414 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

Operability of Folded Microstrip Patch-TypeTag Antenna in the UHF RFID Bands

Within 865–928 MHzLeena Ukkonen, Member, IEEE, Marijke Schaffrath, Student Member, IEEE, Daniel W. Engels, Member, IEEE,

Lauri Sydänheimo, Member, IEEE, and Markku Kivikoski, Member, IEEE

Abstract—The emerging use of passive radio frequency identifi-cation (RFID) systems at ultra-high-frequency (UHF) spectrum re-quires application specific tag antenna designs for challenging ap-plications. Many objects containing conductive material need noveltag antenna designs for reliable identification. The operability offolded microstrip patch-type tag antenna for objects containingconductive material is analyzed and tested within the UHF RFIDbands used at the moment mainly in Europe and in North andSouth America. First the operability of the tag antenna design andthe effects of conductive material are modeled with simulationsbased on finite element method (FEM). The performance of thetag antenna design affixed to a package containing metallic foil isverified with read range measurements. The results show that theantenna design is operable in both of the UHF RFID bands within865–928 MHz.

Index Terms—Conductive materials, radio frequency identifica-tion (RFID), tag antenna design.

I. INTRODUCTION

PASSIVE ultra-high-frequency (UHF) spectrum radiofrequency identification (RFID) systems are increasingly

used in retail supply chain applications [1]. Many objectsin supply chains contain significant amounts of conductivematerials, which set challenges for identifying these objects.Basically, RFID systems consist of a reader, a tag, and a dataprocessing system. Communication in passive UHF RFID sys-tems is based on backscattering of modulated electromagneticwave: reader transmits energy and commands to tag which thenresponds by backscattering its identification data back to readerusing for example amplitude shift keying (ASK) modulation.Passive tags consist of a microchip and an antenna. There is nointernal source of energy in the microchips, and thereby all theenergy for tag to function originates from the electromagneticwave radiated by the reader [2].

Conductive materials are challenging to identify becausethey significantly affect the radiation properties of conven-

Manuscript received April 6, 2006; revised August 8, 2006. This work wasfunded by the National Technology Agency of Finland (TEKES), The GraduateSchool of Electronics, Telecommunications and Automation (GETA), NokiaFoundation, the Finnish Foundation for Technology Development (TES), UllaTuominen Foundation, and Philip Morris USA.

L. Ukkonen, M. Schaffrath, L. Sydänheimo, and M. Kivikoski are with theTampere University of Technology, Institute of Electronics, Rauma ResearchUnit, Rauma FI-26100, Finland (e-mail: leena.ukkonen@tut.fi).

D. W. Engels is with the Massachusetts Institute of Technology, Auto-ID Lab-oratories, Cambridge, MA 02139 USA (e-mail: dragon@csail.mit.edu).

Digital Object Identifier 10.1109/LAWP.2006.883085

tional, usually folded dipole-type tag antennas by shiftingthe resonance frequency, degrading the impedance matching,lowering the radiation efficiency, and changing the radiationpattern [3]. Thereby, new tag antenna designs for identificationof conductive materials must be created.

Also, UHF frequency bands used in RFID vary throughoutthe world. For example, in North and South America the centerfrequency is 915 MHz, whereas Europe, Middle East, and Rus-sian Federation mainly use 866 MHz. Asia and Australia usefrequencies within the band from 866 to 954 MHz [4]. Thereby,the whole bandwidth used in UHF RFID is from 865 to 960MHz. There are also differences for example in transmissionpower and bandwidth regulations in different parts of the world[2], [4]. Therefore, the operability of tags all over the world hasto be taken into account and developed.

This paper presents and analyzes the operation of a novelfolded microstrip patch-type tag antenna affixed to a packagecontaining foiled products within the RFID frequency bandshaving center frequencies at 915 and 866 MHz. The radiationparameters of the tag antenna design affixed to both conduc-tive and nonconductive objects are presented and analyzed. Theperformance of the antenna design affixed to the foil-containingpackage is tested with read range measurements using 915 and866 MHz reader units.

II. ANTENNA DESIGN

Many packages in retail stores are foiled or the products in-side the package are wrapped around with conductive foil. Usu-ally the foil is used to improve the appearance of the packages.These products are difficult to identify with conventional, usu-ally dipole-type tags due to performance degradations caused byconductive material. For identification of products containingmetallic foil a novel folded microstrip patch-type tag antennahas been developed [5]. In this study its performance in the UHFRFID bands within 865–928 MHz was tested affixed to a chal-lenging foil-containing package, cigarette carton.

