A Universal UHF RFID Reader Antenna
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    A Universal UHF RFID Reader Antenna A Universal UHF RFID Reader Antenna Document Transcript

    • IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 1275 A Universal UHF RFID Reader Antenna Zhi Ning Chen, Fellow, IEEE, Xianming Qing, Member, IEEE, and Hang Leong Chung Abstract—A broadband circularly polarized patch antenna is a universal reader antenna with desired performance across the proposed for universal ultra-high-frequency (UHF) RF identifica- entire UHF RFID band would be beneficial for RFID system tion (RFID) applications. The antenna is composed of two corner- configuration and implementation, as well as cost reduction. truncated patches and a suspended microstrip line with open-cir- cuited termination. The main patch is fed by four probes which In this paper, we propose a sequentially fed stacked CP patch are sequentially connected to the suspended microstrip feed line. antenna for UHF RFID applications. The antenna comprises two The measurement shows that the antenna achieves a return loss of suspended truncated patches and a suspended microstrip line. 15 dB, gain of 8.3 dBic, axial ratio (AR) of 3 dB, and 3-dB AR The main patch is sequentially fed by four probes which are con- beamwidth of 75 over the UHF band of 818–964 MHz or 16.4%. nected to the microstrip line. A parasitic patch is positioned right Therefore, the proposed antenna is universal for UHF RFID appli- cations worldwide at the UHF band of 840–960 MHz. In addition, above the main patch for enhancing the bandwidth. The corners a parametric study is conducted to facilitate the design and opti- of the patches are truncated to enhance the axial ratio (AR) per- mization processes for engineers. formance. The proposed antenna is designed to cover the UHF Index Terms—Axial ratio (AR), broadband antenna, circularly RFID band of 840–960 MHz with acceptable performance in polarized (CP), RF identification (RFID), sequential feed, ultra terms of gain, AR, and impedance matching. Meanwhile, the high frequency (UHF). antenna configuration is simple and easy for fabrication. The remainder of this paper is organized as follows. Section II describes the geometry of the proposed antenna. The measured I. INTRODUCTION results, analysis, and discussion are presented in Section III. Section IV demonstrates the results of parametric study. The F IDENTIFICATION (RFID), which was developed R around World War II, is a technology that provides wireless identification and tracking capability. In recent years, validation of the proposed antenna in RFID system applications is exhibited in Section V. Finally, a conclusion is drawn in Sec- tion VI. RFID technology has been rapidly developed and applied to many service industries, distribution logistics, manufacturing II. ANTENNA CONFIGURATION companies, and goods flow systems [1], [2]. In an ultra-high-frequency (UHF) RFID system, the reader CP antennas can be realized when two orthogonal modes emits signals through reader antennas. When an RFID tag com- of equal amplitude are excited with a 90 phase difference prising an antenna and an application-specific integrated circuit [6]. In general, the feeding structures of CP antennas can be (ASIC) is located in the reading zone of the reader antenna, categorized into single and hybrid feeds. A single feed of the tag is activated and interrogated for its content information a CP antenna has the advantages of simple structure, easy by the reader. The querying signal from the reader must have manufacture, and small size in arrays. However, the single-fed enough power to activate the tag ASIC to perform data pro- single-patch CP antenna in its simple form has inherently cessing, and transmit back a modulated string over a required narrow AR and impedance bandwidths of 1%–2% [7]. To reading distance. Since the RFID tags are always arbitrarily ori- improve the bandwidth, a variety of CP antennas have been ented in practical usage and the tag antennas are normally lin- studied, wherein the bandwidth of AR, impedance matching, early polarized, circularly polarized (CP) reader antennas have and gain have been enhanced, e.g., by modifying the radiator been used in UHF RFID systems for ensuring the reliability of