Propag. Soc. Int. Symp. Dig., Boston, MA, 2001, pp. 294 297. [7] M. J. Ammann, Control of the impedance bandwidth of wideband planar monopole antennas using a beveling technique, Microwave Opt. Technol. Lett., vol. 30, pp. 229 232, Aug. 2001. [8] Z. N. Chen, M. J. Ammann, and M. Y. W. Chia, Broadband square annular planar monopoles, Microwave Opt. Technol. Lett., vol. 36, pp. 449 454, Mar. 2003. [9] M. J. Ammann and Z. N. Chen, A wide-band shorted planar monopole with bevel, IEEE Trans. Antennas Propag., vol. 51, no. 4, pp. 901 903, Apr. 2003. [10], Wideband monopole antennas for multi-band wireless systems, IEEE Antennas Propag. Mag., vol. 45, pp. 146 150, Apr. 2003. [11] E. Antonino-Daviu, M. Cabedo-Fabres, M. Ferrando-Bataller, and A. Valero-Nogueira, Wideband double-fed planar monopole antennas, Electron. Lett., vol. 39, pp. 1635 1636, Nov. 2003. [12] The IEEE 802.16 Working Group on Broadband Wireless Access Standards. [Online]. Available: http://grouper.ieee.org/groups/ 802/16/index.html Fig. 13. Measured return loss of the proposed antenna shown in Fig. 1(a) as a function of h; L =40mm, t =15mm, d =1:0 mm. TABLE IV MEASURED RESULTS OF THE PROPOSED ANTENNA SHOWN IN FIG. 1(A) AS A FUNCTION OF h. ANTENNA PARAMETERS ARE THE SAME AS IN FIG. 13 REFERENCES [1] S. Honda, M. Ito, H. Seki, and Y. Jinbo, A disc monopole antenna with 1:8 impedance bandwidth and omnidirectional radiation pattern, in Proc. Int. Symp. Antennas Propag., Sapporo, Japan, 1999, pp. 1145 1148. Kin-Lu Wong (M 91 SM 97) received the B.S. degree in electrical engineering from National Taiwan University, Taipei, Taiwan, R.O.C., and the M.S. and Ph.D. degrees in electrical engineering from Texas Tech University, Lubbock, in 1981, 1984, and 1986, respectively. From 1986 to 1987, he was a visiting Scientist with the Max-Planck Institute for Plasma Physics in Munich, Germany. Since 1987, he has been with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C., where he became a Professor in 1991. He also served as Chairman of the Electrical Engineering Department from 1994 to 1997. From 1998 to 1999, he was a Visiting Scholar with the ElectroScience Laboratory, The Ohio State University, Columbus. He has published more than 330 refereed journal papers and numerous conference articles and has graduated 39 Ph.D. students. He holds 15 U.S. patents, more than 60 Taiwan patents, and has many patents pending. He is the author of Design of Nonplanar Microstrip Antennas and Transmission Lines (New York: Wiley, 1999), Compact and Broadband Microstrip Antennas (New York: Wiley, 2002), and Planar Antennas for Wireless Communication (New York: Wiley, 2003). Dr. Wong is a Member of the National Committee of Taiwan for the International Scientific Radio Union (URSI), the Microwave Society of Taiwan, the Chinese Institute of Electrical Engineers (Taiwan), and the Chinese Institute of Engineers (Taiwan). He received the Outstanding Research Award three times from National Science Council of Taiwan in 1994, 2000, and 2002. He also received the Young Scientist Award from the URSI in 1993, the Outstanding Research Award from National Sun Yat-Sen University in 1994 and 2000, the Outstanding Textbook Award for Microstrip Antenna Experiment (in Chinese) from the Ministry of Education of Taiwan in 1998, the ISI Citation Classic Award for a published paper highly cited in the field in 2001, the Outstanding Electrical Engineer Professor Award from Chinese Institute of Electrical Engineers (Taiwan) in 2003, and the Outstanding Engineering Professor Award from Chinese Institute of Engineers (Taiwan) in 2004. He has been on the editorial board of the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, MICROWAVE OPTICAL TECHNOLOGY LETTERS and the Chinese Journal of Radio Science (China). He has also been on the Board of Directors of the Microwave Society of Taiwan. He is listed in Who s Who of the Republic of China (Taiwan) and Marquis Who s Who in the World.

WONG et al.: ULTRAWIDE-BAND SQUARE PLANAR MONOPOLE ANTENNA 1269 Chih-Hsien Wu was born in Taipei, Taiwan, R.O.C., in 1981. He received the B.S. degree in electrical engineering from Feng-Chia University, Taichung, Taiwan, R.O.C., in 2003. He is currently working toward the Ph.D. degree in the Antenna Laboratory at the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. His main research interests are in planar antennas for wireless communications, especially for the planar antennas for mobile phone, WLAN, and UWB applications. Saou-Wen (Stephen) Su was born in Kaohsiung, Taiwan, R.O.C., in 1977. He received the B.S. and M.S. degrees in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C., in 2001 and 2003, respectively. He is currently working toward the Ph.D. degree in the Antenna Laboratory at the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. His main research interests are in planar antennas for wireless communications, especially for the planar antennas for mobile phone, WLAN, and UWB applications, and also in microwave and RF circuit design. Mr. Su won a one-year full-time School Study Exchange Program scholarship to The University of Auckland, New Zealand from the Asian 2000 Foundation, in 1998. He was awarded the Best Student Paper Award at the 2004 International Conference on Electromagnetic Applications and Compatibility, in Taipei, Taiwan, R.O.C.