Design of Antennas for RFID Application

Transcription

1 Design of Antenns for RFID Applition Ming-To Zhng 1, Yong-Chng Jio, Fu-Shun Zhng nd Wu-Tu Wng 1 1 Chin Ademy of Spe Tehnology (Xi n), Xi n, Shnxi, Ntionl Lbortory of Antenns nd Mirowve Tehnology, Xidin University, Xi n, Shnxi, P. R. Chin Open Aess Dtbse 1. Introdution As vitl nd integrted prt of the rdio-frequeny identifition (RFID) system, RFID ntenns hve been reeived muh ttention over yers, nd their design is very urgent nd signifint. In ft, the development of RFID ntenn is of theoretil signifine nd prtil vlue for the RFID system. In this hpter, the RFID tehnology is briefly introdued, nd the operting priniple of the RFID system is desribed. The ntenn in RFID system is disussed, nd the designing priniple of the ntenns for RFID pplitions is presented. Some ommonly used ntenns in the RFID system re lso displyed.. RFID tehnology nd ntenns As n utomti identifition tehnique without touhing, RFID tehnology uses rdio wves rrying informtion stored bout the identified objet or ommnds to identify objet vi spe oupling, suh s indutive oupling or eletromgneti wve propgtion. For the detils bout the RFID tehnology, refer to some web sites suh s org, hin/, nd As vitl devie for trnsmitting the RF power from the rdio trnseiver to the open spe in the form of eletromgneti wve, or reeiving it from spe nd trnsferring it to the next iruit, ntenn is lwys the key prt of the RF system, nd its performne gretly ffets the performne of the whole system. Thus design of ntenns for the RFID system is very importnt. In the RFID system, ording to their funtions in the system, the ntenns n be divided into two prts: tg ntenn nd reder ntenn. The present RFID systems re pplied t LF, HF (13.56MHz), UHF nd mirowve bnds, nd the ntenn design is foused on these frequeny bnds. In ft, the system working t LF nd HF bnds is bsed on the mgneti field oupling between the tg oil nd reder oil, whose operting priniple is identil with tht of the trnsformer. There is no rdition nd wve trnsmission, nd the ntenn in the system is just oil. The ntenn disussed here is limited to the system tht opertes t UHF bnd, or mirowve bnds. Bsed on the different operting priniples t different bnds, design of the ntenns in the system will be disussed t following setions. Soure: Development nd Implementtion of RFID Tehnology, Book edited by: Cristin TURCU, ISBN , pp. 554, Februry 9, I-Teh, Vienn, Austri

2 14 Development nd Implementtion of RFID Tehnology.1 Antenns in the RFID system Aording to the different funtions in the RFID system, the RFID ntenns n be divided into two lsses: the tg ntenn nd the reder ntenn. The tg ntenn not only trnsmits the wve rrying the informtion stored in the tg, but lso needs to th the wve from the reder to supply energy for the tg opertion. Sine the tg should be tthed to the identified objet, the size of the tg must be smll enough, nd the ntenn should be smll in size. In most ses, the tg ntenn should hve omnidiretionl rdition or hemispheril overge. Generlly the impedne of the tg hip is not 5 ohm, nd the ntenn should relize the onjugte mth with the tg hip diretly, in order to supply the mximum power to the tg hip. In ommon pplitions, the tg ntenn should be lowost nd esy to fbrite for mss prodution. The reder ntenn trnsmits the eletromgneti energy to tivte or wken the tg, relizes the dt trnsfer nd sends the instrutions to the tg. Menwhile, the reder ntenn reeives informtion from the tg. Generlly the position or the orienttion of the identified objet is rndom, nd the mnner for tthing the tg to the identified objet is unfixed. Thus the reder ntenn should be irulrly polrized ntenn, in order to void the polriztion loss when the orienttion of the identified objet is hnged. Menwhile, the reder ntenn should hve low profile nd relize minituriztion, some of whih should operte t more thn one bnd. In some speil ses, multiple ntenn tehnology or smrt ntenn rrys for bem snning will be employed. In pssive RFID system, the energy for mintining the tg opertion omes from the eletromgneti wve trnsmitted by the reder ntenn. Here the pssive system is minly disussed to show the impt of the ntenn prmeters on the system performne (Keskilmmi, Sydnheimo & Kivikoski, 3). To double the reding rnge, the trnsmitted power, the ntenn gin, or the sensitivity of the reeiver should inrese t lest 1dB. First, the impt of the ntenn gin on thte system performne is desribed. When the trnsmitted power is fixed, the mximum reding rnge of the RFID system is minly limited by the ntenn gin nd the operting frequeny. By the RF link nlysis, the eletromgneti wve trnsmitted by the reder ntenn rdites to the tg through the spe loss, nd then reversely propgtes bk to the reder, rrying the informtion stored in the tg. Suppose tht the RF energy ught by the tg n be re-rdited into the spe totlly. Let the power trnsmitted by the reder reder bep trnsmitted, nd the gin of the reder ntenn beg reder. The power density t distne R where the tg is pled n be expressed s S G P 4πR reder reder trnsmitted 1 The power reeived by the tg is lulted by = (1) P tg reeived = SA 1 tg, () where A tg G λ tg = (3) 4π

3 Design of Antenns for RFID Applition 15 Then, we hve λ ( ) 4πR =. (4) tg reder Preeived GrederGtgPtrnsmitted The power density of the return wve from the tg t the position of the reder is Thus the power reeived by the reder is Tht is S GP tg tg reeived = (5) 4πR reder λ Pbk = SA reder = SG (6) reder 4π reder λ Pbk ( ) G G P 4πR = (7) 4 reder reder tg trnsmitted where G reder stnds for the gin of the reder ntenn, Areder the equivlent perture of the reder ntenn, Gtg the gin of the tg ntenn, nd Atg the equivlent perture of the tg ntenn. Define the equivlent trnsmitted power s Then P reder P G P ( EIRP) = (8) reder trnsmitted λ P G G P 4πR =. (9) reder 4 bk ( ) tg reder ( ( EIRP) ) Denote by sensitivity the threshold power of the sensitivity. Then the mximum reding rnge is expressed s R P G G reder λ trnsmitted reder tg = 4 (1) reder 4π Psensitivity Now we nlyze the RFID system by using the rdr priniple. Suppose tht the bksttering setion of the tg, inluding the ntenn nd the hip, is σ, then the bk- tg sttering power of the tg is P BS tg tg GrederPtrnsmittedσ = S1σ = (11) 4πR