shape, designing feeding structures, and optimizing antenna communications between readers and tags [3], [4]. or array configurations [8]–[17]. Usually, a CP antenna with Globally, each country has its own frequency alloca- the hybrid feed features a wide AR bandwidth, but suffers a complicated structure, expansive manufacture, and increased tion for UHF RFID applications, e.g., 840.5–844.5 and 920.5–924.5 MHz in China, 866–869 MHz in Europe, antenna size. 902–928-MHz band in North and South of America, 866–869 Fig. 1 shows the configuration of the proposed antenna. The and 920–925 MHz in Singapore, and 952–955 MHz in Japan, antenna comprises four layers of conductor, which include two suspended radiating patches, a suspended microstrip feed line, and so on, so that the UHF RFID frequency ranges from 840.5 to 955 MHz (a fractional bandwidth of 12.75%) [5]. Therefore, and a finite-size ground plane. Air substrate is used in this con- figuration to achieve higher gain, broader bandwidth, and lower cost. The microstrip feed line of a width of 24 mm is suspended Manuscript received May 24, 2008; revised November 18, 2008. First pub- lished March 27, 2009; current version published May 06, 2009. above the ground plane (250 mm 250 mm) at a height of The authors are with the Institute for Infocomm Research, Singapore 138632 (5 mm). One end of the feed line is connected to an RF input, (e-mail: chenzn@i2r.a-star.edu.sg; qingxm@i2r.a-star.edu.sg; changleo@dso. while the other one is open circuited, which simplifies the an- org.sg). tenna structure. The main radiating patch of 156 mm 156 mm Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. and with a truncation of 24.5 mm at two diagonal corners Digital Object Identifier 10.1109/TMTT.2009.2017290 is placed above the feed line at spacing of mm. The 0018-9480/$25.00 © 2009 IEEE Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • 1276 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 Fig. 1. Configuration of the proposed antenna. (a) Exploded view. (b) Side view. main patch is fed by four probes which are connected to the mi- crostrip line. The probes are of diameter of mm, and positioned along the microstrip feed line with adequate distance to create the 90 phase lag between the probes and brings into sequential rotation of current on the radiation patch for CP ra- diation. To enhance the bandwidth, a truncated parasitic patch with a dimension of 139 mm 139 mm and the truncation of 17 mm is placed right above the main patch with the spacing of mm. The truncated patches produce additional de- generating modes necessary for widening the AR bandwidth. With aid of simulation by Zeland IE3D, which is based on the method of moments (MoM), the antenna is optimized and then prototyped [18]. The prototype and detailed dimensions are shown in Fig. 2. The truncated patches, feed line, and ground plane are all made of copper and fixed using plastic spacers. Four metallic screws are used as the probes to connect the mi- crostrip feed line and the main patch. A coaxial cable is directly connected to the microstrip feed line to simplify the assembly of the antenna, where the coaxial cable is split into two wires (screen and core) and the wires are soldered to the suspended feed line and the ground plane separately. Fig. 2. Antenna prototype and detailed dimensions (h = 5 mm, III. RESULTS AND DISCUSSION h = 20 mm, h = 10 mm, and L = 250 mm). (a) Photograph of the antenna prototype. (b) Main patch. (c) Parasitic patch. (d) Microstrip feed The antenna was measured in an anechoic chamber using the line. Orbit MiDAS far-field measurement system and Agilent 8510C vector network analyzer. Fig. 3(a) shows the simulated and measured return loss of in Fig. 3(c). The antenna exhibits the measured gain of more the antenna. The measured return loss is less than 15 dB over than 8.3 dBic over the band of 815–970 MHz with a peak gain the frequency range of 760–963 MHz (25.6%). Fig. 3(b) ex- of 9.3 dBic at 900 MHz. The measured and simulated return hibits the simulated and measured AR at boresight. The mea- loss, AR, and gain show good agreement. sured 3-dB AR bandwidth of 818–964 MHz or 16.4% is ob- Figs. 4 and 5 show the measured radiation patterns at 840, tained. The simulated and measured boresight gain is illustrated 910, and 955 MHz in the – and – planes, respectively. Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1277 Fig. 3. Simulated and measured results of the proposed antenna. (a) Return loss. (b) AR. (c) Gain. In both planes, symmetrical patterns and wide-angle AR char- acteristics have been observed. The beamwidth of 3-dB AR is more than 75 , which is desirable for wide-coverage RFID applications. The wider 3-dB AR beamwidth is accredited to the sequential feed arrangement. The advantage stems from the symmetry of the feeding structure, which cancelled out the un- wanted cross polarization radiation. The 3-dB AR beamwidth of the antenna prototype at selected frequencies are tabulated in Table I. In addition, the front-to-back ratio of the antenna is better than 15 dB in both the – and – planes at all measured frequen- cies, although a finite-size ground plane is used. Fig. 4. Measured radiation patterns in the x–z plane at: (a) 840, (b) 910, and (c) 955 MHz. IV. PARAMETRIC STUDIES Parametric studies are conducted to provide more detailed in- formation about the antenna design and optimization. The para- the patches, the height of the parasitic patch, the size of feeding metric study is carried out by simulation because good agree- probes, the extension of the open-circuited microstrip line end, ment between the simulation and measurement has been ob- and the size of the ground plane. Since the effects of some pa- served. The parameters under study include the truncation of rameters, such as the size and height of the main patch and the Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • 1278 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 TABLE I 3-dB AXIAL RATIO BEAMWIDTH OF THE PROPOSED ANTENNA Fig. 6. Effect of the truncation of the main patch 1L on the antenna perfor- mance. (a) Return loss. (b) AR. A. Truncation of the Main Patch Fig. 6 shows the effect of on the return loss and AR of the antenna. It is found that the truncation of the main patch shows a significant effect on the AR of the antenna. The nontruncated patch mm exhibits the widest impedance bandwidth, but the narrowest AR bandwidth. The increasing of improves the AR bandwidth and achieves better impedance matching. However, over truncating (such as mm) of the patch will degrade all the bandwidths. The gain of the antenna is hardly affected by so that the results are not exhibited. In practical design, the truncation can Fig. 5. Measured radiation patterns in the y –z plane at: (a) 840, (b) 910, and (c) 955 MHz. be optimized for specific design requirement. B. Truncation of the Parasitic Patch size of the parasitic patch, have been well known, the study of Similar to , has a greater effect on the AR and these parameters is excluded in this paper. To better understand impedance bandwidths, while the gain of the antenna is hardly the influence of the parameters on the performance of the an- affected. As illustrated in Fig. 7, when the parasitic patch be- tenna, only one parameter at a time will be varied, while others comes a square mm , the antenna features dramatic are kept unchanged unless especially indicated. AR bandwidth reduction. The impedance and AR bandwidths Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1279 Fig. 7. Effect of the truncation of the parasitic patch 1L on the qantenna performance. (a) Return loss. (b) AR. change modestly if are kept within 10–20 mm and decline when the patch is over truncated. C. Height of the Parasitic Patch Fig. 8 exhibits the effect of varying height of the para- sitic patch on the performance of the antenna. It is observed that the operating band is shifted down as the height increases. Fur- thermore, the effect is more severe at higher frequencies. When the parasitic patch is placed close to the main patch (such as mm), a slight effect on the performance of the antenna is observed. Increasing makes the antenna size larger, and thus, shifts down the operating band. D. Diameter of Feeding Probes The study shows that the diameter of the feeding probe Fig. 8. Effect of the height of the parasitic patch h on the antenna perfor- has a slight effect on impedance matching, AR, and gain. How- mance. (a) Return loss. (b) AR. (c) Gain. ever, the very thin probe causes poor impedance matching and AR, as shown in Fig. 9. The long