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Design of Antennas for RFID Application

Development and Implementation of RFID Technologyto two classes: theantenna The tag antenna not only transmitcarrying the information stored in the tag, but also needs to catch thefromidentified object, the sis ergy for the tag operation Since the tag should be attached to theize of the tag must be small endand the antenna should be smallGenerally the impedance of the tag chip is not 50 ohm, and theantenna should realize the conjugate match with the tag chip directly, in order to supply thecost and easy to fabricate for mass productio applapplications, the tag antenna should benna receives information from the tag Generally the position or the orientation of theand the nttaching the tag to the identified objectunfixed Thus the reader antenna should be a circularly polarized antenna, in order to avoidhen the orientation of the identified object is changed Meanwhile, theder antenna should have low profile and realize miniaturization, some of which shethan one band

Inspecialultiple antenna technology or smartwillpassive RFID system, the energy for maintaining the tag operation comes from thetromagnetic wave transmitted by the reader antenna Here the passive system is mainly(Keskilammi, Sydanheimo Kivikoski, 200To double the reading range, the transmitted power, the antenna gain, or the sensitivity ofthe receiver should increase at least 12dB First, the impact of the antenna gain on thteystem performance is described When the transmitted power is fixed, the maximumreading range of the RFID system is mainly limited by the antenna gain and the operatingRF link analysis, the electrtes back to theder, carrying the information stored in the tag Suppose that the rF energy caught by thetag can be re-radiated into the space totally Let the power transmitted by the readerbe pd, and the gain of the reader antenna be gThe power density at distance Rwhere the tag is placed can be expressed asS=4tnwsaninteiThe powered by the tag is calculatedS,AwhereA

Design of Antennas for RFID ApplicationRWhen the chip impedance is capacitive, ie Q<0, it follows from(13)that r>0While the chip impedance is inductive, ie Q>0, dr<0

When Q=0while X4R R(R+R)Qc should be as smalfrom the power transmission point of view,when the tag antenna is connected to the tag chirFor the tag antenna, thence chart can be used to guide the degn or to describe thetag antenna The chart is theoretically important and very useful for other applicationsum power transmission can be realized onlyif the antenna inonjugate value of the chip impedance while the

Development and Implementation of RFID Technologyhm or 75ohm the structure of the tag antenna should beThe antenna has the ability to realize several impeder UhF band application thecases with different structure pa912MHz, whose real part is approximately 22ohm, 50ohm, 75ohm, 100ohm respectively Thesimulated results for these four cases are shown in Fig 7

Fig 6 The symmetrical inverted-F Antenna…………x32mmnce results of the antenna in different cases

Design of Antennas for RF ID Application7 shows that the symmetrical inverted-F metallic strip can realize several impedancefamiliar types of tag antennas are theodifications or transformations of this structure( Dobkin &e Weigand, 2005)ement of several tag antennas Antenna B has less inperformance than antenna A, when the antenna is curved (Tikhov Won, 2004) AntC and d are fed by an inductively coupled loop(Son Pyo, 2005)8 Evolvement of the tag antennasy axis243Fig 9

Geometry of a meandered dipole antenna surrounded by the rectangular looUHF band43-j800 ohm impedance is used, and a tagantenna connectedhis chip shthe tag chip Meanwhile the tag antennauld be small ind In Fig 9, a meandereddesigned, and a pair of symmetricalmetallic strips surrounded by a rectangular

Development and Implementation of RFID Technologyloop is fed The higher real part of the impedance can be realized by the meandered dipole,while its hiary part can be supplied by the coupling between rectangular loop andymmetrical meandered dipole In thisa tag antenna with higher absolute valueower transmission The gap of the feeding point is 0Imm, the width of the metallicwandered strip and the hetal part of the rectangular loop is Imm, and the width of itsvertical part is 2mm