Cigarette cartons are a product that is very challenging toidentify with passive UHF RFID systems. They are also a goodexample of a package containing metallic foil for wrapping.Cigarettes in the individual packs are completely wrappedaround with aluminum foil that is highly conductive. Onecigarette carton contains ten individual cigarette packs. Thestructure of a cigarette carton is presented in Fig. 1.

1536-1225/$20.00 © 2006 IEEE

UKKONEN et al.: OPERABILITY OF FOLDED MICROSTRIP PATCH-TYPE TAG ANTENNA 415

Fig. 1. The structure of a cigarette carton.

Fig. 2. Dipole-type tag antennas and printed IFA tag antenna attached to acigarette carton.

If a conventional, usually folded dipole-type tag antenna isattached to a cigarette carton its impedance matching and radi-ation efficiency degrade and its resonance frequency shifts dueto the effects of the aluminum foil, and the read range achievedwith these conventional tags is 0 m. Examples of a dipole-typetag antennas affixed to a cigarette carton are presented in Fig. 2in the upper carton.

Nowadays passive UHF RFID tag antenna designs formetallic objects exist and are being developed [6], [7]. How-ever, usually these tag antennas are fabricated on rigid printedcircuit board (PCB) substrates, and therefore they are notpractical in identification of retail packages, such as cigarettecartons. Fig. 2 presents a metal-mountable printed inverted-F(IFA) tag antenna fabricated on Teflon substrate affixed to acigarette carton. The read range for a cigarette carton when theprinted IFA tag antenna is used is 1.05 m. However, when retailpackages are identified, the tag antenna has to be integratedseamlessly into the existing package. Seamless integrationinto the package without increasing the package dimensions ispractical with folded microstrip patch-type tag antenna.

The dimensions of the flat folded microstrip patch antennabefore it is affixed to the package is presented in Fig. 3. Theexact dimensions of the antenna design are ,

, , and . Fig. 4presents the tag antenna in its final form integrated inside thecigarette carton so that the ground plane is folded around theend of the cigarette carton so that the radiating patch and theground plane are on the opposite sides of the carton.

The input impedance of the microchips used in this study was. The microchips were attached to a plastic strap

with conductive silver ink pads to ease the attaching of the mi-crochips to the antenna designs. The microchips were connectedto the radiating patch and the ground plane using a 1.5-mm-wide

Fig. 3. Flat folded microstrip patch-type tag antenna and its dimensions.

Fig. 4. Folded microstrip patch-type tag antenna mounted on the cigarettecarton.

microstrip feed. The memory of the microchip contains 96 bitelectronic product code (ePC).

III. SIMULATIONS

The simulations were carried out using an advanced com-puter tool based on finite element method (FEM). Two differentsimulation set ups were analyzed. In the first simulation theradiation parameters of the antenna design were modeled af-fixed to a cigarette carton where aluminum blocks (conductivity

) inside the carton represented the individualcigarette packs. In the second simulation the metallic blockswere substituted by dielectric blocks with dielectric constant

and loss tangent to represent the individualcigarette packs without the foils. The simulation model was con-structed with the carton dimensions presented in Fig. 4. The sizeof the dielectric boxes inside the carton was 85 55 23 mmand the size of the metallic boxes was 83 53 22 mm, whichrepresent the actual sizes of a cigarette pack and the metallicblock formed by the aluminum foil wrapping in the pack. Thisway the effects of conductive material to the antenna perfor-mance could be analyzed.

Table I presents the simulated return losses (S11) of the an-tenna design at the studied RFID center frequencies with themetallic and dielectric boxes inside the carton. It can be ob-served that impedance matching to the input impedance of themicrochips is sufficient to guarantee the energysupply for the microchip and to enable efficient backscatteringof the identification data in the microchip. Also, the bandwidthof the antenna design in both cases is wide enough to coverthe desired bandwidth. With the metallic boxes the S11

416 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

TABLE ISIMULATED RADIATION PARAMETERS OF THE ANTENNA DESIGN

bandwidth is 100 MHz from 850 to 950 MHz, and with the di-electric boxes the bandwidth is 500 MHz from 700 MHz to 1.20GHz.

The material between the radiating element and the groundplane also has an effect on the resonance center frequency of theantenna design. When the metallic objects are placed betweenthe radiating patch and the ground plane, the resonance centerfrequency of the antenna design is 890 MHz withreturn loss. With dielectric material the resonance frequency is920 MHz with return loss.