4 16 Development nd Implementtion of RFID Tehnology The power density of the bk sttering wve t the position of the reder is So we hve S tg = P G P σ 4 πr = (4 π) R (1) reder BS reder trnsmitted 4 reder tg reder PtrnsmittedGreder P = bk SA = λ σ λ reder SG reder π = (4 π) R. (13) By djusting the tg hip impedne ording to the stored dt in tg, will be hnged, nd then the return wve oming from the tg nd reeived by the reder will be hnged suh tht the mplitude modultion nd demodultion n be relized. In this mnner, the tg informtion n be red, nd the objet deteted by the tg n be identified. Generlly, the operting frequenies of the norml RFID system bsed on the bksttering inlude: 915MHz,.45GHz, nd 5.8GHz, the orresponding wvelengths re.38m,.1m, nd.51m. Obviously, the mximum reding rnge is diretly proportionl to the wvelength. In ft, for the sme distne the spe loss t higher frequeny is greter thn tht t lower frequeny. The spe loss SL is defined s 4πR SL = λ. (14) Commonly, the size of the ntenn is relevnt to its working frequeny. For lower frequeny, the ntenn will be lrger, nd the size of the tg will inrese. When the ntenn size is fixed, the higher gin will be hieved for higher frequeny. In most ses, the ntenn size is bottlenek for tg minituriztion. In order to ppropritely hoose the operting frequeny for the RFID system, we should onsider simultneously mny ftors suh s the spe loss, the ntenn gin, nd the size of the tg. There lso exists nother loss, lled the polriztion loss, whih is used by the polriztion mismth between the inoming wve nd the ntenn, or between the trnsmitting ntenn nd the reeiving ntenn. The polriztion mismth will mke the ntenn lose the bility to reeive ll the power of the wve. Suppose E = ˆ ρ E i w i is the inoming wve, E = ˆ ρ σ tg is the polriztion orienttion of the reeiving ntenn, nd ˆo ρ is the vetor tht is orthogonl to the polriztion vetor of the reeiving ntenn. The polriztion ftor PLF is defined s PLF = ˆ ρ ˆ ρ = osϕ, or PLF( db) 1lgPLF w p Then, the power reeived by the ntenn is denoted by r mx = (15) P = P PLF, or PdB ( ) = Pmx ( db) + PLFdB ( ) (16) r

5 Design of Antenns for RFID Applition 17 where P mx stnds for the power of the inoming wve, or the mximum power reeived by the ntenn when the polriztions re mthed, ˆ ρ the unit polriztion vetor of the reeiving ntenn, nd ˆw ρ the unit vetor of the inoming wve. Assume tht the inoming wve is irulrly polrized. Then the unit vetor ˆw ρ n be expressed s ˆ ρ ( ˆ ˆ w = ρ ± jρo) (17) PLF = 1/, nd PLF( db) = 3dB. Fig. 1. Polriztions of the ntenn nd the wve As shown in Fig. 1, the polriztion mismth between the ntenn nd the wve redues the reeived power, nd deteriortes the system performne. Thus hoosing suitble polriztion is lso n importnt step for designing the ntenn.. Development of ntenns in the RFID system Potentil pplitions of the RFID tehnology inspired the development of vrious ntenns for the RFID systems. Lots of ntenns with high performne for vrious requirements hve been fbrited. As n identifition system with huge mrket nd potentils, RFID system requires the RFID ntenn to meet some prtiulr speifitions. Design of the RFID ntenns fes mny hllenges, suh s the ntenn struture, the ntenn size, the operting mode, the bndwidth, the rdition pttern, the polriztion, mutul oupling between multiple ntenns, nd the ntenn sttering. In the present RFID system, the reder ntenn is designed to be irulrly polrized ntenn. Pth nd spirl ntenns re typil reder ntenns. In some speil ses, linerly polrized ntenns n lso be used. In the tg, the eroded or printed ntenns re ommonly used, nd the dipole is the typil tg ntenn struture. Some irulrly polrized ntenns for the tg my be required in some speil pplitions. In reent yers, theory for mthing the ntenn with the tg hip is disussed, whih guides the design of the tg ntenn nd the nlysis of the tg onfigurtion. Severl tg ntenns in ommon use re designed with simple impedne trnsformtion for mthing the hip with speil impedne, espeilly for UHF bnd pplition. In the mirowve bnd, some tg ntenns re lso designed to integrte with the lredy existing speifi iruits with 5 ohm impedne.

6 18 Development nd Implementtion of RFID Tehnology Shemes for designing the irulrly polrized reder ntenn re lso presented in some literture. Bsed on two ports for the dul irulr polriztion, the perture-oupled pth ntenn integrted with the mirostrip brnh line oupler is preferred. Some modifitions re performed to hieve the wide bnd, or meet the prtil requirements. The system, in whih multiple reder ntenns re used, is lso disussed. In the design of ntenn for the RFID system, some other problems, suh s the environmentl effets on RFID tg ntenns, espeilly surrounded by metlli objets, should be onsidered. Designing the RFID tg ntenn, whih is mounted on the metlli objets, lso fes hllenge. The inverted-f ntenn nd its modifitions re usully used in the tg for identifying the metlli objets, nd other ntenn strutures n lso be referred in designing ntenn mounted on metlli surfes. The eletromgneti sttering of the tg ntenn is lso introdued nd disussed, nd reltive lultions hve been performed..3 Antenn design softwre for RFID pplition Effiient numeril methods promote the ntenn design. Modern ntenn design beomes mnipultion of urte omputing bsed on reltive theory nd design under the theory instrution or ording to the lulted results. The ntenn design method bsed on numeril methods hs been pplied to design ntenns for vrious systems. Fmilir numeril methods inlude Method of Moment (MoM), Finite Element Method (FEM), nd Finite Differene Time Domin (FDTD). There lredy exist severl design tools bsed on these methods, whih re of different hrteristi nd re widely used. Fig. shows some fmilir methods nd the design tools. These design tools n be hosen for different problems in designing ntenns. The MoM n be used to lulte the ntenn performne quikly nd urtely, espeilly for some lrge ntenn strutures. Some optimiztion methods, suh s the optimiztion tool used in Zelnd IE3D, n be embedded into the nlysis method to mke the ntenn hieve the exellent performne. The FEM nd FDTD methods n be used diretly to nlyze the ntenn performne. However, the FEM method gets more urte results thn the FDTD method. The FDTD method n be used to nlyze some lrger ntenn strutures, solve the wide bnd problems in time domin, nd give dynmi demo bout the eletromgneti field distribution nd rdition. Some tools suh s HFSS, whih re widely used to deign ntenn for the RFID system, dd the bility of utomtilly meshing to filitte the user nd improve the preision. These design tools should be hosen properly for designing ntenn, sine they hve different hrteristis. Some tools n be used to nlyze some types of ntenn suitbly but lose the bility for solving other ntenns or ffording the lrge memory requirement. In designing ntenn, the ntenn onept bsed on the eletromgneti theory should be mixed with the mnipulting softwre skilfully, nd the ntenn prototype of the design sheme hosen for the system requirement is more importnt thn the skill in pplying the softwre. After the ntenn sheme is deided, being fmilir with the softwre nd the reltive numeril methods will help the designer to design ntenn properly, nd djust the struture prmeters to optimize its performne. To sueed in designing ntenn, it is of gret importne to pply softwre under the guidne of ntenn priniple nd eletromgneti theory. Although the funtion of the softwre for designing ntenn is more powerful, the bsi theory nd onept is lso bsolutely neessry. Both the ntenn theory nd the design softwre promote the design of ntenns in the RFID system.

