Interntionl Journl of Electronics Engineering, 4 (), 0, pp. 3 9 Serils Publictions, ISSN : 0973-7383 Novel Drgon Shpe UHF RFID Tg Antenn T.G. Abo-Elng, E.A.F. Abdllh, H. El-Hennwy 3, Microstrip Dept., Electronics Reserch Institute, Giz, Egypt, E-mil: tgber00@yhoo.com, esmt@hotmil.com 3 Electronics nd Communictions Dept., Ain Shms University, Ciro, Egypt, E-mil: helhennwy@ieee.org Abstrct: Novel modified drgon shpe UHF RFID tg ntenn is proposed where both fleibility nd compctness fetures re chieved. Choosing LCP mteril s ntenn substrte nd modified drgon curve s ntenn shpe both fleibility nd compctness fetures re chieved, respectively. Induced EMF method is used considering wire rdius effect to obtin new epression for the input impednce of conventionl dipole. Modified drgon shpe is pplied to the conventionl dipole nd the obtined shpe is conjugte mtched to the RFID tg chip. The obtined results re compred with previous published ones. Structure RCS nd CMF re investigted. PRC of the proposed structure is computed, mesured nd discussed. The proposed structure is novel, chep, fleible, compct, widebnd nd suitble for UHF RFID pplictions. Keywords: Rdio Identifiction (RFID), Induced Electromotive Force (EMF), Rdr Cross Section (RCS), Conjugte Mtch Fctor (CMF), drgon curve, Liquid crystl polymer (LCP).. INTRODUCTION UHF rdio frequency identifiction (RFID) systems hve drwn more nd more ttentions in wide vriety of pplictions such s rel-time trcking in retil stores nd hospitls, wrehouse mngement, ccess control system, electronic toll collection, etc. In this regrd, gret demnd of UHF RFID system is epected to replce the current position of brcode system. UHF frequencies such s 90.5-94.5 MHz in Chin, 90-96MHz in Austrli, 866-869 MHz nd 90-95 MHz in Singpore nd 90-98 MHz in USA, Cnd, Meico, Puerto Rico, Cost Ric, Ltin Americ, nd so on [] re used for RFID pplictions. RFID system consists of n ntenn nd trnsceiver, which red the rdio frequency nd trnsfer the informtion to processing device (reder) nd trnsponder, or RF tg, which contins the RF circuitry nd informtion to be trnsmitted. The ntenn provides the mens for the integrted circuit to trnsmit its informtion to the reder tht converts the rdio wves reflected bck from the RFID tg into digitl informtion tht cn then be pssed to computers tht cn nlyze the dt. The min objective of this pper is to introduce novel, fleible nd compct UHF RFID tg ntenn. Novelty is chieved since the proposed structure hs unique shpe. Compring the proposed ntenn with re of 45 mm with metllic slots RFID tg ntenn which occupies n re of 5776 mm [], n ntenn with brodbnd chrcteristic which occupies n re of 4664 mm [3], dipole with prsitic lines with n re of 400 mm [4], tg ntenn using novel feed method with n re of 50 mm [5], tg ntenn mounted on glss objects with n re of 3600 mm [6], slot RFID tg ntenn with n re of 800 mm [7], modified widebnd dipole ntenn for pssive UHF RFID with n re of 3300 mm [8], conventionl mender line ntenn with re of 69.5 mm nd tpered mender line ntenn with re of 465.85 mm [9] nd U-shped ntenn with re of 9 mm [0], so compctness is lso chieved. Liquid crystl polymer LCP copper cld substrte with dielectric constnt r =.9, height of 0.05 mm nd loss tngent tn = 0.005 is used s ntenn substrte where fleible, relible nd low cost fetures re provided. The proposed modified drgon ntenn chieves frequency bnd from 878 MHz to 7 MHz which covers the UHF RFID frequency bnd for the previous mentioned countries. Computed rdr cross section RCS nd conjugte mtch fctor CMF re considered [] using IE3D 4. simultor. RCS is mesure of how detectble n object is with trnsmitter, lrger RCS indictes tht n object is more esily to be detected by the reder nd CMF is used to judge how good n RFID ntenn is in the relity nd it equls to t perfect conjugte mtching between the tg nd the chip input impednce. Computed nd mesured power reflection coefficient PRC [-4] re introduced nd discussed. Other ntenn prmeters such s rdition efficiency, gin nd rdition pttern re obtined. The pper is orgnized s follows: Section introduces briefly the induced EMF technique which is dopted in this study to predict the conventionl dipole input impednce considering the wire rdius. Net, section 3 presents the T-mtched chrt, while the numericl nd eperimentl results re introduced in section 4 nd 5. Conclusions re given in section 6.. ANALYSIS OF WIRE DIPOLE USING EMF METHOD The induced EMF method is the technique used to derive closed-form epression for the input impednce of finite