The tag antenna has a thickness of 0018mmincluding its impedance and radiation patterns, is calculThe simulated results ain table 1 and Fig 10 These results show that the antenna with smallused as a tag antenna for the UHF band rFiD chip applicationFreq(MHz) impedanceefficientcoefficient T009900009924487+i246222222Table 1 The impedance and power reflection coefficient, power transmission coefficient forTag antenna (chip impedance: 43-j800

Design of Antennas for RFID ApplicationneOtoMAeDOnorFig 10 Radiation pattern of the meandered dipole antennaSince the RFiD technology is applied in wide fields, RFID systems frequently appear in themetallic environment, and the effect of the metallic objects should be considered indesigning the antenna(Penttila et al, 2006) RFID antennas in microwave band have a defectnulls under the impact of metallic enviroTo solve the problemblake thef the rfid antennaxtended part of thets performance Sorting problems should be discussedWhen the traditional dipole antenna is attached to an extrelarge metallic plarradiation will be damaged In general, the tag antenna with arequired In practical application, a tag antenna with low profile is frequently used, and itsis limited

In Fig 11, when a normal dipole antenna approaches closely themetallic surface, an inductive currentpposite dnduced by the current will eliminate the radiation of the dipole, resulting in that the tagnss of antennas, the microstrip antenna mbeing mounted on the metallic surfaces and identifying the metallic objects Fordinary tag chip, a baltther circuit is needed to feed thena Here, based on thepole antenna, two defor the metallic surfacesd oneodification to the Yagi antenna, and the other is a dipole Antenna backed by an EBGstructure A substrate with high dielectric coefficient is sandwiched between the dipthe metallic surface, its thickness will reverse the orientation of the inductive current, andhe radiation is strengthened An EBG structure can depress the primary inductive current,

Development and Implementation of RFID Technologythe radiation of the dipole will be available, and the metallic surface of the identified objectalso the ground of the EBG structureMetallic surfaceMetallic surfacecurrentducurrent8899Fig 11 Design scheme for theantennametallic surfacesa) Excitation current nearby the metallic surface;(b)Scheme based on the Yagi antenna(c)Scheme based on the EBG structureccording to the introduced schemes, three tag antennasp1),6m(chip 3)of the active dipole (Qing &e Yang, 2004a)is also given in Fig 13

In Fig 12, thetive dipole is attached on the substrate with the relative dielectric coefficient er 102 Theth of the metallic strilRFID Chipctive dipole(Metallic strip)2High dielectric coefficientMetallic goundFig 12 The tag antenna for chip 1 based on the Yagi antenna

Design of Antennas for RFID Application4 y axis110mm39mmx axsp137mmFig 13 Geometry of the active dipole(dimensions in mm)npedance matches the chip impedance 15-j20 ohm in UHF band Radiation patterns of thedesign the antenna for chip 2 with 67-j197 ohm impedance, the structure parametedjusted

The designed dipole is shown in Fig 15, and its simulated radiation patterns arepresented in Fig 16Cf the tag antenna for chip 1

Design of Antennas for RFID ApplicationThen we havePg=()G

G Pmemd4TRThe power density of the return wave from the tag at the position of the reader isThus the power received by the readerPS,Amader =s,gThatwhere Gender stands for the gain of the reader antenna, A,moder the equivalent aperturetenna, G, the gain of the tag antenna, and A the equivalent aperture of theDefine the equivalent transmittedPThenGreader(PEIRP)Denote by Psnsitvin the threshold power of the sensitivity Then the maximum readingttering section of the tag, including the antenna and the chip,ring power of the tagPs1)

elopment and Implementation of RFID TechnologyThe power density of the back scattering wave at the position of the reader(12)丌R212PPack=S, Amader=S,grad4By adjusting the tag chip impedance according to the stored data in tag, a will behanged, and then the return wave coming from the tag and received by the reader will bethat the amplitude modulation and demodulation can be realized In thisthe tag information can be read, and the object detected by the03 tering include: 915MHz, 245GHz, and 5 8GHz, the corresponding wavelengths aris directlyproportional to the wavelength In fact, for the same distance the space loss at higherfrequency is greater than that at lower frequency

