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Abstract—A novel wireless local-area network (WLAN) chip antenna suitable to be mounted above the system ground plane of a mobile device is presented.

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 11, NOVEMBER 2005

WLAN Chip Antenna Mountable Above the System Ground Plane of a Mobile Device Kin-Lu Wong, Senior Member, IEEE, and Chih-Hua Chang

Abstract—A novel wireless local-area network (WLAN) chip antenna suitable to be mounted above the system ground plane of a mobile device is presented. The antenna in the study is easily fabricated from folding a single metal plate onto a foam base, and mainly comprises a short-circuited radiating strip and an antenna ground. The antenna ground occupies the bottom surface and two adjacent side surfaces of the foam base. When the antenna is mounted at the corner of the system ground plane, this antenna ground structure is expected to effectively reduce the antenna’s possible fringing electromagnetic fields inside the mobile device. In this case, when the associated element such as the radio-frequency shielding metal case is placed under the proposed antenna, small or negligible variations in the antenna performance are obtained. Design considerations of the proposed antenna for WLAN operation in the 2.4 GHz band are described, and results of the constructed prototypes are presented. Index Terms—Antennas, chip antennas, mobile antennas, wireless local-area network (WLAN) antennas.

I. INTRODUCTION

C

HIP antennas are attractive for their compact size and low profile and are very promising for wireless local-area network (WLAN) operations in mobile communication devices [1]. For practical applications, this kind of conventional WLAN chip antenna is directly mounted on the system circuit board or system ground plane of the mobile device. In addition, when applied inside a mobile device as an internal antenna, an isolation distance [usually about or larger than 7 mm for WLAN operation in the 2.4 GHz band (2400 2484 MHz)] between the antenna and the radio-frequency (RF) shielding metal case, which shields and protects the associated RF components and modules in the mobile device, is usually required. Without the isolation distance, the performance of the antenna will be significantly affected. Therefore, by including the chip antenna itself and the required isolation distance, a certain valuable broad space on the system circuit board is required for embedding the conventional chip antenna to operate as an internal antenna in a mobile device. In this paper, we demonstrate a new chip antenna design for WLAN operation. The proposed antenna is suitable to be mounted above the system circuit board of the mobile communication device, and the associated element such as the RF shielding metal case inside the device can be placed under the proposed antenna. With and without the presence of the

Manuscript received April 4, 2005; revised June 14, 2005. The authors are with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C. (e-mail: [email protected]). Digital Object Identifier 10.1109/TAP.2005.858817

RF shielding metal case, small or negligible variations in the antenna performance are obtained for the proposed antenna. This behavior is because the proposed chip antenna uses a new ground structure, which occupies the bottom surface and two adjacent side surfaces of the chip base of the antenna (see Fig. 1). Thus, when the proposed antenna is placed at the corner of the system circuit board, the possible fringing electromagnetic (EM) fields of the antenna inside the mobile device are expected to be greatly suppressed. In this case, when there are conducting elements placed nearby, the possible coupling between the chip antenna and the conducting elements will be eliminated. A design example of the proposed chip antenna for WLAN operation in the 2.4 GHz band for a mobile device is demonstrated, and experimental results of constructed prototypes are presented. Effects of the distance between the proposed chip antenna and the system ground plane on the antenna performance are also studied. In addition, a parametric study for analyzing the dimensions of the system ground plane on the performance of the antenna is conducted. II. ANTENNA DESIGN Fig. 1(a) shows the configuration of the proposed chip antenna mounted at the corner of the system ground plane (length and width ) of a mobile communication device. Note that the proposed chip antenna is supported by three grounding strips of width 1.5 mm and length above the system ground plane. The proposed antenna in the study is easily fabricated by folding a single metal plate [see Fig. 1(b)] onto a foam base of size 12 20 3 mm . The single metal plate was obtained by line-cutting a 0.2-mm-thick copper plate in the study. Instead of using the line-cutting technique, one can also use a stamping technique to fabricate the single metal plate shown in Fig. 1(b) Also note that instead of using a foam base [2], whose permittivity is very close to that of air, one can use a plastic base [3] or a ceramic base [4]–[6] for the proposed antenna. For the latter cases, a smaller antenna size can be achieved. However, the achievable operating bandwidth and radiation efficiency will be decreased for the proposed antenna. To test the antenna in the study, a 50 mini coaxial line is used to feed the antenna. Across a 1 mm feed gap, the central pin of the coaxial line is connected to point A (the feeding point of the antenna), and the outer grounding sheath of the coaxial line is connected to point B, the grounding point. The proposed antenna mainly consists of a short-circuited radiating strip and an antenna ground. The radiating strip has a uniform width of 2 mm and a mean length of 31 mm, close to a quarter-wavelength of the desired center operating frequency (2442 MHz) of the 2.4 GHz WLAN band. The radiating strip

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WONG AND CHANG: WLAN CHIP ANTENNA MOUNTABLE ABOVE THE SYSTEM GROUND PLANE

Fig. 2. Measured and simulated return loss. h = 6 mm.