4 Interntionl Journl of Electronics Engineering liner dipole with rdius nd length l through the clcultion of the current distribution nd tngentil electric field long the surfce of the wire. Bsed on [5] nd follow the sme procedure but with considering the wire rdius, generl induced EMF input impednce z of dipole is derived referring to Figure nd is found to be: 4 ( e ) ( Ci ( k ) jsi ( k )) kl cos ( Ci ( k ) j Si ( k ( e ) ( C ( k ) js ( k )) ( e ) ( C ( k ) js ( k )) i 3 i 3 i 4 i 4 ( e ) ( C ( k ) j S ( k )) Where i 5 i 5 3,,, ( ) nd ( ), 4 5 C ( ) i cos ( ) d () j { Si ( kl) cos( kl) [ Si ( kl) Si ( kl)] sin( kl) 4 k Ci ( kl) Ci ( kl) Ci } Bsed on Eq. () nd Eq. () mtlb code is built nd the results re shown in Figure. As shown in Figure () good greement is found for the impednce rective prt which is clculted using Eq. () nd Eq. (), noting tht the curves re very close t wire rdius equl 0.005 nd 0.0005. As shown in Figure 3(b) s rdius becme less thn 0.075 ll computed impednce resistive prt curves using Eq. () or Eq. () re nerly the sme. It should be mentioned tht using Eq. (), the resistive prt of the wire dipole input impednce is.3,.688,.696 nd.693 for rdius equls to 0.075, 0.05, 0.005, 0.0005, respectively. The mimum difference between the curves obtined by both equtions t / = 0.9 is bout 0 (which is pproimtely 5% from tht obtined using Eq. (). This mens tht Eq. () is the generl eqution for clculting for ny vlue of, while Eq. () is specil cse when is very smll s compred to wvelength. () S ( ) i 0 sin ( ) d = 0, k = / nd C = 0.5775665 For / << the following pproimtion could be used s: = 3 Figure : Dipole Geometry 4 l 5 l Ci ( ) c ln( ), The obtined result will be ectly s [7]: { C ln( kl) Ci ( kl) sin( kl) [ Si ( kl) kl Si ( kl)] cos( kl) C ln Ci ( kl) Ci ( kl) } Figure (): Dipole Rectnce Versus Normlized Length / for Different rdii Using Induced EMF Method
Novel Drgon Shpe UHF RFID Tg Antenn 5 u w w v re re b 0.5( w ) 0.5 ( w) 0.5 ( w) Figure (b): Dipole Resistnce Versus Normlized Length / for Different Rdii using induced EMF method A conventionl / dipole with rdius = 0.005 (prcticl vlue) nd input impednce of 73.3 + j4.54 will be suitble for the RFID tg ntenn since it could be conjugte mtched esily with the chip s input impednce of 3.9 j43.6 [6] t 900 MHz by simply dding prllel RC circuit. As given by [6], it is worth to mention tht the operting frequency for the chip will cover the bnd from 860MHz to 960 MHz, nd its input impednce vries from 5.69 j5.6 Ohm t 860 MHz to.6 j37 t 960 MHz. Chip input impednce of 3.9 j43.6 t 900 MHz is chosen s the vlue to be mtched with the designed ntenn. By choosing this vlue the designed ntenn input impednce will be remin in n cceptble level of vrition reltive to the vrince of the chip input impednce with frequency. 3. T-MATCHED CHART T-mtched chrt is n effective method to mtch the chip input impednce with dipole input impednce. Referring to Figure 3, dipole of length l is connected to second dipole of length d, plced t close distnce b from the first nd lrger one. It cn be proved, tht the impednce t the source point is given by [7]: Where in 0 t t ( ) ( ) cosh cosh t j0 tn cosh v v k d b / r r e u v u uv e (3) Figure 3: T-mtched Configurtion for Plnr Dipoles nd its Equivlent 4. PROPOSED ANTENNA NUMERICAL RESULTS A drgon curve is member of fmily of self-similr frctl curves, which cn be pproimted by recursive methods such s Lindenmyer systems [8]. The Heighwy drgon (lso known s the Hrter-Heighwy drgon) ws first investigted by NASA physicists John Heighwy, Bruce Bnks, nd Willim Hrter. It cn be described this wy, strting from bse segment, replce ech segment by segments with right ngle nd with rottion of 45 lterntively to the right nd to the left s shown in Figure 4 for first, second, third, fourth nd fifth order drgon curve. A mtlb code ws built to generte drgon curve of ny order n, s shown in Figure 5 for n = 8. The implemented Mtlb code in ddition to the IE3D simultor re used to optimized the prototype drgon dipole ntenn input impednce to be mtched with the chip input impednce of 3.9 j43.6. As first step the wire dipole structure ws trnsferred to printed one using [9]. Figure 6 shows the optimized modified drgon structure (n = 8). Compring the proposed modified drgon dipole ntenn with length of 7.5 mm with the printed / dipole with length of 66.666 mm t 900 MHz, it is cler tht 4.9% length reduction ws chieved scrificing the ntenn width which reches 30 mm. Figure 4: Drgon Curve Implementtion
6 Interntionl Journl of Electronics Engineering surveillnce pplictions. Other ntenn prmeters re shown in Figures. () nd (b) where, conjugte mtch gin nd rdition efficiency re t cceptble level. Figure 5: Drgon Curve for n = 8. Figure 7: Drgon Dipole Input Impednce Versus Figure 6: The Proposed Modified Drgon Dipole Antenn Structure The ntenn width spce would contin the T-mtched section so the proposed ntenn still hs the length reduction dvntge. As shown in Figure 7, the drgon dipole ntenn ws found to hve n input impednce of.70 j89.4 t 900 MHz which will be mtched through using the T mtch digrm shown in Figure 8. T mtch digrm shows tht mtched condition could be met through the vlue of 7.54 s rel prt, 68.95 s imginry prt nd these vlues will be decresed s d / decreses. Since we could not determine the drgon ntenn equivlent length very ccurtely we begin by choosing b/w =.5 nd using the IE3D simultor to optimize the input impednce to the chip input impednce of 3.9-j43.6. Figure 9 shows the proposed designed modified drgon dipole ntenn with its T mtched circuit nd its reflection coefficient reltive to the chip input impednce of 3.9 j 43.6 is shown in Figure 0 which indictes tht the bndwidth covers the bnd from 878 MHz to 6 MHz which is very suitble for the world wide UHF RFID frequency bnds. As shown in Figure () t 900 MHz the ntenn s RCS is 0.004 m which mkes its detection n esy tsk for the reder. Also, the proposed ntenn introduces CMF = 0.755 t 900 MHz s shown in Figure (b) which indictes tht there is good mtching between tg nd the chip. Figure (c) shows the rdition pttern t 900 MHz which is lmost omnidirectionl t the H-plne nd figure of 8 t the E-plne. This rdition pttern is very suitble for the RFID Figure 8: The T Mtched Digrm for the Modified Drgon Dipole Antenn Figure 9: The Proposed Plnner Modified Drgon Dipole Antenn with Its T Mtched Circuit
Novel Drgon Shpe UHF RFID Tg Antenn 7 Figure 0: Modified Drgon Dipole Antenn PRC Versus Figure (): Modified Drgon Shpe Antenn RCS Versus Figure (b): Modified Drgon Shpe Antenn CMF Versus Figure (c): Modified Drgon Dipole Antenn Rdition Pttern t 900 MHz Figure (): Modified Drgon Dipole Antenn Conjugte Mtch Gin Figure (b): Modified Drgon Dipole Antenn Rdition Efficiency
8 Interntionl Journl of Electronics Engineering Figure (c): Modified Drgon Dipole Antenn Conjugte Mtch Efficiency 5. PROPOSED ANTENNA EXPERIMENTAL RESULTS The modified drgon dipole is fbricted using LCP substrte s shown in Figure 3(). Input impednce of the modified drgon dipole ws mesured by mesuring the hlf of the structure over ground plne nd multiply the resultnt input impednce by two, then, simply PRC ws clculted. As shown in Figure 3(b), there is different between the simulted PRC denoted by PRCs nd the mesured PRC denoted by PRCm. Inserting n ir gp of mm t the symmetry plne of the proposed structure, PRCs ws obtined which grees well with the mesured one. Air gp ws inserted in the