and thin feeding probes in- troduce a large inductance to degrade the impedance matching. effect on the AR has been observed. Optimal AR is achieved Furthermore, the large inductance also disturbs the phase char- when the last probe is positioned at the edge of the strip line. In- acteristic at the feeding point, and thus, degrades the AR per- creasing greatly degrades the AR. When reaches 25 mm, formance. The feeding probes with a diameter of 2–3 mm are the AR is larger than 3 dB over the entire frequency band. recommended in practical design. F. Size of the Ground Plane E. Extension of the Open-Circuited Strip The effect of the size of ground plane on the performance of The open-circuited feed line configuration simplifies the an- the antenna is exhibited in Fig. 11. As expected, the antenna tenna implementation and reduces the fabrication cost. How- with the larger ground plane has superior performance over the ever, the open-circuited termination will cause reflection on the smaller ones. When the ground plane is smaller than 200 mm feed line, and thus, affect the magnitudes and phase difference 200 mm, the performance of the antenna degrades in terms of the feeding currents at the four probes. The effect of the exten- of impedance, gain, and AR, especially at the lower frequen- sion of the open-circuited strip is illustrated in Fig. 10. A severe cies. For instance, the AR bandwidth is reduced to less than Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • 1280 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 Fig. 9. Effect of the probe diameter d on the antenna performance. (a) Return loss. (b) AR. Fig. 11. Effect of the size of the ground plane L on the performance of the antenna. (a) Return loss. (b) AR. (c) Gain. The change of the ground plane size offers a simple way to im- prove the antenna performance, but at the price of increasing the overall antenna volume. Unfortunately, practical antenna de- signs are always subject to certain size constraints. V. RFID VALIDATION: READING-RANGE MEASUREMENT To validate the superior features of the proposed antenna Fig. 10. Effect of the extension of the open-circuited strip d on the antenna in RFID reader applications, the reading-range measurement performance. (a) Return loss. (b) AR. was carried out using the proposed antenna incorporated into a UHF RFID reader to detect a UHF RFID tag. The Omron 750 5%. Increasing the ground plane size properly, for example, series reader and an in-house developed UHF tag were used; up to 250 mm 250 mm, achieves better performance. Fur- the Omron 750 series reader can operate at different frequency ther increasing the ground plane size only enhances the gain. bands of 865.6–867.6, 902.75–927.75, and 952–954 MHz with Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1281 TABLE II [6] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. New READING RANGE OF THE ANTENNA (EIRP OF THE READER: 4 W) York: Wiley, 2005, pp. 859–864. [7] Y. T. Lo and W. F. Richards, “Perturbation approach to design of cir- cularly polarized microstrip antennas,” Electron. Lett., vol. 17, no. 6, pp. 383–385, May 1981. [8] S. Egashira and E. Nishiyama, “Stacked microstrip antenna with wide bandwidth and high gain,” IEEE Trans. Antennas Propag., vol. 44, no. 11, pp. 1533–1534, Nov. 1996. [9] K. L. Chung and A. S. Mohan, “A systematic design method to ob- tain broadband characteristics for singly-fed electromagnetically cou- pled patch antennas for circular polarization,” IEEE Trans. Antennas Propag., vol. 51, no. 12, pp. 3239–3248, Dec. 2003. [10] R. B. Waterhouse, “Stacked patches using high and low dielectric con- stant material combinations,” IEEE Trans. Antennas Propag., vol. 47, 4-W effective isotropic radiated power (EIRP). The reading no. 12, pp. 1767–1771, Dec. 1999. range indicates the maximum distance of the tag from the reader [11] H. Kim, B. M. Lee, and Y. J. Yoon, “A single-feeding circularly polar- antenna, where the tag can be detected properly by the reader. ized microstrip antenna with the effect of hybrid feeding,” IEEE Trans. Antennas Propag. Lett., vol. 2, no. 4, pp. 74–77, Apr. 2003. The measurement was conducted in a full anechoic chamber at [12] K. L. Ong and T. W. Chiou, “Broad-band single-patch circularly po- boresight and 30 offset from the boresight