> The space loss SL is defined as4TR4)Commonly, the size of the antenna is relevant to its working frequency For lowerfrequency, the antenna will be larger, and the size of the tag will increase When the antennaize is fixed, the higher gain will be achieved for higher frequency Ing miniaturization In order to appropriately choose theoperating frequency for the RFID system, we should consider simultaneously many factorson mismatch will make theantenna lose the ability to receive all the power of the wavereceiving antenna, and p, is the vector that is orthogonal to the polarization vector of theeceiving antenna The polarization factor PLF is defined asPLFcosp, or PLF(dB)=10lg PLFThen, the power received by the antenna is denoted byP=P PLF, or P(dB)=P(dB)+ PLF(dB)

Design of Antennas for RF ID Applicationstands for the power of the incoming wave, or the maximum power receiby the antenna when the polarizations are matched, p, the unit polarization vectreceiving antenna, and p the unit vector of the incominAssume that the incoming wave is circularly polarized Then the unit vectorexpressed asPLFand PLF(dB)=-3dBantennFig 1 Polarizations of the antenna and thesmatch between the antenna and the wave reducesthe system perforhus choosing a suitablpolarization is also an important step for designing the antenna2

2 Deveent of antennas in the rthe rFid technology inspired the development of various antennasfor the RFID systems Lots of antennas with high perRFID antenna to meet some particular specificatign of the RFIDfaces many challenges, such as the antenna structure, the antenna size, theoperating mode, the bandwidth, the radiation pattern, the polarization, mutual couplingbetween multiple antennas, and the antenna scattering In theRFID system, theenna is designed to be a circularly polarized antenna Patchiral antennaspecial cases, linearly polarized antennused In the tag, the eroded or printed antennas are commonly used, and the dipole is theypical tag antenna structure Some circularly polarized antennas for the tag may betheory for matching the antenna with the tag chip is discussed which guiche design of the tag antenna and the analysis of the tag configuration, Several tag antennasdesigned withimpedance transformation for matching the chipspecial impedance, especially for UHF band application In thetag antennas are also designed to integrate with the already existing specific circuits with 50

Development and Implementation of RFID TechnologySchemes for designing the circularly polarized reader antenna are also presented in sometwo ports for the dual circular polarization, the aperture-coupled patcha integrated with the microstrip branch line coupler is preferred Some modificationsd to achieve the wide band, or meet the practical requirements The system, inwhich multiple reader antennas are used, is also discussedIn the design of antennathe rFidother pchd Designing theerred in designing antenna mounted on metallic surfaces The electromagnetic scatteringof the tag antenna is also introduced and discussed, and relative calculations23 Antennaor RFID applicatEfficient numerical methods promote the antenna design Moderndesign becomescomputing based on relative theory anheory instruction or according to the calculated results The antenna design method basedon numerical methods hasgn antennas for variousFinite Element Method(FEM), andthese methods, which are of different characteristic and are widely used

Fig 2 shows somefamiliar methodsthe design tools These design toThe MoMbe usede quickly and accurately, especially for some large antenna structures, Sorptimization methods, such as the optimization tool used in Zeland IE3D, can be embto the analysis metto make the antenna achThe Femnd FDTD methods can be used directly to analyze the antenna performance However, theFEM method getsaccurate results than the fdtd method the fdtd method can beused to analyze some larger antenna structures, solve the wide band problems in tirnamic demo about the electromagnetic field distributioradiation Some tools such as HFSS, which are widely used to deign antenna for the rFIDtem, add the ability of automatically meshing to facilitate thend improve theharacteristics Some toolsd to analyze sores of antenna suitablybut lose the ability for solving other antennas or affording the large memory requirement Innd the design software promote the design of antennas in the RFID systerm a ntem pppa aa ro bdesigning antenna, the antenna concept based on themixed with the manipulating software skilfully, and the antenna prototype of the designpotant thanlative numerical methods will help the designer tots perfoTo succeed in designingly software under the guidance of antenna prinAlthough the function of the softwar