simultion minly due to the presence of the ir gp between the hlf modified drgon structure nd the ground plne which is hrdly to be eliminted due to the fleible nture of the LCP substrte. Figure 3(): Fbricted Modified Drgon Shpe Dipole on LCP Substrte Figure 3(b): Mesured, Computed nd Verified PRC of Modified Drgon Shpe Dipole 6. CONCLUSIONS In this pper modified drgon fleible UHF RFID tg ntenn ws introduced. Induced EMF technique ws used for wire dipole input impednce clcultion. Size reduction ws chieved through n re of 46 mm tht modified drgon dipole shpe ntenn occupies compred with previous published results. The proposed modified drgon dipole covers the frequency bnd from 878 MHz to 7 MHz indicting tht the ntenn is suitble for the UHF RFID frequency bnd t different countries. Computed rdr cross section RCS nd conjugte mtch fctor CMF were introduced nd discussed for the proposed ntenn to insure tht the proposed ntenn structure is esy to be detectble nd chieves cceptble mtching level. Both computed nd mesured power reflection coefficients PRC were investigted. Other ntenn prmeters such s rdition efficiency, gin nd rdition pttern were clculted nd found to be t cceptble level. The proposed ntenn hs unique shpe nd is chep, fleible nd introduces wide frequency bnd suitble for UHF RFID pplictions. References [] H. Brthel, Regultory Sttus for RFID in the UHF Spectrum, EPC Globl, Brussels, Belgium, Mrch 009. [] X. eng, J. Siden, G. Wng nd H. Nilsson, Slots in Metllic Lbel s RFID Tg Antenn, IEEE Antenns nd Propgtion Society Interntionl Symposium, pp. 749-75, 007. [3] L. Xu, B.J. Hu nd J. Wng, UHF RFID Tg Antenn with Brodbnd Chrcteristic, Electronics Letters, 44(), pp. 79-80, 008. [4] P. Wongsiritorn, C. Phongchroenpnich, D. Torrungrueng nd M. Kririksh, UHF-RFID Tg Antenn Design Using Dipole with Prsitic Lines, IEEE 6th Interntionl Conference on Electricl Engineering/Electronics, Computer,
Novel Drgon Shpe UHF RFID Tg Antenn 9 Telecommunictions nd Informtion Technology ECTI-CON, pp. 794-797, 009. [5] P. Yng, J. Hung, W. Chew nd T. Ye, Design of Compct nd Reusble Pltform-tolernt RFID Tg Antenn Using Novel Feed Method, Asi Pcific Microwve Conference APMC, pp. 64-644, 009. [6] J. Lu, J. Wu, UHF Bnd RFID Tg Antenn Mounted on Glss Objects, Asi Pcific Microwve Conference APMC, pp. 6-64, 009. [7] W. eng nd J. ho, A Comb-shped Slot RFID Tg Antenn, Antenns nd Propgtion Society Interntionl Symposium, pp. -4, 00. [8] A. Sbwi nd K. Qubo, Widebnd Modified Dipole Antenn for Pssive UHF RFID Tgs, IEEE 7th Interntionl Multi-Conference on Systems, Signls nd Devices (SSD), pp. -4, 00. [9] B. Sho, Q. Chen, R. Liu, L. heng, Linerly-tpered RFID Tg Antenn with 40% Mteril Reduction for Ultr-lowcost Applictions, IEEE Interntionl Conference on RFID, pp. 45-49, 0. [0] S. Lee, H. Jung, H. Choo nd I. Prk, Design of U-shped RFID Tg Antenn with n Isotropic Rdition Chrcteristic, Interntionl Workshop on Antenn Technology (IWAT), pp. 306-309, 0. [] IE3D User s Mnul Relese 4., elnd Softwre, Inc. My 008. [] L. Mo nd C. Qin, Plnr UHF RFID Tg Antenn with Open Stub Feed for Metllic Objects, IEEE Trns. Antenns Propgtion, 58(9), pp. 3037-3043, Sep. 00. [3] S. Chen nd R. Mittr, Indirect Coupling Method for RFID Tg Antenn Design, Electronics Letters, 46(), pp. 8-0, 00. [4] C.S. Lin nd R. Mittr, A Triple Feed nd Ner Omnidirectionl (3D) RFID Tg Antenn Design, Asi Pcific Microwve Conference APMC, pp. 39-393, 009. [5] C.A. Blnis, Antenn Theory Anlysis nd Design, John Wiley & Sons, Inc, 3rd Ed., 005. [6] Higgs- Integrted Single Chip UHF RFID Tg IC Dt Sheet, EPC Globl Clss Gen, Alien Technology Corportion, July 008. [7] Q.X. Chu, L. Wng, J. K. hou, A Novel Folded T-mtched Dipole in Bse Sttion, Int. Conf. Microwve nd Millimeter Wve Technology ICMMT, pp. -3, 007. [8] G. Rozenberg nd A. Slom, The Mthemticl Theory of L Systems, Acdemic Press Inc., New York, 980. [9] C.H. Butler, The Equivlent Rdius of Nrrow Conducting Strip, IEEE Trns. Antenns Propgtion, AP-30(4), pp. 755-758, July 98.