A Universal UHF RFID Reader Antenna Zhi Ning Chen, Fellow, IEEE, Xianming Qing, Member, IEEE, and Hang Leong Chung

Transcription

1 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 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 proposed for universal ultra-high-frequency (UHF) RF identification (RFID) applications. The antenna is composed of two cornertruncated patches and a suspended microstrip line with open-circuited termination. The main patch is fed by four probes which are sequentially connected to the suspended microstrip feed line. The measurement shows that the antenna achieves a return loss of 15 db, gain of 8.3 dbic, axial ratio (AR) of 3 db, and 3-dB AR beamwidth of 75 over the UHF band of MHz or 16.4%. Therefore, the proposed antenna is universal for UHF RFID applications worldwide at the UHF band of MHz. In addition, a parametric study is conducted to facilitate the design and optimization processes for engineers. Index Terms Axial ratio (AR), broadband antenna, circularly polarized (CP), RF identification (RFID), sequential feed, ultra high frequency (UHF). I. INTRODUCTION R F IDENTIFICATION (RFID), which was developed around World War II, is a technology that provides wireless identification and tracking capability. In recent years, RFID technology has been rapidly developed and applied to many service industries, distribution logistics, manufacturing companies, and goods flow systems [1], [2]. In an ultra-high-frequency (UHF) RFID system, the reader emits signals through reader antennas. When an RFID tag comprising an antenna and an application-specific integrated circuit (ASIC) is located in the reading zone of the reader antenna, the tag is activated and interrogated for its content information by the reader. The querying signal from the reader must have enough power to activate the tag ASIC to perform data processing, and transmit back a modulated string over a required reading distance. Since the RFID tags are always arbitrarily oriented in practical usage and the tag antennas are normally linearly polarized, circularly polarized (CP) reader antennas have been used in UHF RFID systems for ensuring the reliability of communications between readers and tags [3], [4]. Globally, each country has its own frequency allocation for UHF RFID applications, e.g., and MHz in China, MHz in Europe, MHz band in North and South of America, and MHz in Singapore, and MHz in Japan, and so on, so that the UHF RFID frequency ranges from to 955 MHz (a fractional bandwidth of 12.75%) [5]. Therefore, Manuscript received May 24, 2008; revised November 18, First published March 27, 2009; current version published May 06, The authors are with the Institute for Infocomm Research, Singapore ( chenzn@i2r.a-star.edu.sg; qingxm@i2r.a-star.edu.sg; changleo@dso. org.sg). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMTT a universal reader antenna with desired performance across the entire UHF RFID band would be beneficial for RFID system configuration and implementation, as well as cost reduction. In this paper, we propose a sequentially fed stacked CP patch antenna for UHF RFID applications. The antenna comprises two suspended truncated patches and a suspended microstrip line. The main patch is sequentially fed by four probes which are connected to the microstrip line. A parasitic patch is positioned right above the main patch for enhancing the bandwidth. The corners of the patches are truncated to enhance the axial ratio (AR) performance. The proposed antenna is designed to cover the UHF RFID band of MHz with acceptable performance in terms of gain, AR, and impedance matching. Meanwhile, the 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 results, analysis, and discussion are presented in Section III. Section IV demonstrates the results of parametric study. The validation of the proposed antenna in RFID system applications is exhibited in Section V. Finally, a conclusion is drawn in Section VI. II. ANTENNA CONFIGURATION CP antennas can be realized when two orthogonal modes of equal amplitude are excited with a 90 phase difference [6]. In general, the feeding structures of CP antennas can be categorized into single and hybrid feeds. A single feed of a CP antenna has the advantages of simple structure, easy manufacture, and small size in arrays. However, the single-fed single-patch CP antenna in its simple form has inherently narrow AR and impedance bandwidths of 1% 2% [7]. To improve the bandwidth, a variety of CP antennas have been studied, wherein the bandwidth of AR, impedance matching, and gain have been enhanced, e.g., by modifying the radiator shape, designing feeding structures, and optimizing antenna or array configurations [8] [17]. Usually, a CP antenna with the hybrid feed features a wide AR bandwidth, but suffers a complicated structure, expansive manufacture, and increased antenna size. Fig. 1 shows the configuration of the proposed antenna. The antenna comprises four layers of conductor, which include two suspended radiating patches, a suspended microstrip feed line, and a finite-size ground plane. Air substrate is used in this configuration to achieve higher gain, broader bandwidth, and lower cost. The microstrip feed line of a width of 24 mm is suspended above the ground plane (250 mm 250 mm) at a height of (5 mm). One end of the feed line is connected to an RF input, while the other one is open circuited, which simplifies the antenna structure. The main radiating patch of 156 mm 156 mm and with a truncation of 24.5 mm at two diagonal corners is placed above the feed line at spacing of mm. The /$ IEEE