7 Design of Antenns for RFID Applition 19 Fig.. Numeril methods nd softwre 3. Power trnsmission between tg hip & ntenn Generlly, the RFID system minly onsists of reder nd tg. The tg design is the most importnt loop in the RFID pplition, nd lso the most diffiult prt in the funtion reliztion. Performne of the tg usully deides the performne of the whole system. The tg is omposed of the tg ntenn nd the hip, between whih good onnetion nd power trnsmission diretly impt on the system onfigurtion, the reltive funtion reliztion nd lso the system performne. Thus, it is neessry to nlyze the onnetion of the tg ntenn to the RFID tg hip, nd to disuss the impedne mth problem. 3.1 Theory of impedne mth The most importnt ftor in the tg is the reding rnge, whih is the mximum distne between the reder nd the tg suh tht the reder n detet the bksttering signl from the tg. Compred with the tg, the reder is lwys of high sensitivity, nd the reding rnge is minly limited by the performne of the tg. Espeilly for the pssive tg, both the energy for mintining or rousing the tg nd the power of signl retrnsmitted by the tg re from the RF energy, whih is trnsmitted by the reder nd ught by the tg. The impedne mth between the ntenn nd the hip hs diret influene on whether the tg iruit n operte well nd the hip is ble to retrnsmit enough energy to implement the bksttering ommunition, nd limits the reding rnge. To mximize the power trnsfer between the ntenn nd the hip, the impedne of the hip onneted to the ntenn should be onjugte to the ntenn impedne. When the working frequeny omes into the mirowve bnd, the impedne mth problem beomes

8 Development nd Implementtion of RFID Tehnology more serious. Ordinrily, the impedne of the ntenn prototype designed for the tg is 5 ohm or 75 ohm, while the hip impedne my be rndom vlue, or vry with frequeny, nd hve differene when the driving power is hnged. It is extremely ruil to hieve suitble impedne mth between the ntenn nd the hip. New integrted iruit hip design nd development need lrge investment nd long reserh period, however, designing ntenn to mth the existing hip is more onvenient nd prtil. Due to the requirements suh s esy mnufture, low ost nd smll size, dding the mthing network is infesible. To solve this problem, the ntenn should be ble to mth the hip diretly by djusting its struture. How to design n ntenn to mth hip of rbitrry impedne is n inevitble mission in designing ntenn for the RFID system (Nikitin et l., 5; Ro, Nikitin & Lm, 5). By nlyzing the tg, its equivlent iruit is shown in Fig. 3. Denote by Z the ntenn impedne, ndz = R + jx, by Z the hip impedne, ndz = R + jx. ntenn hip Fig. 3. Equivlent iruit of the tg Define the omplex power refletion oeffiient s s Z Z s = Z + Z Then the power refletion oeffiient is lulted by * (18) * + + Z Z ( R R ) jx ( X ) s = = Z + Z ( R + R ) + jx ( + X ) Let R X + X + j 1 [ R + jx ( + X) ] R R R = = [ R + jx ( + X) ] + R R X + X + j + 1 R R (19) R R X + X + j = r+ jy = Z () R

9 Design of Antenns for RFID Applition 1 be the ntenn impedne normlized to the rel prt of the hip impedne, then s Z 1 = Z + 1, or s = Z Z (1) On the bsis of the trnsformtion, the trditionl Smith Chrt n be used to desribe the impedne mth between the ntenn nd the hip. Z n be mrked ording to its rel prt nd imginry prt on Smith Chrt like the trditionl normlized impedne. The distne between the point of eh Z nd the entre point of Smith Chrt expresses the mgnitude of the omplex power refletion oeffiient s, while the tre of impedne points, whih hve onstnt distne to the entre point, forms the onentri irle, whih is lled s the equivlent power refletion irle. The entre point of Smith Chrt is the perfet impedne mth point, while the most outer irle denotes the omplete mismth se, i.e. s = 1. The power trnsmission oeffiient (Ro, Nikitin & Lm, 5b) n lso be defined s τ, nd P = Pτ, where P stnds for the power from reder ught by tg ntenn, P the power trnsmitted from the tg ntenn to the tg hip. It follows from Fig. 3 tht 4RR τ =, τ 1 Z + Z () Let x X =, R r R =, R trnsmission oeffiient is expressed s follows. Q X τ + s = 1 (3) =, then eqution of the irle with onstnt power R 4 τ + + = τ (4) [ r ( 1)] [ x Q] (1 τ ) From eqution (4), the impedne hrt with the onstnt power trnsmission oeffiient is drw, s shown in Fig. 4. In Fig. 4, the x xis expresses the normlized rel prt r = R / R, nd y xis the normlized imginry prt x = X / R. The irles with onstnt power trnsmission oeffiients τ =1,.75,.5,.5 re drw in Fig. 4. The x xis is lled s the resonnt line withx = X, while the y xis is lled s the omplete mismth line. When τ s derese, the rdius of the irles with onstnt power trnsmission oeffiient inrese. While τ, the irle with onstnt power trnsmission oeffiient pprohes to its tngent, tht is the y xis, on whih the impedne point nnot hieve the power trnsmission. When the hip nd the ntenn re resonnt, (4) beomes X = X, nd x = Q, then eqution

10 Development nd Implementtion of RFID Tehnology Fig. 4. The impedne hrt with the onstnt power trnsmission oeffiient 4 r τ τ = τ (5) [ ( 1)] (1 ) = (6) [ τr ( τ)] 4(1 τ) Mking the derivtive for the both sides of eqution (6), we hve Obviously 1 dτ dτ dτ [ τr ( τ)]( τ + r + ) = 4 (7) dr dr dr dτ [( r + 1) τ ] τ = (8) dr r τ = dτ mens perfet mth, nd =. τ = dr mens omplete mismth, dτ nd =. Thus either the perfet mth or the omplete mismth is stedy point of τ dr with r dτ, i.e. =. dr

