A Universal UHF RFID Reader Antenna

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 1275

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

Abstract—A broadband circularly polarized patch antenna isproposed for universal ultra-high-frequency (UHF) RF identifica-tion (RFID) applications. The antenna is composed of two corner-truncated patches and a suspended microstrip line with open-cir-cuited termination. The main patch is fed by four probes whichare 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 ARbeamwidth of 75 over the UHF band of 818–964 MHz or 16.4%.Therefore, the proposed antenna is universal for UHF RFID appli-cations worldwide at the UHF band of 840–960 MHz. In addition,a parametric study is conducted to facilitate the design and opti-mization processes for engineers.

Index Terms—Axial ratio (AR), broadband antenna, circularlypolarized (CP), RF identification (RFID), sequential feed, ultrahigh frequency (UHF).

I. INTRODUCTION

R F IDENTIFICATION (RFID), which was developedaround World War II, is a technology that provides

wireless identification and tracking capability. In recent years,RFID technology has been rapidly developed and applied tomany service industries, distribution logistics, manufacturingcompanies, and goods flow systems [1], [2].

In an ultra-high-frequency (UHF) RFID system, the readeremits signals through reader antennas. When an RFID tag com-prising 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 informationby the reader. The querying signal from the reader must haveenough power to activate the tag ASIC to perform data pro-cessing, and transmit back a modulated string over a requiredreading distance. Since the RFID tags are always arbitrarily ori-ented in practical usage and the tag antennas are normally lin-early polarized, circularly polarized (CP) reader antennas havebeen used in UHF RFID systems for ensuring the reliability ofcommunications between readers and tags [3], [4].

Globally, each country has its own frequency alloca-tion for UHF RFID applications, e.g., 840.5–844.5 and920.5–924.5 MHz in China, 866–869 MHz in Europe,902–928-MHz band in North and South of America, 866–869and 920–925 MHz in Singapore, and 952–955 MHz in Japan,and so on, so that the UHF RFID frequency ranges from 840.5to 955 MHz (a fractional bandwidth of 12.75%) [5]. Therefore,

Manuscript received May 24, 2008; revised November 18, 2008. First pub-lished March 27, 2009; current version published May 06, 2009.

The authors are with the Institute for Infocomm Research, Singapore 138632(e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMTT.2009.2017290

a universal reader antenna with desired performance across theentire UHF RFID band would be beneficial for RFID systemconfiguration and implementation, as well as cost reduction.

In this paper, we propose a sequentially fed stacked CP patchantenna for UHF RFID applications. The antenna comprises twosuspended truncated patches and a suspended microstrip line.The main patch is sequentially fed by four probes which are con-nected to the microstrip line. A parasitic patch is positioned rightabove the main patch for enhancing the bandwidth. The cornersof the patches are truncated to enhance the axial ratio (AR) per-formance. The proposed antenna is designed to cover the UHFRFID band of 840–960 MHz with acceptable performance interms of gain, AR, and impedance matching. Meanwhile, theantenna configuration is simple and easy for fabrication.

The remainder of this paper is organized as follows. Section IIdescribes the geometry of the proposed antenna. The measuredresults, analysis, and discussion are presented in Section III.Section IV demonstrates the results of parametric study. Thevalidation of the proposed antenna in RFID system applicationsis exhibited in Section V. Finally, a conclusion is drawn in Sec-tion VI.

II. ANTENNA CONFIGURATION

CP antennas can be realized when two orthogonal modesof equal amplitude are excited with a 90 phase difference[6]. In general, the feeding structures of CP antennas can becategorized into single and hybrid feeds. A single feed ofa CP antenna has the advantages of simple structure, easymanufacture, and small size in arrays. However, the single-fedsingle-patch CP antenna in its simple form has inherentlynarrow AR and impedance bandwidths of 1%–2% [7]. Toimprove the bandwidth, a variety of CP antennas have beenstudied, wherein the bandwidth of AR, impedance matching,and gain have been enhanced, e.g., by modifying the radiatorshape, designing feeding structures, and optimizing antennaor array configurations [8]–[17]. Usually, a CP antenna withthe hybrid feed features a wide AR bandwidth, but suffers acomplicated structure, expansive manufacture, and increasedantenna size.