of the antenna for larized microstrip antenna with dual capacitively coupled feeds,” IEEE all the frequency bands. The results are tabulated in Table II, Trans. Antennas Propag., vol. 49, no. 1, pp. 41–44, Jan. 2001. [13] F. S. Chang, K. L. Wong, and T. Z. Chiou, “Low-cost broadband cir- the maximum reading range of 7.1–7.5 m has been achieved at cularly polarized patch antenna,” IEEE Trans. Antennas Propag., vol. boresight and 6.1–6.5 m is achieved at the directions of 30 51, no. 10, pp. 3006–3009, Oct. 2003. offset from the boresight. The reading range is comparable with [14] K. L. Lau and K. M. Luk, “A novel wide-band circularly polarized patch antenna based on L-probe and aperture-coupling techniques,” that achieved by reader with single band antennas. IEEE Trans. Antennas Propag., vol. 53, no. 1, pp. 577–580, Jan. 2005. [15] R. L. Li, G. DeJean, J. Laskar, and M. M. Tentzeris, “Investigation of circularly polarized loop antennas with a parasitic element for band- VI. CONCLUSIONS width enhancement,” IEEE Trans. Antennas Propag., vol. 53, no. 12, pp. 3930–3939, Dec. 2005. In this paper, a broadband sequentially fed CP stacked patch [16] R. L. Li, D. C. Thompson, J. Papapolymerou, J. Laskar, and M. M. antenna has been presented for universal UHF RFID appli- Tentzeris, “A circularly polarized short backfire antenna excited by an cations. By using a simple feeding structure and combining unbalance-fed cross aperture,” IEEE Trans. Antennas Propag., vol. 54, no. 3, pp. 852–859, Mar. 2006. several band broadening techniques, the optimized antenna [17] W. K. Lo, J. L. Hu, C. H. Chan, and K. M. Luk, “Bandwidth enhance- has achieved the desired performance over the UHF band of ment of circularly polarized microstrip patch antenna using multiple 818–964 MHz or 16.4% with the gain of more than 8.3 dBic, L-shaped probe feeds,” Microw. Opt. Technol. Lett., vol. 42, no. 4, pp. 263–265, Aug. 2004. AR of less than 3 dB, return loss of less than 15 dB, and [18] IE3D User’s Manual Release 12. Fremont, CA: Zeland Softw. Inc., 3-dB AR beamwidth of larger than 75 . Therefore, this uni- Oct. 2006. versal design can be applied to all the UHF RFID applications worldwide. The reading-range measurement has validated that Zhi Ning Chen (M’99–SM’05–F’08) received the the proposed antenna can be incorporated into the multiband B.Eng., M.Eng., and Ph.D. degrees in electrical RFID readers or/and readers operating at different RFID bands engineering from the Institute of Communications to achieve desired reading ranges. This feature will benefit Engineering (ICE), Nanjing, China, and the DoE degree from the University of Tsukuba, Tsukuba, RFID system configuration and implementation, as well as cost Japan. reduction. In 1988, he joined ICE as a Teaching an Assis- Furthermore, the parametric studies have addressed the ef- tant, a Lecturer, and then an Associate Professor. He subsequently joined Southeast University, Nanjing, fects of the truncations of the patches, height of the parasitic China, as a Postdoctoral Fellow and then an Asso- patch, size of the feeding probes, extension of the open-circuited ciate Professor. In 1995, he continued his research feed line, and size of the ground plane on the performance of the with the City University of Hong Kong, China. From 1997 to 1999, he was with the University of Tsukuba, Tsukuba, Japan, as a Research Fellow awarded antenna. The information derived from the study will be helpful by Japan Society for the Promotion of Science (JSPS). In 2001 and 2004, he for antenna engineers to design and optimize the antennas for visited the University of Tsukuba, again under Invitation Fellowship Program UHF RFID applications. (senior level) of the JSPS. In 2004, he conducted his research with the Thomas J. Watson Research Center, International Business Machines Corporation (IBM), Yorktown Heights, NY, as an Academic Visitor (Antenna Designer). In 1999, REFERENCES he joined the Institute for Infocomm Research (I R) [formerly known as the Centre for Wireless Communications (CWC) and Institute for Communications [1] R. Want, “An introduction to RFID technology,” IEEE Pervasive Research (ICR)] as a Member of Technical Staff (MTS), and then the Principal Comput., vol. 5, no. 1, pp. 25–33, Jan.