Since the separation of the radiating patch from the groundplane, 48 mm, is relatively large, the structure of the antenna de-sign is a broadband planar monopole antenna [8]. The relativelywide bandwidth especially when the dielectric boxes are placedbetween the patch and the ground plane is due to this broad-band structure of the antenna design and the dielectric loadingbetween the patch and the ground plane.

Gain, bandwidth, and other radiation properties of microstripantennas can be enhanced for example by using parasitic ele-ments placed between the radiating patch and the ground plane[9], [10]. In this case, the metallic blocks, i.e., the cigarettepacks, can bee seen as floating parasitic elements between thepatch and the ground plane. In this structure, the individualcigarette packs working as floating parasitic elements enhancethe gain of the antenna design. With dielectric loading the max-imum gain of the antenna design at 915 MHz is 4.72 dBi intothe line-of-sight direction with the reader antenna. As presentedin Fig. 5, the metallic blocks increase the maximum gain to 6.0dBi. However, in this case the gain into the line-of-sight direc-tion with the reader antenna is 3.90 dBi, which is lower thanwith dielectric loading.

IV. READ RANGE MEASUREMENTS

To verify the operation of the tag antennas in practice, readrange measurements were carried out. Tag antenna prototypesfor cigarette cartons were fabricated of aluminum on polyesterlaminate, and they were integrated on the inside surface of thecarton as presented in Fig. 4 to keep the outside cardboard sur-face free of the tag. This enables printing on the outside surfaceof the carton, and the tag is also shielded inside the box [5]. Theability to print on the outside surface of the box is especiallyimportant if the product, for example, cigarette packs, are soldin the box containing several individual items.

The 915 and 866 MHz reader units manufactured by AlienTechnology were used in the measurements. As mentioned inSection I, the admitted transmission power level and bandwidthregulations vary around the world. In Europe the new European

Fig. 5. Gain pattern of the folded microstrip patch antenna at 915 MHz affixedto the simulation model of a foil-containing cigarette carton in dBi (���H-plane(� = 0 ),�� E-plane (� = 90 )).

Telecommunications Standards Institute (ETSI) 302–208 reg-ulation allows 2 W effective radiated power (ERP) to be trans-mitted between 865.6 and 867.6 MHz, 500 mW ERP to be trans-mitted between 867.6 and 868 MHz and 100 mW to be trans-mitted between 865 and 865.5 MHz. This 3 MHz frequencyband is also divided into 15 channels of 200 kHz. In the UnitedStates the Federal Communications Commission (FCC) allows4 W effective isotropic radiated power (EIRP) to be transmittedbetween 902 and 928 MHz if frequency hopping across a min-imum of 50 channels is used. Also, the data rate allowed in Eu-rope is approximately 30% of the data rate in America [2], [4].

Though the bandwidth and data rate allowed in the 915 MHzband are larger than in the 866 MHz band, the difference in thetransmission power levels is not as remarkable as it used to bewhen the allowed transmission power in Europe was only 500mW ERP in a bandwidth of 200 kHz. ERP relates to a dipoleantenna whereas EIRP relates to a spherical emitter. Thereby,an ERP power figure expresses the transmission power at whicha dipole antenna must be supplied in order to generate a definedemitted power at a distance of . The gain of the dipole antennain relation of the isotropic emitter is , and thereby [2]

(1)

According to (1), the 2 W ERP transmission power in Europecorresponds to 3.28 W EIRP. Therefore, the ETSI power regu-lations are relatively close to the 4 W EIRP transmission powerallowed by FCC.

Read ranges of the tag antenna with both the 915 and 866MHz reader units were measured to verify the interoperabilitywithin the FCC and ETSI regulations. To verify the simulationresults, the measurements were carried out with and without thefoil wrapping inside the individual cigarette packs. Tables II andIII present the maximum reliable read ranges of the tag proto-types measured with linearly and circularly polarized reader an-tennas and 915 and 866 MHz reader units. These results are not

UKKONEN et al.: OPERABILITY OF FOLDED MICROSTRIP PATCH-TYPE TAG ANTENNA 417

TABLE IIREAD RANGES OF A CIGARETTE CARTON WITH FOILS IN THE PACKS

TABLE IIIREAD RANGES OF A CIGARETTE CARTON WITHOUT FOILS IN THE PACKS

absolute values since they may vary when different measure-ment setup is used. However, they are comparable with eachother. The maximum reliable read range was defined as theline-of-sight distance at which the reader continuously identi-fied the tag antenna for at least one minute. No drops to zeroin the read rate (the amount of reads per second) were allowed.It should be noted that the read rate started to drop occasion-ally to zero immediately after the maximum reliable read range.Thereby, drawing the line between the reliable and unreliableidentification was straightforward in this case.