11 Design of Antenns for RFID Applition 3 For the fixed R X R nd X, R R 4 4 R R τ = = R X R X jq (1 + ) (1 + ) + Q (1 + ) R X R X (9) dτ X R R X = 8 Q(1 + ) [(1 + ) + Q (1 + ) ] (3) dq X R R X When the hip impedne is pitive, i.e. Q < dτ, it follows from (13) tht >. dq While the hip impedne is indutive, i.e. Q > dτ, <. When Q =, i.e. X = dq nd menwhile X =, we hve 4RR τ = ( R + R ) The urve of τ versus Q is shown in Fig.5. From this figure, we n see tht for the fixed R R nd X, Q should be s smll s possible from the power trnsmission point of view, X when the tg ntenn is onneted to the tg hip. For the tg ntenn, the impedne hrt n be used to guide the design or to desribe the tg ntenn. The hrt is theoretilly importnt nd very useful for other pplitions. (31) Fig. 5. Curve of τ versus Q 3. Impedne design for the tg ntenn Aforementioned results indite tht the mximum power trnsmission n be relized only if the ntenn impedne is equl to the onjugte vlue of the hip impedne. While the

12 4 Development nd Implementtion of RFID Tehnology hip impedne is not norml 5 ohm or 75ohm, the struture of the tg ntenn should be refully hosen. In this setion, symmetril inverted-f metlli strip with simple struture shown in Fig. 6 is proposed. The ntenn hs the bility to relize severl impednes. For UHF bnd pplition, the impedne of the ntenn in four ses with different struture prmeters is nlyzed t 91MHz, whose rel prt is pproximtely ohm, 5ohm, 75ohm, 1ohm respetively. The simulted results for these four ses re shown in Fig. 7. Fig. 6. The symmetril inverted-f Antenn Z =R +jx x L1 R X Z =R +jx R X L1 (A) W=3mm, L=1mm (B) W=5mm, L=5mm 1 Z =R +jx 1-1 R X Z =R +jx -1 - R X L L1 (C) W=64mm, L=3mm (D) W=73mm, L=3mm Fig. 7. Impedne results of the ntenn in different ses

13 Design of Antenns for RFID Applition 5 Fig. 7 shows tht the symmetril inverted-f metlli strip n relize severl impedne vlues by djusting its short brnh. A lot of fmilir types of tg ntenns re the modifitions or trnsformtions of this struture (Dobkin & Weignd, 5). Fig. 8 shows the evolvement of severl tg ntenns. Antenn B hs less influene on its performne thn ntenn A, when the ntenn is urved (Tikhov & Won, 4). Antenns C nd D re fed by n indutively oupled loop (Son & Pyo, 5). A B C D Fig. 8. Evolvement of the tg ntenns Fig. 9. Geometry of mendered dipole ntenn surrounded by the retngulr loop (dimensions in mm) In our pplition, n UHF bnd tg hip with 43-j8 ohm impedne is used, nd tg ntenn onneted to this hip should mth the tg hip. Menwhile the tg ntenn should be smll in size nd esily fbrited. In Fig. 9, mendered dipole ntenn is designed, nd pir of symmetril mendered metlli strips surrounded by retngulr

14 6 Development nd Implementtion of RFID Tehnology loop is fed. The higher rel prt of the impedne n be relized by the mendered dipole, while its high imginry prt n be supplied by the oupling between retngulr loop nd symmetril mendered dipole. In this wy, tg ntenn with higher bsolute vlue impedne nd higher Q vlue is designed nd onneted to the hip, to ensure the good power trnsmission. The gp of the feeding point is.1mm, the width of the metlli mendered strip nd the horizontl prt of the retngulr loop is 1mm, nd the width of its vertil prt is mm. The tg ntenn hs thikness of.18mm. The tg ntenn is nlyzed by the HFSS softwre, the performne of the ntenn, inluding its impedne nd rdition ptterns, is lulted. The simulted results re shown in Tble 1 nd Fig. 1. These results show tht the ntenn with smll size n be used s tg ntenn for the UHF bnd RFID hip pplition. Freq(MHz) Antenn impedne (ohm) Power refletion oeffiient s Power trnsmission oeffiientτ j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j Tble 1. The impedne nd power refletion oeffiient, power trnsmission oeffiient for Tg ntenn hip impedne: 43-j8ohm

15 Design of Antenns for RFID Applition E pl ne H pl ne Fig. 1. Rdition pttern of the mendered dipole ntenn 3.3 Tg ntenn mountble on metlli objets Sine the RFID tehnology is pplied in wide fields, RFID systems frequently pper in the metlli environment, nd the effet of the metlli objets should be onsidered in designing the ntenn (Penttilä et l, 6). RFID ntenns in mirowve bnd hve defet of stnding wve nulls under the impt of metlli environment. To solve the problem brought by the metlli objets, some speil tg ntenns should be designed. These ntenns usully hve metlli ground. Some metlli objets, whih mke the performne of the RFID ntenn worse, re modified to be s n extended prt of the ntenn to improve its performne. Some existing problems should be disussed. When the trditionl dipole ntenn is tthed to n extremely lrge metlli plne, its rdition will be dmged. In generl, the tg ntenn with hemispheril overge is required. In prtil pplition, tg ntenn with low profile is frequently used, nd its vertil urrent is limited. In Fig. 11, when norml dipole ntenn pprohes losely the metlli surfe, n indutive urrent in opposite diretion is exited, nd the rdition indued by the urrent will eliminte the rdition of the dipole, resulting in tht the tg nnot be deteted or red. As lss of ntenns, the mirostrip ntenn my be good hoie for being mounted on the metlli surfes nd identifying the metlli objets. For ordinry tg hip, blun or other iruit is needed to feed the ntenn. Here, bsed on the dipole ntenn, two design shemes for the metlli surfes re proposed. One is modifition to the Ygi ntenn, nd the other is dipole Antenn bked by n EBG struture. A substrte with high dieletri oeffiient is sndwihed between the dipole nd the metlli surfe, its thikness will reverse the orienttion of the indutive urrent, nd the rdition is strengthened. An EBG struture n depress the primry indutive urrent,