–Mar. 2006. MTS. He is currently Principal Scientist and Department Head for RF and Op- [2] K. Finkenzeller, RFID Handbook, 2nd ed. New York: Wiley, 2004. tical. He is concurrently an Adjunct Associate Professor with the National Uni- [3] H. L. Chung, X. Qing, and Z. N. Chen, “A broadband circularly polar- versity of Singapore (NUS) and Nanyang Technologies University (NTU), Sin- ized stacked probe-fed patch antenna for UHF RFID applications,” Int. gapore, and an Adjunct/Guest Professor with Zhejiang University, Nanjing Uni- J. Antennas Propag., vol. 2007, 2007, Art. ID 76793, 8 pp. versity, Shanghai Jiao Tong University, and Southeast University. Since 1990, [4] H. W. Kwa, X. Qing, and Z. N. Chen, “Broadband single-fed he has authored or coauthored over 220 technical papers published in interna- single-patch circularly polarized antenna for UHF RFID applications,” tional journals and presented at international conferences. He holds two patents in IEEE AP-S Int. Antennas Propag. Symp., San Diego, CA, Jul. 5–11, with seven patent applications filed. He authored Broadband Planar Antennas 2008, pp. 1072–1075. (Wiley, 2006), coedited UWB Communications (Wiley, 2006), and edited An- [5] H. Barthel, “Regulatory status for u RFID in the UHF spectrum,” tennas for Portable Devices (Wiley, 2007). He is the Editor for the “Field of EPCGlobal, Brussels, Belgium, Sep. 2007. [Online]. Available: Microwaves, Antennas and Propagation” for International Journal on Wireless http://www.epcglobalinc.org/tech/freq_reg/RFID_at_UHF_Regula- and Optical Communications. He is an Associate Editor for Research Letters in tions_20070504.pdf Communications and Journal of Electromagnetic Waves and Applications. He Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.
    • 1282 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 also reviews papers for many prestigious journals and conferences. His main for Infocomm Research (formerly known as CWC and ICR), Singapore. He research interests include applied electromagnetics, antenna theory, and design. is currently a Research Scientist with the RF and Optical Department. He has In particular, his research and development focuses on small and broadband an- authored or coauthored over 60 papers in international journals and confer- tennas and arrays for wireless systems, such as multiinput multioutput (MIMO) ences. He has authored two book chapters. His current research interest includes systems and UWB systems, bio-implanted systems, and RF imaging systems. RFID reader/tag antennas, ultra-wideband (UWB) antennas, antenna measure- Dr. Chen founded the IEEE International Workshop on Antenna Technology ment technology, and antenna co-design. (IEEE iWAT) and as general chair, organized the first IEEE iWAT: Small An- Mr. Qing has been a member of the IEEE Antennas and Propagation Society tennas and Novel Metamaterials, 2005, Singapore. He chairs the iWAT Steering (IEEE AP-S) since 1990. He received seven Awards of Advancement of Sci- Committee. He has been invited to deliver keynote addresses and talks at several ence and Technology in China. He was also the recipient of the IES Prestigious international events and serves many international conferences as key organizers Engineering Achievement Award 2006, Singapore. He currently serves New Technology Directions of the IEEE Antenna and Prop- agation Society (IEEE AP-S) (2005–2010) as a member. Hang Leong Chung was born in Singapore, in 1979. He received the B.E. degree in electrical engineering from the University of Queensland, Brisbane, Xianming Qing (M’90) received the B.Eng. degree Qld., Australia, in 2005. from the University of Electronic Science and Tech- From 2005 to 2007, he was a Research Engineer with the Institute for Inf- nology of China (UESTC), Chengdu, China, in 1985. comm Research (I R), A*Star, Singapore. He is currently with DSO National From 1985 to 1996, he was with UESTC, where Laboratories, Singapore. he taught and performed research, became a Lecturer in 1990, and then an Associate Professor in 1995. In 1997, he joined the Physics Department, National University of Singapore (NUS), Singapore, as a Re- search Scientist, where he focused on development of high-temperature superconductor (HTS) microwave devices. Since 1998, he has been with the Institute Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.