2 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 microstrip 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 radiation. 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 degenerating 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 microstrip 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. III. RESULTS AND DISCUSSION The antenna was measured in an anechoic chamber using the Orbit MiDAS far-field measurement system and Agilent 8510C vector network analyzer. Fig. 3(a) shows the simulated and measured return loss of the antenna. The measured return loss is less than 15 db over the frequency range of MHz (25.6%). Fig. 3(b) exhibits the simulated and measured AR at boresight. The measured 3-dB AR bandwidth of MHz or 16.4% is obtained. The simulated and measured boresight gain is illustrated Fig. 2. Antenna prototype and detailed dimensions (h = 5 mm, 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 line. in Fig. 3(c). The antenna exhibits the measured gain of more than 8.3 dbic over the band of MHz with a peak gain of 9.3 dbic at 900 MHz. The measured and simulated return loss, AR, and gain show good agreement. Figs. 4 and 5 show the measured radiation patterns at 840, 910, and 955 MHz in the and planes, respectively.

3 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 characteristics 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 unwanted 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 frequencies, although a finite-size ground plane is used. IV. PARAMETRIC STUDIES Parametric studies are conducted to provide more detailed information about the antenna design and optimization. The parametric study is carried out by simulation because good agreement between the simulation and measurement has been observed. The parameters under study include the truncation of Fig. 4. Measured radiation patterns in the x z plane at: (a) 840, (b) 910, and (c) 955 MHz. the patches, the height of the parasitic patch, the size of feeding probes, the extension of the open-circuited microstrip line end, and the size of the ground plane. Since the effects of some parameters, such as the size and height of the main patch and the

4 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 on the antenna perfor- Fig. 6. Effect of the truncation of the main patch 1L mance. (a) Return loss. (b) AR. Fig. 5. Measured radiation patterns in the y z plane at: (a) 840, (b) 910, and (c) 955 MHz. size of the parasitic patch, have been well known, the study of these parameters is excluded in this paper. To better understand the influence of the parameters on the performance of the antenna, only one parameter at a time will be varied, while others are kept unchanged unless especially indicated. 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 be optimized for specific design requirement. B. Truncation of the Parasitic Patch Similar to, has a greater effect on the AR and impedance bandwidths, while the gain of the antenna is hardly affected. As illustrated in Fig. 7, when the parasitic patch becomes a square mm, the antenna features dramatic AR bandwidth reduction. The impedance and AR bandwidths

5 CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1279 Fig. 7. Effect of the truncation of the parasitic patch 1L performance. (a) Return loss. (b) AR. on the qantenna change modestly if are kept within 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 parasitic patch on the performance of the antenna. It is observed that the operating band is shifted down as the height increases. Furthermore, 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 has a slight effect on impedance matching, AR, and gain. However, the very thin probe causes poor impedance matching and AR, as shown in Fig. 9. The long and thin feeding probes introduce a large inductance to degrade the impedance matching. Furthermore, the large inductance also disturbs the phase characteristic at the feeding point, and thus, degrades the AR performance. The feeding probes with a diameter of 2 3 mm are recommended in practical design. E. Extension of the Open-Circuited Strip The open-circuited feed line configuration simplifies the antenna implementation and reduces the fabrication cost. However, the open-circuited termination will cause reflection on the feed line, and thus, affect the magnitudes and phase difference of the feeding currents at the four probes. The effect of the extension of the open-circuited strip is illustrated in Fig. 10. A severe on the antenna perfor- Fig. 8. Effect of the height of the parasitic patch h mance. (a) Return loss. (b) AR. (c) Gain. effect on the AR has been observed. Optimal AR is achieved when the last probe is positioned at the edge of the strip line. Increasing greatly degrades the AR. When reaches 25 mm, the AR is larger than 3 db over the entire frequency band. F. Size of the Ground Plane The effect of the size of ground plane on the performance of the antenna is exhibited in Fig. 11. As expected, the antenna with the larger ground plane has superior performance over the smaller ones. When the ground plane is smaller than 200 mm 200 mm, the performance of the antenna degrades in terms of impedance, gain, and AR, especially at the lower frequencies. For instance, the AR bandwidth is reduced to less than

