band resonant type metamaterial transmission lines, IEEE MicrowaveWireless Compon Lett 17 (2007), 97–99.

20. M. Gil, J. Bonache, J. García-García, J. Martel, and F. Martín, Com-posite right/left-handed (CRLH) metamaterial transmission lines basedon complementary split-rings resonators and their applications to verywide band and compact filter design, IEEE Trans Microwave TheoryTech 55 (2007), 1296–1304.

21. M. Gil, J. Bonache, and F. Martín, Metamaterial filters with attenua-tion poles in the pass band for ultra wide band (UWB) applications,Microwave Opt Technol Lett 49 (2007), 2909–2913.

22. J. Baena, J. Bonache, F. Martín, R. Marques, F. Falcone, T. Lopetegi,M. Laso, J. García-García, I. Gil. and M. Flores-Portillo, Equivalent-circuit models for split-ring resonators and complementary split-ringresonators coupled to planar transmission lines, IEEE Trans Micro-wave Theory Tech 53 (2005), 1451–1461.

23. J. Bonache, M. Gil, O. García-Abad, and F. Martín, Parametric anal-ysis of microstrip lines loaded with complementary split ring resona-tors, Microwave Opt Technol Lett 50 (2008), 2093–2096.

24. J. Bonache, M. Gil, I. Gil, J. García-García, and F. Martín, On theelectrical characteristics of complementary metamaterial resonators,IEEE Microwave Wireless Compon Lett 16 (2006), 543–545.

© 2009 Wiley Periodicals, Inc.

METAMATERIAL APPLICATIONS INRFID

A. Toscano and L. VegniUniversita di Roma Tre, Dipartimento di Elettronica Applicata, Roma,Italia; Corresponding author: alessandro.toscano@gmail.com

Received 17 May 2009

ABSTRACT: In this article, we discuss various requirements of an-tenna design for passive radio frequency identification (RFID) tags, weoutline a generic design process and show how RFID tags loaded witha metamaterial slab are able to increase the read range and to decouplethe tag with the tagged objects. © 2009 Wiley Periodicals, Inc.Microwave Opt Technol Lett 51: 2745–2748, 2009; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24728

Key words: RFID; metamatrials

1. INTRODUCTION

The main aim of this article is to provide new insights andsolutions to problems related to small antennas for radio frequencyidentification (RFID) and wireless sensor applications by metama-terials. RFID is a rapidly developing technology which uses RFsignals for automatic identification of objects. In order for wirelesscommunication to take place, the transmitter and receiver must becoupled, or connected across the air interface. There are severalmodes of coupling, but the two most common modes in RFIDapplications are inductive and far field or radiative (see Fig. 1).Now, RFID finds many applications in various areas such aselectronic toll collection, asset identification, retail item manage-ment, access control, animal tracking, and vehicle security [1, 2].Globally, each country has its own frequency allocation for RFID.For example, RFID UHF bands are: 866–869 MHz in Europe,902–928 MHz in North and South America, and 950–956 MHz inJapan and some Asian countries (see Fig. 1). A typical passiveRFID transponder, often called tag, consists of an antenna and anApplication Specific Integrated Circuit (ASIC) chip. RFID tagscan be active (with batteries) or passive (batteryless).

In this article, we briefly review design requirements for pas-sive UHF RFID tag antennas, we outline the design process andpropose a performance protocol for tag analysis. We present aspecific application example: a passive UHF tag design for a RFIDtag placed on a metamaterial slab (see Fig. 2). In RFID, the desiredinput impedance of the antenna in the UHF band (see Fig. 1) isusually strongly inductive and the resistance is low due to thedirect matching to the capacitive input impedance of the ASIC. Inthis contribution, we show how a metamaterial slab [3] can be usedto increase the read range of the tag antenna without using aground plane and a quarter wavelength spacing between the radi-ating element and the ground plane which is not acceptable atUHF. We consider a Philips EPC 1.19 G2 RFID ASIC chipmounted on antenna terminals as shown in Figure 2. In Figure 3,we have reported the equivalent circuit representation of the wholeRFID system where Ra � jXa and Rc � jXc are the complex inputimpedances of the tag antenna and the ASIC, respectively.