16 8 Development nd Implementtion of RFID Tehnology the rdition of the dipole will be vilble, nd the metlli surfe of the identified objet is lso the ground of the EBG struture. Fig. 11. Design sheme for the tg ntenn on metlli surfes () Exittion urrent nerby the metlli surfe; (b) Sheme bsed on the Ygi ntenn () Sheme bsed on the EBG struture Aording to the introdued shemes, three tg ntenns re designed for three tg hips with impednes 15-j ohm (hip 1), 6.7-j197ohm (hip ), nd 43-j8 ohm (hip 3), respetively. The tg ntenn bsed on the Ygi ntenn is shown in Fig. 1, nd the geometry of the tive dipole (Qing & Yng, 4) is lso given in Fig. 13. In Fig.1, the tive dipole is tthed on the substrte with the reltive dieletri oeffiient εr=1.. The width of the metlli strip is.8mm. Fig. 1. The tg ntenn for hip 1 bsed on the Ygi ntenn

17 Design of Antenns for RFID Applition 9 Fig. 13. Geometry of the tive dipole (dimensions in mm) The ntenn shown in Fig. 1 is nlyzed by the HFSS softwre. The lulted ntenn impedne mthes the hip impedne 15-j ohm in UHF bnd. Rdition ptterns of the tg ntenn re lso lulted nd shown in Fig. 14. To design the ntenn for hip with 6.7-j197 ohm impedne, the struture prmeters re djusted. The designed dipole is shown in Fig. 15, nd its simulted rdition ptterns re presented in Fig E pl ne H pl ne Fig. 14. Rdition ptterns of the tg ntenn for hip 1

18 3 Development nd Implementtion of RFID Tehnology (1) The tg ntenn nd the substrte () The tive dipole Fig. 15. Geometry of the tg ntenn for hip E plne H plne Fig. 16. Rdition ptterns of the tg ntenn for hip Similr tg ntenn n lso be designed bsed on the EBG struture (Abedin & Ali, 5, 5b, 6; Yng & Rhmt-Smii, 3) like the tg ntenn shown in Fig. 1. The EBG struture is tthed to the surfe of the metlli objet, nd the tg dipole ntenn like the tive dipole in Fig. 13 is pled on the EBG struture formed by 5 7 elements, s shown in Fig. 17. This struture is nlyzed t frequeny 915MHz in the UHF bnd, nd its rdition ptterns re lulted, whih re shown in Fig. 18. The simulted impedne vlues show tht the tg ntenn mthes the hip 3 with impedne 43-j8 ohm. The reltive dieletri oeffiient of the substrte of the EBG struture is.65, its thikness is mm, nd the totl thikness of the tg ntenn is 15mm. The low ost tg ntenn with low profile will be fbrited.

19 Design of Antenns for RFID Applition 31 Fig. 17. The tg ntenn bked by the EBG struture for hip E plne H plne Fig. 18. Rdition ptterns of the dipole bked by the EBG struture for hip 3 In this setion, design of the tg ntenn for the metlli surfe is presented, nd severl ses re desribed nd disussed. Other types of tg ntenn mounted on the metlli objets, suh s the inverted-f ntenn nd its modifitions re lso populr. For the detils bout these ntenns, refer to Kim et l., 5; Son et l., 6; Ukkonen, Sydänheimo et l., 4; Hirvonen et l., 4; nd Ukkonen, Engels et l., Cirulr polriztion modultion nd design of the irulrly polrized ntenns 4.1 Cirulrly polrized reder ntenn nd irulr polriztion modultion Generlly the objet to be identified or the tg does not point to ertin diretion, so the irulrly polrized reder ntenns re usully used (Rumonen et l., 4) to reeive signls from ll diretions nd do not miss the mismthed polrized signls of the moving objet. The linerly polrized reder reeives more thn 3dB power, when the polriztions of the tg nd the reder re mthed. In some wireless ommunition systems, the irulr

20 3 Development nd Implementtion of RFID Tehnology polriztion modultion (Fries et l., ; Kossel, Kung, et l., 1999), whih is well dpted to the low rte RFID systems, is nother hoie tht n redue the requirement of the frequeny bnd, nd simplifies the dt ommunition, s shown in Fig. 19. Therefore, the ntenns, used for the reder nd the tg, should be dul irulr polriztion ntenns with two ports in the RFID system. Fig. 19. Priniple hrt of the irulr polriztion modultion Helix ntenns nd mirostrip ntenns re widely used s the irulrly polrized reder ntenn for one-port pplitions. The helix ntenn hs some dvntges, suh s low ost nd simple design, exept its lrger physil size. The low profile helix ntenn with the EBG struture insted of the metl ground plne n be used for the RFID reder (Rumonen et l., 4). The irulr polriztion modultion is lwys used in the RFID system, nd its bsi priniple is tht logil zero is trnsmitted s the left-hnd irulrly polrized (LHCP) wve, nd logil one is represented by right-hnd irulrly polrized (RHCP) wve. Both reder nd tg n use irulrly polrized ntenns with swithble polriztions. Cross polriztion isoltion hs the signifint effet on the performne of the whole

21 Design of Antenns for RFID Applition 33 system. The mximum trnsmission distne n be expnded more thn %, if the ross polriztion level (XPL) rehes up to db from 5dB. In the bksttering modultion system, the inident LHCP wve illuminted to the tg is modulted nd bksttered into the RHCP wve, nd then retrnsmitted to the reder. Reltive to the system where the linerly polrized tg ntenns re used, the signl reeived by the reder in the irulr polriztion modultion system will rise 6dB. In spite of wht kind of the modultion is used, the system should hve higher polriztion isoltion. At the sme time, the tg ntenn should hve higher port isoltion, whih n redue the interferene between the trnsmission hnnel nd the reeive hnnel. Fig.. The 3dB brnh line diretionl oupler struture Fig. 1. Mirostrip ntenn with oupling slot bsed on the brnh line oupler The trditionl design of the dul-port dul-polriztion ntenn (Kossel, Benedikte et l, 1999; Qing & Yng, 4b; Shrm et l., 4) is bsed on the brnh line diretionl oupler, in whih the eletril fields in two output brnhes hve identil voltges nd 9º phse shift, nd hs high isoltion between two output ports, s shown in Fig.. When the impednes of the four ports re mthed very well nd the signl inputs from Port 1, Port 4, lled the isoltion port, hs no output signl, nd there is 9º phse shift between Port nd Port 3. The dul irulrly polrized ntenn, s shown in Fig. 1, is mirostrip

22 34 Development nd Implementtion of RFID Tehnology pth ntenn, whih uses brnh line oupler to feed the orthogonl slot pertures nd to relize the required 9º phse shift. Four different irulrly polrized ntenns re shown in Fig.. The multilyered ntenns employ two substrtes, the pth lyer nd the feed lyer, nd ground plne with slot pertures between two substrtes, s shown in Fig. 3. The pth ntenns n relize the dul irulr polriztion by using the brnh line oupler or the mirowve brnhes to feed the slot pertures with the required phse shift. Fig.. Four dul-port dul irulrly polrized ntenns Fig. 3. Multilyered mirostrip ntenn struture