The results show that a maximum of 1.15 m read range isachieved with 915 MHz reader unit and a linearly polarizedreader antenna when the conductive foils are in place insidethe cigarette packs. As expected from the simulated return lossand bandwidth, the read range achieved with 866 MHz reader isshorter, approximately one half of the read range achieved with915 MHz reader unit. It can be observed that if the metallic foilsare removed, the read ranges are significantly longer, at leastfourfold compared to the read ranges with the metallic foils inthe packs. In addition, differences in the read ranges with 915and 866 MHz readers are not significant. This was also expectedaccording to the simulation results with the dielectric boxes in-side the carton: the bandwidth is broad and impedance matchingis sufficient in both of the center frequencies. Somewhat betterimpedance matching, broad bandwidth, and larger gain into theforesight direction explain the increase in the read ranges whenthe foils are removed.

The results show also that if a circularly polarized reader an-tenna is used in the measurements the read ranges drop to ap-proximately one half of the read ranges achieved with a linearlypolarized reader antenna. This is due to increased polarization

losses [11]. However, with a circular reader antenna the tag an-tenna can be read at any orientation relative to the reader an-tenna. The measurement results prove that the antenna design isoperable at the RFID center frequencies of 866 and 915 MHz.This is due to the structure of the antenna design, which takesinto account both the dielectric and conductive materials insidethe identified package.

V. CONCLUSION AND FUTURE WORK

In this paper we analyze the operability of the folded mi-crostrip patch-type tag antenna within the 865–928 MHz RFIDband affixed to packages with and without metal. The antennadesign and the effects of the dielectric and conductive materialsinside the package are analyzed with simulations and read rangemeasurements. The results show that the antenna design is in-teroperable in both Europe and Americas despite the differencesin the frequency, transmission power, and communication regu-lations. This is due to the structure of the antenna design, whichtakes into account both the dielectric and conductive materialsinside the identified package. In the future, the folded microstrippatch-type tag antenna will be further modified to achieve re-liable operation worldwide within the UHF RFID bandwidthfrom 865 to 960 MHz.

REFERENCES

[1] K. Michael and L. McCathie, “The pros and cons of RFID in supplychain management,” in Proc. IEEE Int. Conf. Mobile Business (ICMB),2005, pp. 623–629.

[2] K. Finkenzeller, RFID Handbook, 2nd ed. New York: Wiley, 2003.[3] P. Raumonen et al., “Folded dipole antenna near metal plate,” in Proc.

IEEE Antennas and Propagation Society Int. Symp., vol. 1, Columbus,OH, 2003, pp. 848–851.

[4] EPCGlobal. (2006) Regulatory Status for Using RFID in theUHF Spectrum 20. [Online]. Available: http://www.epcglobal-canada.org/docs/RFIDatUHFRegulations20 050 720.pdf

[5] L. Ukkonen et al., “Folded microstrip patch antenna for rfid tagging ofobjects containing metallic foil,” in Proc. IEEE Int. Conf. Antennas andPropagation, vol. 1B, Washington, DC, USA, Jul. 2005, pp. 211–214.

[6] H. Kwon and B. Lee, “Compact slotted planar inverted-F rfid tagmountable on metallic objects,” IEE Electron. Lett., vol. 41, no. 24, pp.994–996, Nov. 2005.

[7] H.-W. Son et al., “Design of wideband rfid tag antenna for metallic sur-faces,” IEE Electron. Lett., vol. 42, no. 5, pp. 1308–1310, Mar. 2006.

[8] G. Kumar and K. P. Ray, Broadband Microstrip Antennas. Norwood,MA: Artech House, 2003, pp. 357–368.

[9] Z. Qi and B. Liang, “Design of microstrip antenna with broader band-width and beam,” in Proc. IEEE Int. Conf. Antennas and Propagation,vol. 3A, Washington, DC, USA, Jul. 2005, pp. 617–620.

[10] C. S. Lee et al., “Gain enhancement of a thick microstrip antenna by sup-pressing surface waves,” in Proc. IEEE Int. Conf. Antennas and Propa-gation, vol. 1, Jun. 1994, pp. 460–463.

[11] B. Y. Toh et al., “Understanding and measuring circular polarization,”IEEE Trans. Educ., vol. 46, no. 3, pp. 313–318, 2003.

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