Fig. 3. mm.

Fig. 1. Configuration of the proposed WLAN chip antenna mounted above the system ground plane of a mobile device. (b) Detailed dimensions of a single metal plate for fabricating the proposed antenna.

is folded to achieve a compact size to fit on the top surface of the foam base. The radiating strip is also short-circuited to the antenna ground through a shorting strip of width 1.5 mm and mean length 7.5 mm. By bending line 1 in the figure, the radiating strip and the shorting strip are attached onto the top surface of the foam base. The short-circuited radiating strip controls the excitation of a resonant mode for WLAN operation in the 2.4 GHz band. For the antenna ground, it includes a ground plane of size 12 20 mm on the bottom surface of the foam base and two 3 mm and 12 3 mm smaller ground planes of sizes 20 on two side surfaces of the foam base. Note that the two smaller ground planes are attached onto the two side surfaces by bending lines 2 and 3 shown in Fig. 1(b) Mainly due to the presence of the two side ground planes, the antenna’s possible EM fringing fields inside the mobile device are expected to be greatly suppressed. This property allows the presence of the conducting elements in close proximity to the proposed antenna. Thus, when placing a RF shielding metal case below the proposed antenna, small or negligible effects in the performance of the antenna are expected. The related results will be discussed in more detail in Section III with the aid of Fig. 4.

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L

Measured return loss as a function of h;

= 100 mm,

W

= 70 mm,

L = 100 mm and W

= 70

There are also three grounding/supporting strips of length formed by bending three protruded strips at three corners of the antenna ground [see Fig. 1(b)] for mounting the antenna above the system ground plane. By varying the length , the height of the proposed chip antenna mounted above the system ground plane can be varied. Effects of the length on the impedance matching of the proposed antenna are found to be small, and more detailed results will be discussed with the aid of Fig. 3 in Section III. III. EXPERIMENTAL RESULTS AND DISCUSSION Based on the design dimensions shown in Fig. 1, the proposed chip antenna was constructed and tested. The ground plane is first selected to have dimensions of 100 mm 70 mm , which are reasonable dimensions of a practical personal digital assistant phone. The length of the grounding/supporting pins is chosen to be 6 mm. Fig. 2 shows the measured and simulated return loss for the constructed prototype. The simulated results are obtained using Ansoft High Frequency Structure Simulator (HFSS) software,1 and agreement between the measured and simulated results is obtained. The measured impedance bandwidth, defined by 10 dB return loss, reaches 126 MHz (2382–2508 MHz) and covers the required bandwidth for WLAN operation in the 2.4 GHz band. Fig. 3 shows the measured return loss as a function of length . Four different cases with , and mm are studied. 1http://www.ansoft.com/products/hf/hfss/

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 11, NOVEMBER 2005

Fig. 4. 40

Measured return loss for the case with an RF shielding metal case (30 ) directly placed under the proposed antenna. L = 100 mm, = 70 mm, h = 6 mm.

2 2t mm W

Fig. 6. Fig. 2.

Measured antenna gain versus frequency for the antenna studied in

TABLE I MEASURED RESULTS OF THE PROPOSED ANTENNA SHOWN IN FIG. 1 WITH VARIOUS GROUND-PLANE LENGTHS; W = 70 mm, h = 6 mm. f AND f ARE, RESPECTIVELY, THE UPPER AND LOWER EDGE FREQUENCIES OF THE MEASURED 10 dB RETURN-LOSS IMPEDANCE BANDWIDTH (BW) AND f IS THE CENTER OPERATING FREQUENCY OF THE IMPEDANCE BANDWIDTH

Fig. 5. Measured radiation patterns at 2442 MHz for the antenna studied in Fig. 2.

For , it indicates that the proposed chip antenna is in direct contact with the system ground plane. Results show that the variations in the measured return loss are small, and the obtained impedance bandwidths remain about the same. Effects of the length on the impedance matching of the proposed chip antenna are thus negligible. Fig. 4 shows the measured return loss for the case with an RF 40 mm ) directly placed under shielding metal case (30 the proposed antenna. Other parameters are the same as studied in Fig. 2. With the presence of the RF shielding metal case, it is and seen that the measured impedance bandwidths for mm are slightly decreased. However, the impedance bandwidth still covers the required bandwidth for WLAN operation in the 2.4 GHz band. The obtained results are expected as discussed in Section II. Radiation characteristics of the proposed antenna were also studied. Fig. 5 plots the measured radiation patterns for the proposed antenna at 2442 MHz; the antenna parameters are the same as studied in Fig. 2. Note that the measured results for the cases with and without the RF shielding metal case are almost identical, thus only the results for the case without the RF shielding metal case are shown. From the measured and components are seen. radiation patterns, comparable This characteristic is advantageous for practical applications, because the propagation environment for WLAN operation is usually complex. For other frequencies over the operating