23 Design of Antenns for RFID Applition A ompt dul irulrly polrized ntenn In order to relize low profile nd prt the feed line from the pth, the slot perture mirostrip ntenn is ommonly used. For this kind of ntenns, the designer ould selet different substrtes for the feed nd pth lyers, ording to the pplition requirements of the mirowve integrte iruits. As shown in lst setion, dul irulrly polrized ntenns for the RFID system in mirowve bnd re fed by two orthogonl nd isolted slot pertures, bsed on the brnh line diretionl oupler or other omplex mirowve networks. However, the onfigurtion of the ntenn presents struturl bottlenek, i.e. the isolted slots nd feeding network limit the minituriztion of the ntenns, nd the mirowve network with omplex iruits oupies the lrger spe. It is well known tht RFID ntenns n hieve long distne propgtion of eletromgneti wves, but sometimes hve the problem suh s stnding wve nulls. Therefore, the ntenns should be integrted with the loop, whih ould trnsmit power to the low frequeny system through the indutne oupling, nd redue the size of the feed network. In order to get rid of the bottlenek on the minituriztion of the ntenns, we should design the ompt slot perture mirostrip ntenn with simple feed network to omplish the dul irulr polriztion. In this setion, we present ompt dul irulrly polrized ntenn for RFID systems. In the RFID system, the rte of the dt ommunition is not so high, sometimes just few bites. Therefore the irulr polriztion modultion n be used in the nrrow bndwidth ommunition to simplify the dt ommunition. It is neessry to design dul irulrly polrized ntenn with two well-isolted ports for the irulr polriztion modultion. In order to miniturize the dimensions of the ntenn, s shown in Fig. 4, dul irulrly polrized mirostrip ntenn fed by rossed slots without the brnh line oupler is proposed (Zhng, Chen., Jio & Zhng, 6), whih is n optiml hoie for the RFID system with lrger bndwidth nd the smller size. The oupling perture for the irulrly polrized ntenn omprises two rossed slots (Aloni E. & Kstener, 1994) in the ground plne, with four rms of the perture fed serilly by single mirostrip line loted underneth the ground plne. The mirostrip line feeds the four rms with 9º progressive () Top view Fig. 4. Struture of the dul irulr polrized ntenn (b) Side view

24 36 Development nd Implementtion of RFID Tehnology phse differene. The symmetri rrngement mkes the ntenn hieve esily the dul irulr polriztion. The design method hs been widely used for the ntenn t.45ghz in the RFID system. In order to redue the ost, the ir lyer used to reple the fom mteril, s shown in Fig. 4, is sndwihed between two substrte lyers with the sme dieletri onstnt εr=.65. HFSS simultion results show tht the performne of the ntenn nnot stisfy the requirement for the RFID system. Thus, the struture of the ntenn should be modified to improve its performne. As result, orner-trunted squre pth (Wng, 1989) is used to reple the norml squre pth, whih will improve the irulr polriztion performne of the ntenn nd its port hrteristis. At the sme time, we ut squre perture in the entre of the pth to restrit the urrent nd to improve the port isoltion. Steps of the pth evolution from the squre to the orner-trunted squre with squre perture re shown in Fig. 5, nd the finl ntenn struture is shown in Fig. 6. Simulted performne indies of these three pth ntenns re given in Tble, whih indite the effetiveness of the pth modifitions. Fig. 5. Steps for the pth modifitions Fig. 6. Geometry of the ntenn fter modifitions

25 Design of Antenns for RFID Applition 37 We now determine the dimensions of the orner-trunted squre pth ntenn with squre slot. First we djust the dimension of the pth to get the mximum gin, keeping the other prmeters of the ntenn fixed. We then djust the length nd width of the slots to improve the port hrteristis. Beuse it ffets the oupling between the mirostrip nd the pth more seriously, the length of the slots should be djusted in dvne. Finlly, by djusting the squre perture nd the orner of the pth, the ntenn with better performne is obtined. The totl size of the ntenn is 6mm 6mm 3mm. The width of the orner-trunted squre pth with squre perture is 51mm, nd eh lyer is 1mm thik. The mirostrip line hs width tht mkes the trnsmission line hve 5Ω hrteristi impedne. The length nd width of the perture re 4mm nd.316mm, respetively. Pth modifitions Axil Rtio (db) S11 (db) S1 (db) Primry squre pth Corner-trunted squre pth Finl pth struture Tble. Antenn prmeters during the modifition We hve tested prototype of the ompt dul irulrly polrized ntenn shown in Fig. 9. The mesured results re shown in Figs. 7 nd 8. Comprison between the simulted results nd the mesured dt shows tht the mesured S prmeters t two ports gree well with the simulted results. The mesured S1 is better thn the simulted one, however the mesured S11 is worse thn the simulted one. There is trdeoff between the return loss nd the port isoltion. From the mesured rdition ptterns, we n see tht the ross polriztion levels better thn -15 db re hieved. Although the mesured ross polriztion levels nnot reh the simulted irulr polriztion performne, they meet the requirements of the RFID system. The ntenn n be used to relize the irulr polriztion modultion for the RFID systems db S11 S S Fig. 7. Mesured S prmeters t two ports

26 38 Development nd Implementtion of RFID Tehnology Simulted nd mesured results for the ompt dul irulrly polrized perture oupled pth ntenn show tht the ompt struture meets the requirements for the RFID system. For the ntenn with smller size, port deoupling better thn db nd good irulr polriztion re hieved by the oupling nd feeding tehnique, without using the mirostrip brnh line oupler or other omplex feed networks. The design n sve more spe for the IC lyout, nd the minituriztion of the ntenn is relized, whih is very importnt for the integrtion of the RFID system t the mirowve frequeny nd low frequeny bnds. The dul irulrly polrized ntenn with the ompt struture is not only pplible to the norml RFID systems, espeilly in some identifition rd pplitions, but lso suitble for using in some wireless ommunition systems Rel.Power (db) - -3 Co-Polriztion Cross-Polriztion Rel.Power (db) - -3 Co-Polriztion Cross-Polriztion () Fig. 8. Mesured rdition ptterns t two ports () Exittion in LHCP port; (b) Exittion in RHCP port (b) Fig. 9. Photogrph of the ompt dul irulrly polrized ntenn prototype 5. Design of ntenns for the RFID tg in mirowve bnds Aording to the design of the tg nd the reder t the mirowve bnd, speil hips for the pplition of the RFID system re immture nd seldom used in industry. Thus engineers usully use speifi ASICs vilble in the mrket for some speil RFID systems,