2. RFID DESIGN REQUIREMENTS

The most important tag performance characteristic is read rangei.e., the maximum distance at which RFID reader can detect thebackscattered signal from the tag. The read range can be calculatedusing Friis free-space formula as:

r ��

4��PtGtGr�

Pth(1)

where � is the wavelength, Pt is the power transmitted by thereader, Gt is the gain of the transmitting antenna (reader antenna),Gr is the gain of the receiving antenna (tag antenna), Pth is thethreshold power, i.e., the minimum power to activate the chip inthe tag, and � is the power transmitting coefficient defined as (seeFig. 3):

� �4RcRa

�Zc � Za�2(2)

�max � 1 in presence of conjugate matching for Ra � Rc andXa � �Xc.

Figure 1 RFID frequency bands

Figure 2 Geometry of the loaded meander tag antenna

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 2745

The range in (1) can be normalized with a factor r0 : r� r0r�. r0 is the range of the tag with 0 dBi antenna perfectlymatched (� � 1 ) to the chip impedance at a fixed frequency:

r0 ��

4��PtGt

Pth(3)

r� � �Gr� (4)

In this article, we used simulated values for gains and powers.

3. METAMATERIALS AND ANTENNA DESIGN

In antenna design to improve the antenna gain by 3 dB, andconsequently the read range, a ground plane as a planar metal sheetis sometimes used. The presence of a ground plane is also used topartially shield the objects below the antenna. This is a veryimportant feature in RFID applications where tag design mustmeet range requirements in spite of variations in tagged objects.The straightforward solution to increase the radiation efficiency ofgrounded antennas is to include a quarter wavelength spacingbetween the radiating element and the ground plane. But, at UHF,this would require a minimum thickness of 10 cm, which is notacceptable. To circumvent the problem, we propose the use of anew class of metamaterials known as artificial magnetic materials

(AMM) made by a periodic arrangement of multiple split ringresonators (MSRRs) [3] (see Fig. 4) without a metallic groundplane.

In Ref. [3], we have shown that a few rings are enough toobtain a good reduction of the resonant frequency down to theEuropean RFID UHF bands, giving a typical miniaturization rateof the order of �/40–�/50 in the linear dimensions of the inclusion.

The RFID tag antenna geometry is shown in Figure 2. Ameandered dipole antenna was chosen due to its size and tunabil-ity. Meandering allowed the antenna to be compact and to provideomnidirectional performance in the plane perpendicular to the axisof the meander. To have a better control over the antenna resis-tance, one loading bar with the same width as the meander tracehas been added. The RFID tag antenna design process involvesinevitable tradeoffs between antenna gain, impedance, and band-width. The loaded meander antenna has several key parameters:loading bar width w, distance d, spacing s, meander step width a,and meander step height b, which influence antenna gain, imped-ance, tag resonance, peak range, and bandwidth.

Antenna power transmission characteristics can be controlledby varying the spacing s as it is illustrated in Figure 5. The optimalrealizable combinations of tag antenna parameters were identifiedby plotting and examining the families of curves showing thepower transmission coefficients for different values of the spacings. After optimization via CST Microwave Studio package, wearrived to the final parameter values for tag antenna as given inTable 1.

The dispersion characteristics of the optimized metamaterialslab are shown in Figure 6.

Tag range was computed from (1) taking into account thatrequirements to the tag design impose PtGt � 3.3 W (transmitterEIRP) in 866–869 MHz frequency band. The minimum power Pth

needed by the chip to turn on is �10 dBm. So, r0 is:

Figure 3 Equivalent circuit representation of the loaded meander tagantenna and RFID chip

Figure 4 Multiple split ring resonator being m1 the strip width, m2 thelength of the split, m3 the separation between adjacent strips, and m4 thelength of the outer ring. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

Figure 5 Power transmission coefficient � for different values of thespacing s. [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com]

TABLE 1 Parameters of the Loaded MeanderTag Antenna (mm)

Parameters � w s d a b

Values 96 0.7 0.4 9 5.6 14

2746 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 DOI 10.1002/mop

r0 ��

4��PtGt

Pth� 5 m (5)

The input impedance of the ASIC chip at f � 0.868 GHz is givenby [4]:

Zc � Rc � jXc � 18 � j366� (6)

At the end of the optimization process, the antenna reactance andresistance are found to be (Figs. 7 and 8):

Za � Ra � jXa � 14.48 � j354.2� (7)

which lead to:

● a very good conjugate matching with the input impedance of

the ASIC chip (Figs. 9 and 10), giving a greater than onevalue for r� :

r� � �Gr� � �2.94 � 0.91 � 1.63 (8)

● a power absorbed by the ASIC chip given by �8.61 dBm(see Fig. 11) which is well above the minimum power neededby the chip to turn on (�10 dBm ),

● an effective read range given by:

r � r� � r0 � 8.15 m (9)

In the case of presence of only the optimized tag antenna withoutthe metamaterial slab the chip is still on (see Fig. 12), we have

Figure 6 Dispersion characteristics of the metamaterial used as sub-strate, �r � 3.8, m1 � m2 � m3 � 0.1 mm, m4 � 8 mm. [Colorfigure can be viewed in the online issue, which is available at www.interscience.wiley.com]

Figure 7 Real part of the input impedance of the optimized tag antenna.[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com]

Figure 8 Imaginary part of the input impedance of the optimized tagantenna. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

Figure 9 Power transmission coefficient of the tag antenna loaded witha metamaterial slab. [Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 2747

r� � �Gr� � �1.47 � 0.95 � 1.18 and an effective read rangegiven by:

r � r� � r0 � 5.90 m

This means that the insertion of a proper designed metamaterialslab allows an increase of the effective read range parameter ofabout 40%.

4. CONCLUSIONS

In this article, new insights and solutions to problems related tosmall antennas for RFID and wireless sensor applications bymetamaterials have been provided. We have reviewed designrequirements for passive UHF RFID tag antennas; we outlined the

design process, and proposed a performance protocol for taganalysis. We have also shown that a metamaterial slab can be usedto increase of about 40% the read range of the tag antenna withoutusing a ground plane.

REFERENCES

1. K. Finkenzeller, RFID handbook: Radio-frequency identification fun-damentals and applications, 2nd ed., Wiley, New York, 2004.

2. A. Alu, C. Sapia, A. Toscano, and L. Vegni, Radio frequency animalidentification: Electromagnetic analysis and experimental evaluation ofthe transponder-gate system, Int J Radio Freq Identification TechnolAppl 10 (2006), 90–106.

3. F. Bilotti, A. Toscano, and L. Vegni, Design of Spiral and MultipleSplit-Ring Resonators for the Realization of Miniaturized MetamaterialSamples, IEEE Trans Antennas Propagat 55 (2007), 2258–2267.

4. www.semiconductors.philips.

© 2009 Wiley Periodicals, Inc.

NUMERICAL AND EXPERIMENTALINVESTIGATION OF BASICPROPERTIES OF WIRE MEDIUM-BASEDSHORTENED HORN ANTENNAS

Silvio Hrabar, Davor Bonefacic, and Damir MuhaDepartment of Wireless Communications, Faculty of ElectricalEngineering and Computing, University of Zagreb, Unska 3, Zagreb,HR-10000, Croatia; Corresponding author: silvio.hrabar@fer.hr

Received 10 June 2009

ABSTRACT: Recent numerical study predicted a phenomenon of gainincrease of a shortened horn antenna by embedding a wire medium-based slab. Here, we report an experimental verification of this idea bythe development of two shortened horn antennas that operate in 10 GHzband: the horn with embedded single-wire ENZ slab and the horn withembedded double-wire ENZ slab. These two horn antennas had lengthsof 52% and 33% of the length of the optimal horn, respectively. Mea-sured gain was found to be very similar to the gain of the full-lengthoptimal horn (within 0.1 dB), but in a narrow band (5–12%). In addi-tion, we present numerical results that show that a Drude model is a

Figure 10 Zoom of the power transmission coefficient in the frequencyband of interest. [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com]

Figure 11 Power received (dBm) by the passive UHF ASIC vs fre-quency (GHz) in presence of the metamaterial slab. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com]

Figure 12 Power received (dBm) by the passive UHF ASIC vs fre-quency (GHz) without the metamaterial slab. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com]

2748 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 DOI 10.1002/mop