6 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 improve the antenna performance, but at the price of increasing the overall antenna volume. Unfortunately, practical antenna designs are always subject to certain size constraints. Fig. 10. Effect of the extension of the open-circuited strip d on the antenna performance. (a) Return loss. (b) AR. 5%. Increasing the ground plane size properly, for example, up to 250 mm 250 mm, achieves better performance. Further increasing the ground plane size only enhances the gain. V. RFID VALIDATION: READING-RANGE MEASUREMENT To validate the superior features of the proposed antenna in RFID reader applications, the reading-range measurement was carried out using the proposed antenna incorporated into a UHF RFID reader to detect a UHF RFID tag. The Omron 750 series reader and an in-house developed UHF tag were used; the Omron 750 series reader can operate at different frequency bands of , , and MHz with

7 CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1281 TABLE II READING RANGE OF THE ANTENNA (EIRP OF THE READER: 4 W) 4-W effective isotropic radiated power (EIRP). The reading range indicates the maximum distance of the tag from the reader antenna, where the tag can be detected properly by the reader. The measurement was conducted in a full anechoic chamber at boresight and 30 offset from the boresight of the antenna for all the frequency bands. The results are tabulated in Table II, the maximum reading range of m has been achieved at boresight and m is achieved at the directions of 30 offset from the boresight. The reading range is comparable with that achieved by reader with single band antennas. VI. CONCLUSIONS In this paper, a broadband sequentially fed CP stacked patch antenna has been presented for universal UHF RFID applications. By using a simple feeding structure and combining several band broadening techniques, the optimized antenna has achieved the desired performance over the UHF band of MHz or 16.4% with the gain of more than 8.3 dbic, AR of less than 3 db, return loss of less than 15 db, and 3-dB AR beamwidth of larger than 75. Therefore, this universal design can be applied to all the UHF RFID applications worldwide. The reading-range measurement has validated that the proposed antenna can be incorporated into the multiband RFID readers or/and readers operating at different RFID bands to achieve desired reading ranges. This feature will benefit RFID system configuration and implementation, as well as cost reduction. Furthermore, the parametric studies have addressed the effects of the truncations of the patches, height of the parasitic patch, size of the feeding probes, extension of the open-circuited feed line, and size of the ground plane on the performance of the antenna. The information derived from the study will be helpful for antenna engineers to design and optimize the antennas for UHF RFID applications. REFERENCES [1] R. Want, An introduction to RFID technology, IEEE Pervasive Comput., vol. 5, no. 1, pp , Jan. Mar [2] K. Finkenzeller, RFID Handbook, 2nd ed. New York: Wiley, [3] H. L. Chung, X. Qing, and Z. N. Chen, A broadband circularly polarized stacked probe-fed patch antenna for UHF RFID applications, Int. J. Antennas Propag., vol. 2007, 2007, Art. ID 76793, 8 pp. [4] H. W. Kwa, X. Qing, and Z. N. Chen, Broadband single-fed single-patch circularly polarized antenna for UHF RFID applications, in IEEE AP-S Int. Antennas Propag. Symp., San Diego, CA, Jul. 5 11, 2008, pp [5] H. Barthel, Regulatory status for u RFID in the UHF spectrum, EPCGlobal, Brussels, Belgium, Sep [Online]. Available: [6] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. New York: Wiley, 2005, pp [7] Y. T. Lo and W. F. Richards, Perturbation approach to design of circularly polarized microstrip antennas, Electron. Lett., vol. 17, no. 6, pp , May [8] S. Egashira and E. Nishiyama, Stacked microstrip antenna with wide bandwidth and high gain, IEEE Trans. Antennas Propag., vol. 44, no. 11, pp , Nov [9] K. L. Chung and A. S. Mohan, A systematic design method to obtain broadband characteristics for singly-fed electromagnetically coupled patch antennas for circular polarization, IEEE Trans. Antennas Propag., vol. 51, no. 12, pp , Dec [10] R. B. Waterhouse, Stacked patches using high and low dielectric constant material combinations, IEEE Trans. Antennas Propag., vol. 47, no. 12, pp , Dec [11] H. Kim, B. M. Lee, and Y. J. Yoon, A single-feeding circularly polarized microstrip antenna with the effect of hybrid feeding, IEEE Trans. Antennas Propag. Lett., vol. 2, no. 4, pp , Apr [12] K. L. Ong and T. W. Chiou, Broad-band single-patch circularly polarized microstrip antenna with dual capacitively coupled feeds, IEEE Trans. Antennas Propag., vol. 49, no. 1, pp , Jan [13] F. S. Chang, K. L. Wong, and T. Z. Chiou, Low-cost broadband circularly polarized patch antenna, IEEE Trans. Antennas Propag., vol. 51, no. 10, pp , Oct [14] K. L. Lau and K. M. Luk, A novel wide-band circularly polarized patch antenna based on L-probe and aperture-coupling techniques, IEEE Trans. Antennas Propag., vol. 53, no. 1, pp , Jan [15] R. L. Li, G. DeJean, J. Laskar, and M. M. Tentzeris, Investigation of circularly polarized loop antennas with a parasitic element for bandwidth enhancement, IEEE Trans. Antennas Propag., vol. 53, no. 12, pp , Dec [16] R. L. Li, D. C. Thompson, J. Papapolymerou, J. Laskar, and M. M. Tentzeris, A circularly polarized short backfire antenna excited by an unbalance-fed cross aperture, IEEE Trans. Antennas Propag., vol. 54, no. 3, pp , Mar [17] W. K. Lo, J. L. Hu, C. H. Chan, and K. M. Luk, Bandwidth enhancement of circularly polarized microstrip patch antenna using multiple L-shaped probe feeds, Microw. Opt. Technol. Lett., vol. 42, no. 4, pp , Aug [18] IE3D User s Manual Release 12. Fremont, CA: Zeland Softw. Inc., Oct Zhi Ning Chen (M 99 SM 05 F 08) received the B.Eng., M.Eng., and Ph.D. degrees in electrical engineering from the Institute of Communications Engineering (ICE), Nanjing, China, and the DoE degree from the University of Tsukuba, Tsukuba, Japan. In 1988, he joined ICE as a Teaching an Assistant, a Lecturer, and then an Associate Professor. He subsequently joined Southeast University, Nanjing, China, as a Postdoctoral Fellow and then an Associate Professor. In 1995, he continued his research 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 by Japan Society for the Promotion of Science (JSPS). In 2001 and 2004, he visited the University of Tsukuba, again under Invitation Fellowship Program (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, he joined the Institute for Infocomm Research (I R) [formerly known as the Centre for Wireless Communications (CWC) and Institute for Communications Research (ICR)] as a Member of Technical Staff (MTS), and then the Principal MTS. He is currently Principal Scientist and Department Head for RF and Optical. He is concurrently an Adjunct Associate Professor with the National University of Singapore (NUS) and Nanyang Technologies University (NTU), Singapore, and an Adjunct/Guest Professor with Zhejiang University, Nanjing University, Shanghai Jiao Tong University, and Southeast University. Since 1990, he has authored or coauthored over 220 technical papers published in international journals and presented at international conferences. He holds two patents with seven patent applications filed. He authored Broadband Planar Antennas (Wiley, 2006), coedited UWB Communications (Wiley, 2006), and edited Antennas for Portable Devices (Wiley, 2007). He is the Editor for the Field of Microwaves, Antennas and Propagation for International Journal on Wireless and Optical Communications. He is an Associate Editor for Research Letters in Communications and Journal of Electromagnetic Waves and Applications. He