Design of Antennas for RFID ApplicationNumerical methodMoMFEMFDTDIE3D FEKOHFSSXFDTDSome simulatorEmpire FDTDSome simulatorin designerin ADsg 2 Numerical methods and software3 Power transmission between tag chip& antennaGenerally, the RFID system mainly consists of reader and tag

The tag design is the mostrealization, Performance of the tag usually decides the perfof theThe tag is composed of the tag antenna and the chiptransmission directlthe system configuration, the relative functionrealization and also the system performance Thus, it is necessary to analyze the connectionof the tag antenna to the rFid tag chip, and to discuss the impedance match proble31 Theory of impedancThe most important factor in thethe reading range, which is the和 m the tag Compared with the tag, the reader is alw etect the backscatterbetweenboading range is mainly limited by the performance of the tag Especially for the passive tag,g the tag and the power of signal retransmittedby the tag are from the rF energy, which is transmitted by thThe impedance match between the antenna and the chip haas a directwhetherthe tag circuit can operate well and the chip is able tomplement the backscattering communication, and limits the rethe power transfer between the antenna and the chip, the impedance of theorking frequeninto thee band, the impedance match problem becomes

Development and Implementation of RFID Technologyore serious Ordinarily, the impedance of the antenna prototype designed for the tag is 50design and development need large investment and Chip Ateremeary with frequencand have a difference when the driving power is changed Itcrucial to achieveen the antenna and thedesigning antenna to match the existing chip is more convenient and practical

Due to theuirements such as easy manufacture, low cost and small size, adding thdirectly by adjustingmatch a chip of arbitraryion in designing antenna for the RFID system(Nikitin et2005: Rao, Nikitin lam 2005lent circuit is shown in Fig 3 Denote by Z the antennapedance, andZ,=R+ jX,, by Z the chip impedance, and ZantennaDefine the complex power reflection coefficient s asZ+ZThen the power reflection coefficient is calculated byR,+r)+j(XRRX+Xr+ⅳy=Z

Design of Antennas for RF ID Applicationbe the antenna impedance normalized to the real part of the chip impedance, thenz o z +lpart and imaginary part on Smith Chart like the traditional normalized impedance Thedistance between thef each Z, and the centre point of Smith Chart expthemagnitude of the complex power reflection coefficient s, while the trace of impedancpoints, which have a constant distance to the centre point, forms the concentric circle, whichcalled as the equivalent power reflection circle The centre point of Smith Chart is theperfect impedance match point, while the most outer circle denotes the complete mismatchThe power transmission coefficienO, Nikitin Lam, 2005b) can also be defined as IP,T, where P, stands for the power from reader caught by tag antenna, Pthepower transmitted from the tag antennatag chip It follows from Fig 3 that0≤r≤lXRX2=-, then equation ofcircle with constant poweransmission coefficient is expressed as follows), the impedance chart with theR/Rnormalized imaginary part x,=X / R The circles with constant

75, 05, 025 are draw in Fig 4 ThetheX while the ycalled as the complete mismatch line Whendecrease, the radius of the circles with constant power transmission coefficient increaseWhile r→0theconstanttransmission coefficient approaches to itstangent, that is the y axis, on which the impedanint cannot achieve thewhen the chip and the antennaonant XO, then equ(24)bece

Development and Implementation of RFID Technology69282-qc28284-Qcr=0524944-c13333Qc1-3333-e8283

928=249d4-028284Q-44140cPerfect Match56500E9282-eFig 4 The impedance chart with the constant power transmispefficient==(1-)5[rr-(2-r)=4(1-r)Making the derivative for the both sides of equation(2Obviouslymeans perfect match, andO means complete mismatch,Thus either the perfect match or the complete mismatch is a steady point of r