Fig. 1 shows the configuration of the proposed antenna. Theantenna comprises four layers of conductor, which include twosuspended radiating patches, a suspended microstrip feed line,and a finite-size ground plane. Air substrate is used in this con-figuration to achieve higher gain, broader bandwidth, and lowercost. The microstrip feed line of a width of 24 mm is suspendedabove 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 an-tenna structure. The main radiating patch of 156 mm 156 mmand with a truncation of 24.5 mm at two diagonal cornersis placed above the feed line at spacing of mm. The

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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) Sideview.

main patch is fed by four probes which are connected to the mi-crostrip line. The probes are of diameter of mm, andpositioned along the microstrip feed line with adequate distanceto create the 90 phase lag between the probes and brings intosequential rotation of current on the radiation patch for CP ra-diation. To enhance the bandwidth, a truncated parasitic patchwith a dimension of 139 mm 139 mm and the truncationof 17 mm is placed right above the main patch with the spacingof 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 onthe method of moments (MoM), the antenna is optimized andthen prototyped [18]. The prototype and detailed dimensions areshown in Fig. 2. The truncated patches, feed line, and groundplane 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 directlyconnected to the microstrip feed line to simplify the assemblyof the antenna, where the coaxial cable is split into two wires(screen and core) and the wires are soldered to the suspendedfeed line and the ground plane separately.

III. RESULTS AND DISCUSSION

The antenna was measured in an anechoic chamber using theOrbit MiDAS far-field measurement system and Agilent 8510Cvector network analyzer.

Fig. 3(a) shows the simulated and measured return loss ofthe antenna. The measured return loss is less than 15 dB overthe frequency range of 760–963 MHz (25.6%). Fig. 3(b) ex-hibits the simulated and measured AR at boresight. The mea-sured 3-dB AR bandwidth of 818–964 MHz or 16.4% is ob-tained. The simulated and measured boresight gain is illustrated

Fig. 2. Antenna prototype and detailed dimensions (� � � mm,� � �� mm, � � �� mm, and � � �� mm). (a) Photograph ofthe antenna prototype. (b) Main patch. (c) Parasitic patch. (d) Microstrip feedline.

in Fig. 3(c). The antenna exhibits the measured gain of morethan 8.3 dBic over the band of 815–970 MHz with a peak gainof 9.3 dBic at 900 MHz. The measured and simulated returnloss, 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.


CHEN et al.: UNIVERSAL UHF RFID READER ANTENNA 1277

Fig. 3. Simulated and measured results of the proposed antenna. (a) Returnloss. (b) AR. (c) Gain.

In both planes, symmetrical patterns and wide-angle AR char-acteristics have been observed. The beamwidth of 3-dB ARis more than 75 , which is desirable for wide-coverage RFIDapplications. The wider 3-dB AR beamwidth is accredited tothe sequential feed arrangement. The advantage stems from thesymmetry of the feeding structure, which cancelled out the un-wanted cross polarization radiation. The 3-dB AR beamwidthof the antenna prototype at selected frequencies are tabulated inTable I.

In addition, the front-to-back ratio of the antenna is better than15 dB in both the – and – planes at all measured frequen-cies, although a finite-size ground plane is used.

IV. PARAMETRIC STUDIES

Parametric studies are conducted to provide more detailed in-formation about the antenna design and optimization. The para-metric study is carried out by simulation because good agree-ment between the simulation and measurement has been ob-served. The parameters under study include the truncation of

Fig. 4. Measured radiation patterns in the �–� plane at: (a) 840, (b) 910, and(c) 955 MHz.

the patches, the height of the parasitic patch, the size of feedingprobes, the extension of the open-circuited microstrip line end,and the size of the ground plane. Since the effects of some pa-rameters, such as the size and height of the main patch and the



Fig. 5. Measured radiation patterns in the �–� plane at: (a) 840, (b) 910, and(c) 955 MHz.

size of the parasitic patch, have been well known, the study ofthese parameters is excluded in this paper. To better understandthe influence of the parameters on the performance of the an-tenna, only one parameter at a time will be varied, while othersare kept unchanged unless especially indicated.

TABLE I3-dB AXIAL RATIO BEAMWIDTH OF THE PROPOSED ANTENNA

Fig. 6. Effect of the truncation of the main patch �� 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 ofthe 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 widestimpedance bandwidth, but the narrowest AR bandwidth. Theincreasing of improves the AR bandwidth and achievesbetter 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 theresults are not exhibited. In practical design, the truncation canbe optimized for specific design requirement.

B. Truncation of the Parasitic Patch

Similar to , has a greater effect on the AR andimpedance bandwidths, while the gain of the antenna is hardlyaffected. As illustrated in Fig. 7, when the parasitic patch be-comes a square mm , the antenna features dramaticAR bandwidth reduction. The impedance and AR bandwidths



Fig. 7. Effect of the truncation of the parasitic patch �� on the qantennaperformance. (a) Return loss. (b) AR.

change modestly if are kept within 10–20 mm and declinewhen 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 thatthe operating band is shifted down as the height increases. Fur-thermore, the effect is more severe at higher frequencies. Whenthe parasitic patch is placed close to the main patch (such as

mm), a slight effect on the performance of the antennais observed. Increasing makes the antenna size larger, andthus, shifts down the operating band.