8 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 research interests include applied electromagnetics, antenna theory, and design. In particular, his research and development focuses on small and broadband antennas and arrays for wireless systems, such as multiinput multioutput (MIMO) systems and UWB systems, bio-implanted systems, and RF imaging systems. Dr. Chen founded the IEEE International Workshop on Antenna Technology (IEEE iwat) and as general chair, organized the first IEEE iwat: Small Antennas and Novel Metamaterials, 2005, Singapore. He chairs the iwat Steering Committee. He has been invited to deliver keynote addresses and talks at several international events and serves many international conferences as key organizers He currently serves New Technology Directions of the IEEE Antenna and Propagation Society (IEEE AP-S) ( ) as a member. Xianming Qing (M 90) received the B.Eng. degree from the University of Electronic Science and Technology of China (UESTC), Chengdu, China, in From 1985 to 1996, he was with UESTC, where he taught and performed research, became a Lecturer in 1990, and then an Associate Professor in In 1997, he joined the Physics Department, National University of Singapore (NUS), Singapore, as a Research Scientist, where he focused on development of high-temperature superconductor (HTS) microwave devices. Since 1998, he has been with the Institute for Infocomm Research (formerly known as CWC and ICR), Singapore. He is currently a Research Scientist with the RF and Optical Department. He has authored or coauthored over 60 papers in international journals and conferences. He has authored two book chapters. His current research interest includes RFID reader/tag antennas, ultra-wideband (UWB) antennas, antenna measurement technology, and antenna co-design. Mr. Qing has been a member of the IEEE Antennas and Propagation Society (IEEE AP-S) since He received seven Awards of Advancement of Science and Technology in China. He was also the recipient of the IES Prestigious Engineering Achievement Award 2006, Singapore. Hang Leong Chung was born in Singapore, in He received the B.E. degree in electrical engineering from the University of Queensland, Brisbane, Qld., Australia, in From 2005 to 2007, he was a Research Engineer with the Institute for Infcomm Research (I R), A*Star, Singapore. He is currently with DSO National Laboratories, Singapore.