27 Design of Antenns for RFID Applition 39 nd sometimes use the oxil ble to onnet the ASIC with the ntenn, whose impedne is 5Ω. A rdio-frequeny identifition system onsists of tgs nd reders, nd reders ommunite wirelessly with the tgs to obtin or trnsfer the informtion. The dt sent by the reder is modulted nd bksttered from number of tgs. In some ses, the reder hnges the dt stored in the tg. Severl frequeny bnds, suh s 15 KHz, MHz, 869 MHz, 9-98 MHz,.45GHz nd 5.8GHz bnds, hve been ssigned to the RFID pplitions. As the operting frequeny for the RFID systems rises into the mirowve bnds, the ntenn design beomes more ute nd essentil (Chen & Hsu, 4; Liu & Hu, 5). The tg, whih inludes the ntenn nd mirohip trnsmitter, must be low in profile, low in ost nd smll in size for the vluble nd esy use, when it is tthed to n objet to be identified. Therefore, suitble ntenn used in the tg beomes more nd more importnt. As the RFID tehnology ontinues to be widely used in pplitions, espeilly meeting the relibility, the nti-interfere nd the other speil requirements, the systems tht re ble to work t two bnds, suh s.45ghz nd 5.8GHz bnds, re expeted. The reder with single ntenn is used for both dt trnsmission nd reeiving, whih needs severe opertionl requirements to the reder RF front end, sine the ommunition tkes ple in both diretions t the sme time. The reder my use two ntenns for the ommunition, one ntenn for the dt trnsmission nd the other ntenn for the dt reeiving (Penttilä et l, 6). This hoie n redue the reliztion diffiulty from the hrdwre point of view. In ddition, there my hve more thn two ntenns in the reder. In this se, the reder must follow ertin sequene to swith on n ntenn t time, while keeping other ntenns swithed off, to void interferenes between these ntenn signls. The pproh for using two ntenns in the reder is bsed upon the following resons: 1. The trnsmitting eletromgneti wve from the reder does not vnish, when the refleting wve from the tg rehes the ntenn of the reder in the single ntenn systems.. The reder definitely hs less sensitivity thn the rdr, nd the trnsmitting wve of the reder hs muh more power thn the reeiving wve from the tg. Thus the irultor or the diretionl oupler should be designed to meet higher requirements. 3. The bksttered wve hs the lower intensity thn the trnsmitting wve, so the irultor or the diretionl oupler should meet higher isoltion in order to seprte the signls. 4. The reder must be inexpensive. Bsed on the forementioned resons, the reder is hrd to relize. Two ntenns, one for trnsmitting nd the other for reeiving, n overome these problems. However, higher isoltion between the ntenns for the ommunition should be required, nd the smller tg mkes the isoltion hrd to relize. Mirowve frequeny bnds used in the RFID system inlude.45ghz nd 5.8GHz bnds, whih hve the similr trnsmission hrteristi. Therefore the design method for the ntenn operting in two frequeny bnds is lso similr. If the trnsmitting ntenn nd the reeiving ntenn work t two frequeny bnds seprtely, it is esy to relize nrrow frequeny bnd ntenn for the reder, nd then higher isoltion n be hieved between the trnsmitting ntenn nd the reeiving ntenn. On the other hnd, the RFID system only uses these two frequeny

28 4 Development nd Implementtion of RFID Tehnology bnds, does not interferes signls t other lose frequeny bnds ssigned by globl Interntionl Orgniztion for Stndrdiztion (ISO), thus stisfies the EMC requirements. We present two-ntenn system whih n operte simultneously t.45 nd 5.8GHz bnds, s shown in Fig. 3. The tg ntenn, worked t two frequeny bnds, not only n reeive the trnsmitting signls from the reder t 5.8GHz, but lso n trnsmit signls with the tg ode t.45ghz, whih re reeived nd demodulted by the reder to obtin the tg informtion. Fig. 3. The dul ntenn system in the RFID system We present dul-bnd folded-slot ntenn with RF performne suitble for the RFID tg use t.45/5.8 GHz (Zhng, Jio & Zhng, 6), whih onsists of folded slot with open end nd oplnr wveguide (CPW)-fed struture suh tht only single-lyer substrte is required for the ntenn. By properly djusting the folded slot on the retngulr pth, ompt ntenn size, good mth t two frequeny bnds nd the rdition hrteristis suitble for the RFID pplition t.45 nd 5.8 GHz ould be hieved. The geometry of the proposed CPW-fed folded-slot monopole ntenn with the open end is shown in Fig. 31. The ntenn hs simple struture with only one lyer of FR4 dieletri substrte (thikness 1 mm nd reltive permittivity 4.4) nd metlliztion. The ntenn is symmetril with respet to the longitudinl diretion; folded slot splits the retngle pth into double C-shped ground, nd blne-shped strip tht is fed by the CPW nd onnets to n SMA forms monopole struture. Clerly, s the rditing element of this ntenn, the blne-shped strip is thus seprted from the ground plne by the folded slot with the open end. The strip n produe two resonnt frequenies by djusting the lotion of its double rms. The blne-shped strip is hosen to be of height 31 mm, whih is lose to one-qurter wvelength in free spe t.45 GHz, while the top prt of the blne-shped strip bove the lotion of the double rms is hosen to be of height 1 mm, whih is lso lose to one-qurter wvelength in free spe t 5.8 GHz.

29 Design of Antenns for RFID Applition 41 Fig. 31. Geometry of the proposed ntenn with open end. The dimensions (in millimetres) shown in this figure re not to sle. We first study the dimensions of the ntenn by the simultion with the id of HFSS eletromgneti softwre, nlyze its performne by XFDTD simultion tool, nd then djust them by the experiment. Finlly, the dimensions of the fbrited ntenn re hosen with height 3 mm nd width mm, nd detils of the struture re shown in Fig. 31. For the blne-shped strip, the top end of the vertil setion with strip width 1.6 mm nd length 31 mm is hosen to be open, nd the other setions of the strip re djusted to hieve good mth t these two frequenies. The CPW feedline, with signl strip of width 1.6 mm nd length 18 mm, nd gp distne of 1.7 mm between the signl strip nd the oplnr ground plne, is hosen to feed the dul-bnd monopole ntenn entrlly from its bottom edge. The prototype of the proposed dul-bnd CPW-fed folded-slot ntenn with optiml geometril prmeters, s shown in Fig. 3, is fbrited nd tested. The performne of the ntenn is lso simulted with the id of two eletromgneti simultors, HFSS nd XFDTD. In Fig. 33, the mesured nd simulted frequeny responses of the return loss t two bnds for the proposed design re ompred, nd the mesurement is mde with Wiltron 3769A network nlyzer. As n be seen from the mesured results, the ntenn is exited t.45 GHz with 1 db impedne bndwidth of 3 MHz ( GHz) nd t 5.8 GHz with n impedne bndwidth of 6 MHz ( GHz). However, the mesured results show tht the resonnt modes re exited t.51 nd 5.85 GHz simultneously, whih re