band, the radiation patterns were also measured, and the results are about the same as shown here. This behavior indicates that stable radiation patterns over the operating band are obtained for the proposed antenna. Fig. 6 presents the measured antenna gain versus frequency. An antenna gain level of about 3.2 dBi is obtained over the operating band. For the radiation efficiency, it is found to be larger than 90% from the simulated results obtained from HFSS. In addition, it is found that the radiation efficiency is almost the same for varied from 0 to 6 mm in this paper. This good radiation efficiency is largely owing to the use of the low-loss, low-permittivity foam base for the proposed WLAN chip antenna. Finally, a parametric study for analyzing the effects of the system ground-plane dimensions on the performance of the antenna is conducted experimentally. The case with various ground-plane lengths is first studied. The measured center and impedance bandwidth (BW) for the length frequency varied from 40 to 150 mm are listed in Table I for comparison. The measured center frequencies are seen to vary in

WONG AND CHANG: WLAN CHIP ANTENNA MOUNTABLE ABOVE THE SYSTEM GROUND PLANE

TABLE II MEASURED RESULTS OF THE PROPOSED ANTENNA SHOWN IN Fig. 1 WITH VARIOUS GROUND-PLANE WIDTHS. L = 100 mm, h = 6 mm

a small range from 2445 to 2456 MHz, an 11 MHz or 0.5% variation only. As for the impedance bandwidth, it varies from 112 to 126 MHz, a 14 MHz or 11% variation. Although a relatively large variation in the impedance bandwidth is seen, the obtained bandwidths still easily cover the 2.4 GHz band (2400–2484 MHz) for WLAN operation. The case with various ground-plane widths is also studied, and the results for the varied from 40 to 150 mm are listed in Table II. width The variations in the center frequency and the impedance bandwidth are seen to be about the same as those for various ground-plane lengths studied in Table I. These results indicate that the proposed chip antenna is suitable for applications in mobile devices with various possible system ground-plane dimensions. IV. CONCLUSION A novel WLAN chip antenna suitable to be mounted above the system ground-plane of a mobile device for operating as an internal antenna has been proposed. Prototypes of the proposed antenna have been successfully implemented. Experimental results indicate that with the presence of an RF shielding metal case placed under the proposed antenna, small or negligible effects on the antenna performance have been observed. Good radiation characteristics for the proposed antenna have also been obtained. The variations in the dimensions of the system ground plane have also been found to result in small effects on the center operating frequency and impedance bandwidth of the proposed antenna. In addition, since the proposed antenna is mountable over the RF shielding metal case or other RF components, no valuable board space on the system ground plane is occupied for employing the proposed antenna. This can lead to a compact integration of the proposed antenna with associated components in mobile communication devices.

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REFERENCES [1] K. L. Wong, Planar Antennas for Wireless Communications. New York: Wiley, 2003, ch. 5. [2] S. W. Su, T. W. Wu, Y. T. Cheng, and K. L. Wong, “A foam-base surfacemountable shorted monopole antenna for WLAN application,” Microw. Opt. Technol. Lett., vol. 38, pp. 501–503, Sep. 20, 2003. [3] K. L. Wong, S. W. Su, T. W. Chiou, and Y. C. Lin, “Dual-band plastic chip antenna for GSM/DCS mobile phones,” Microw. Opt. Technol. Lett., vol. 33, pp. 330–332, Jun. 5, 2002. [4] K. Kawahata, K. Okada, A. Yuasa, and S. Nagumo, “Surface-mountable type antenna, antenna device, and communication device including the antenna device,” U.S. Patent 6 177 908, Jan. 23, 2001. [5] H. Mandai, K. Asakura, T. Tsuru, S. Kanba, and T. Suesada, “Chip antenna,” U.S. Patent 5 977 927, Nov. 2, 1999. [6] C. M. Su and K. L. Wong, “Surface-mountable dual side-feed circularly polarized ceramic chip antenna for GPS operation,” Microw. Opt. Technol. Lett., vol. 35, pp. 137–138, Oct. 20, 2002.

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 Max-Planck-Institute for Plasma Physics, Munich, Germany. Since 1987 he has been with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, where he became a Professor in 1991. He was Chairman of the Electrical Engineering Department from 1994 to 1997 and is now Dean of the Office of Research Affairs. From 1998 to 1999, he was a Visiting Scholar with the ElectroScience Laboratory, The Ohio State University, Columbus. He has published more than 350 refereed journal papers and numerous conference articles. He has graduated 39 Ph.D. students. He has received 15 U.S. patents and more than 70 Taiwan patents, with 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 became a Commissioned Research Fellow of the National Science Council of Taiwan in 2005. 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.

Chih-Hua Chang 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, in 2004. He is currently working toward the M.S. degree at the Antenna Laboratory, Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan. His main research interests are in planar antennas for wireless communications, especially for the planar antennas for mobile phone and WLAN applications, and also in microwave and RF circuit design.

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