D. Diameter of Feeding Probes

The study shows that the diameter of the feeding probehas a slight effect on impedance matching, AR, and gain. How-ever, the very thin probe causes poor impedance matching andAR, as shown in Fig. 9. The long and thin feeding probes in-troduce a large inductance to degrade the impedance matching.Furthermore, the large inductance also disturbs the phase char-acteristic at the feeding point, and thus, degrades the AR per-formance. The feeding probes with a diameter of 2–3 mm arerecommended in practical design.

E. Extension of the Open-Circuited Strip

The open-circuited feed line configuration simplifies the an-tenna implementation and reduces the fabrication cost. How-ever, the open-circuited termination will cause reflection on thefeed line, and thus, affect the magnitudes and phase differenceof the feeding currents at the four probes. The effect of the exten-sion of the open-circuited strip is illustrated in Fig. 10. A severe

Fig. 8. Effect of the height of the parasitic patch � on the antenna perfor-mance. (a) Return loss. (b) AR. (c) Gain.

effect on the AR has been observed. Optimal AR is achievedwhen the last probe is positioned at the edge of the strip line. In-creasing 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 ofthe antenna is exhibited in Fig. 11. As expected, the antennawith the larger ground plane has superior performance over thesmaller ones. When the ground plane is smaller than 200 mm

200 mm, the performance of the antenna degrades in termsof impedance, gain, and AR, especially at the lower frequen-cies. For instance, the AR bandwidth is reduced to less than



Fig. 9. Effect of the probe diameter � on the antenna performance. (a) Returnloss. (b) AR.

Fig. 10. Effect of the extension of the open-circuited strip � on the antennaperformance. (a) Return loss. (b) AR.

5%. Increasing the ground plane size properly, for example,up to 250 mm 250 mm, achieves better performance. Fur-ther increasing the ground plane size only enhances the gain.

Fig. 11. Effect of the size of the ground plane � on the performance of theantenna. (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 increasingthe 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 antennain RFID reader applications, the reading-range measurementwas carried out using the proposed antenna incorporated into aUHF RFID reader to detect a UHF RFID tag. The Omron 750series reader and an in-house developed UHF tag were used;the Omron 750 series reader can operate at different frequencybands of 865.6–867.6, 902.75–927.75, and 952–954 MHz with



TABLE IIREADING RANGE OF THE ANTENNA (EIRP OF THE READER: 4 W)

4-W effective isotropic radiated power (EIRP). The readingrange indicates the maximum distance of the tag from the readerantenna, where the tag can be detected properly by the reader.The measurement was conducted in a full anechoic chamber atboresight and 30 offset from the boresight of the antenna forall the frequency bands. The results are tabulated in Table II,the maximum reading range of 7.1–7.5 m has been achieved atboresight and 6.1–6.5 m is achieved at the directions of 30offset from the boresight. The reading range is comparable withthat achieved by reader with single band antennas.

VI. CONCLUSIONS

In this paper, a broadband sequentially fed CP stacked patchantenna has been presented for universal UHF RFID appli-cations. By using a simple feeding structure and combiningseveral band broadening techniques, the optimized antennahas achieved the desired performance over the UHF band of818–964 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, and3-dB AR beamwidth of larger than 75 . Therefore, this uni-versal design can be applied to all the UHF RFID applicationsworldwide. The reading-range measurement has validated thatthe proposed antenna can be incorporated into the multibandRFID readers or/and readers operating at different RFID bandsto achieve desired reading ranges. This feature will benefitRFID system configuration and implementation, as well as costreduction.

Furthermore, the parametric studies have addressed the ef-fects of the truncations of the patches, height of the parasiticpatch, size of the feeding probes, extension of the open-circuitedfeed line, and size of the ground plane on the performance of theantenna. The information derived from the study will be helpfulfor antenna engineers to design and optimize the antennas forUHF RFID applications.

REFERENCES

[1] R. Want, “An introduction to RFID technology,” IEEE PervasiveComput., vol. 5, no. 1, pp. 25–33, Jan.–Mar. 2006.

[2] K. Finkenzeller, RFID Handbook, 2nd ed. New York: Wiley, 2004.[3] H. L. Chung, X. Qing, and Z. N. Chen, “A broadband circularly polar-

ized 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-fedsingle-patch circularly polarized antenna for UHF RFID applications,”in IEEE AP-S Int. Antennas Propag. Symp., San Diego, CA, Jul. 5–11,2008, pp. 1072–1075.