30 4 Development nd Implementtion of RFID Tehnology lmost the sme s tht from simultions. The mesured rdition ptterns t these two operting frequenies re presented in Figs. 34 nd 35, respetively. The mesured results show tht the rdition ptterns of the ntenn re brodside nd bidiretionl in the E- plne nd lmost omnidiretionl in the H-plne (x y plne). The mesured pek ntenn gins of the ntenn t.45 nd 5.8 GHz re -1.8 nd.3 dbi, respetively. Agreement between mesurement nd simultion is generlly good, nd the proposed design hs suffiient bndwidth to over the requirement of the RFID dul-bnd.45/5.8 GHz system. Fig. 3. Photogrph of the dul bnd tg ntenn prototype HFSS XFDTD mesured HFSS XFDTD mesured ().45GHz bnd (b) 5.8GHz bnd Fig. 33. Mesured nd simulted frequeny responses of the input return loss for the proposed ntenn A dul-bnd CPW-fed monopole ntenn hs been proposed nd implemented. With the open end nd the blne-shped strip fed by the CPW onneting to n SMA, the proposed ntenn n be designed to operte t the.45 nd 5.8 GHz bnds, nd to hve orresponding bndwidth of 13.1% nd 4.5%, respetively. A good rdition performne is lso hieved. The low-ost ntenn is only 3mm mm in size, mehnilly robust, nd esy to fbrite nd integrte with the pplition-speifi iruit. This design is not only suitble for the dul-bnd RFID systems, but lso pplible to the dul-bnd ommunition systems for WLAN pplitions.

31 Design of Antenns for RFID Applition 43 () E-plne pttern (b) H-plne pttern Fig. 34. Mesured fr-field rdition ptterns t.45 GHz for the proposed ntenn () E-plne pttern (b) H-plne pttern Fig. 35. Mesured fr-field rdition ptterns t 5.8 GHz for the proposed ntenn 6. Summry nd outlook In this hpter, the ntenn in RFID system is disussed, nd the design of ntenn is lso desribed. The min ontents inlude the sttus of the ntenn in the RFID system, the design method for the ntenn, the power trnsmission between the tg hip nd the tg ntenn, the tg ntenn design, the sheme nd design for the irulr polriztion, nd the design of ntenn for mirowve bnd RFID tg. These reserhes lmost over ll problems of the ntenn enountering in the RFID pplition. The onsidertions nd the design method re lso signifint for prtil pplitions. The development of the RFID tehnology for the prtil pplitions impels the dvnement of the ntenn in the RFID system, nd the progress of the ntenn lso promotes the spred of the RFID systems ll over our life nd soiety. In the future, the RFID system my require the multi-bnd ntenns for the high relibility, or the integrtion of severl ntenns for multifuntion. The unnotied ntenn is lso preferred for some RFID pplitions. The minituriztion of the ntenn is n eternl design trget in

32 44 Development nd Implementtion of RFID Tehnology designing the ntenn, nd reduing the ost of the ntenn for lrge-sle pplitions is lso signifint onsidertion in the ntenn design, whih will promote the development of the ntenn tehnology. The RFID pplitions in speil situtions my initite new reserh field for designing the RFID ntenns. 7. Referenes Abedin M. F. & Ali M. (5). Effets of smller unit ell plnr EBG struture on the mutul oupling of printed dipole rry, IEEE Antenns nd Wireless Propgtion Letters, Vol. 4, pp Abedin M. F. & Ali M.(5b). Effets of EBG refletion phse profiles on the input impedne nd bndwidth of ultr-thin diretionl dipoles, IEEE Trnstions on Antenns nd Propgtion, Vol. 53, No. 11, pp Abedin M.F. & Ali M. (6). A low profile dipole ntenn bked by plnr EBG struture, 6 IEEE Interntionl Workshop on Antenn Tehnology Smll Antenns nd Novel Metmterils, pp , Mrh 6-8, 6. Aloni E. & Kstener R. (1994). Anlysis of dul irulrly polrized mirostrip ntenn fed by rossed slots, IEEE Trnstions on Antenns nd Propgtion, Vol. 4, No. 8, pp Chen, S.Y. & Hsu, P. (4). CPW-fed folded-slot ntenn for 5.8 GHz RFID tgs, Eletronis Letters, Vol. 4, No. 4, pp Dobkin D. M. & Weignd S. M. (5). Environmentl effets on RFID tg ntenns, 5 IEEE MTT-S Interntionl Mirowve Symposium Digest, pp , June 1-17, 5. Fries M., Kossel M, Vhldiek R. & Bhtold W. (). Aperture oupled pth ntenns for n RFID system using irulr polriztion modultion. Proeedings of the Millennium Conferene on Antenns nd Propogofion, p. 358, Dvos, Switzerlnd, April 9-14,. Hirvonen M., Pursul P., Jkkol K. & Lukknen K.(4). Plnr inverted-f ntenn for rdio frequeny identifition, Eletronis Letters, Vol. 4, No. 14, pp Keskilmmi M, Sydänheimo L. & Kivikoski M. (3). Rdio frequeny tehnology for utomted mnufturing nd logistis ontrol. Prt 1: Pssive RFID system nd the effets of ntenn prmeters on opertionl distne, The Interntionl Journl of Advned Mnufturing Tehnology, Vol.1, No. 1-11, pp Kim S.-J., Yu B., Lee H.-J., Prk M.-J., Hrkiewiz F. J., & Lee B. (5). RFID Tg Antenn Moutble on Metlli Pltes, 5 Asi-Pifi Mirowve Conferene (APMC 5) Proeedings, Vol. 4, pp Kossel M., Benedikter H. & Behtold W. (1999). Cirulr polrized perture oupled pth ntenns for n RFID system in the.4 GHz ISM bnd, 1999 IEEE Rdio nd Wireless Conferene (RAWCON 99), pp , August 1-4, Kossel M.A., Kung R., Benedikter H. & Bhtold W. (1999). An tive tgging system using Cirulr-polriztion modultion, IEEE Trns. Mirowve Theory nd Tehniques, Vol. 47, No. 1, pp Liu, W.C. & Hu, Z.K. (5). Brodbnd CPW-fed folded-slot monopole ntenn for 5.8 GHz RFID pplition, Eletronis Letters, Vol. 41, No. 17, pp. 5 6.