[5] H. Barthel, “Regulatory status for u RFID in the UHF spectrum,”EPCGlobal, Brussels, Belgium, Sep. 2007. [Online]. Available:http://www.epcglobalinc.org/tech/freq_reg/RFID_at_UHF_Regula-tions_20070504.pdf

[6] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. NewYork: 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 widebandwidth and high gain,” IEEE Trans. Antennas Propag., vol. 44, no.11, pp. 1533–1534, Nov. 1996.

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[17] W. K. Lo, J. L. Hu, C. H. Chan, and K. M. Luk, “Bandwidth enhance-ment of circularly polarized microstrip patch antenna using multipleL-shaped probe feeds,” Microw. Opt. Technol. Lett., vol. 42, no. 4, pp.263–265, Aug. 2004.

[18] IE3D User’s Manual Release 12. Fremont, CA: Zeland Softw. Inc.,Oct. 2006.

Zhi Ning Chen (M’99–SM’05–F’08) received theB.Eng., M.Eng., and Ph.D. degrees in electricalengineering from the Institute of CommunicationsEngineering (ICE), Nanjing, China, and the DoEdegree from the University of Tsukuba, Tsukuba,Japan.

In 1988, he joined ICE as a Teaching an Assis-tant, a Lecturer, and then an Associate Professor. Hesubsequently joined Southeast University, Nanjing,China, as a Postdoctoral Fellow and then an Asso-ciate Professor. In 1995, he continued his research

with the City University of Hong Kong, China. From 1997 to 1999, he waswith the University of Tsukuba, Tsukuba, Japan, as a Research Fellow awardedby Japan Society for the Promotion of Science (JSPS). In 2001 and 2004, hevisited 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 theCentre for Wireless Communications (CWC) and Institute for CommunicationsResearch (ICR)] as a Member of Technical Staff (MTS), and then the PrincipalMTS. He is currently Principal Scientist and Department Head for RF and Op-tical. He is concurrently an Adjunct Associate Professor with the National Uni-versity of Singapore (NUS) and Nanyang Technologies University (NTU), Sin-gapore, and an Adjunct/Guest Professor with Zhejiang University, Nanjing Uni-versity, Shanghai Jiao Tong University, and Southeast University. Since 1990,he has authored or coauthored over 220 technical papers published in interna-tional journals and presented at international conferences. He holds two patentswith seven patent applications filed. He authored Broadband Planar Antennas(Wiley, 2006), coedited UWB Communications (Wiley, 2006), and edited An-tennas for Portable Devices (Wiley, 2007). He is the Editor for the “Field ofMicrowaves, Antennas and Propagation” for International Journal on Wirelessand Optical Communications. He is an Associate Editor for Research Letters inCommunications and Journal of Electromagnetic Waves and Applications. He



also reviews papers for many prestigious journals and conferences. His mainresearch interests include applied electromagnetics, antenna theory, and design.In particular, his research and development focuses on small and broadband an-tennas 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 An-tennas and Novel Metamaterials, 2005, Singapore. He chairs the iWAT SteeringCommittee. He has been invited to deliver keynote addresses and talks at severalinternational events and serves many international conferences as key organizersHe currently serves New Technology Directions of the IEEE Antenna and Prop-agation Society (IEEE AP-S) (2005–2010) as a member.

Xianming Qing (M’90) received the B.Eng. degreefrom the University of Electronic Science and Tech-nology of China (UESTC), Chengdu, China, in 1985.

From 1985 to 1996, he was with UESTC, wherehe taught and performed research, became a Lecturerin 1990, and then an Associate Professor in 1995.In 1997, he joined the Physics Department, NationalUniversity of Singapore (NUS), Singapore, as a Re-search Scientist, where he focused on development ofhigh-temperature superconductor (HTS) microwavedevices. Since 1998, he has been with the Institute

for Infocomm Research (formerly known as CWC and ICR), Singapore. Heis currently a Research Scientist with the RF and Optical Department. He hasauthored or coauthored over 60 papers in international journals and confer-ences. He has authored two book chapters. His current research interest includesRFID reader/tag antennas, ultra-wideband (UWB) antennas, antenna measure-ment technology, and antenna co-design.

Mr. Qing has been a member of the IEEE Antennas and Propagation Society(IEEE AP-S) since 1990. He received seven Awards of Advancement of Sci-ence and Technology in China. He was also the recipient of the IES PrestigiousEngineering Achievement Award 2006, Singapore.

Hang Leong Chung was born in Singapore, in 1979. He received the B.E.degree in electrical engineering from the University of Queensland, Brisbane,Qld., Australia, in 2005.

From 2005 to 2007, he was a Research Engineer with the Institute for Inf-comm Research �I R�, A*Star, Singapore. He is currently with DSO NationalLaboratories, Singapore.


A Universal UHF RFID Reader Antenna

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