Design of UHF Fractal Antenna for Localized Near-Field RFID Application

Transcription

1 1 Design of UHF Frctl Antenn for Loclized Ner-Field RFID Appliction Yonghui To, Erfu Yng, Yxin Dong, nd Gng Wng, Memer, IEEE Astrct In this pper, frctl structure is proposed for loclized ner-field UHF ntenn of rdio frequency identifiction (RFID) reder. In the design, trditionl Koch frctl structure is modified y converting the direction of one frctl structure of ech line segment. Prmeters of the frctl structure re optimized to cquire strong nd uniform mgnetic field distriution within n interrogtion zone t designted height over the ntenn. Two Koch frctl ner-field ntenns fed from center or side re presented. And the side-fed frctl ner-field ntenn is fricted nd tested to demonstrte loclized coverge re with perimeter of two wvelengths within reding distnce of 35mm ove the ntenn. For distnce ove 35mm, shrp decrese of reding rte is oserved, which demonstrtes stisfctory ner-field UHF ntenn. I Index Terms Antenn, Koch Frctl, RFID, UHF I INTRODUCTION N quite some of RFID-sed pplictions, UHF RFID tgs on ojects must e identified in ner field of UHF RFID reder ntenn [1, 2]. For exmple, UHF RFID reder ntenn on level of smrt shelf should restrict its coverge without interrogting tgs of ojects on other levels. With loclized coverge of ner-field reder ntenn, re on the level cn e further prtitioned with severl loclized RFID ntenns so tht exct oject position on the shelf level cn e cquired y interrogting logiclly the loclized RFID ntenns. Ordinry directionl UHF ntenn my not e suitle for such pplictions ecuse it is generlly designed for fr-field opertion. In recent yers, design of ner-field UHF ntenns for such RFID pplictions hs received much ttention, nd severl successful designs hve een reported sed on loop structure. Segmented loop with lumped cpcitors [3], coupled lines [4], distriuted cpcitor [5,6], dsh line [7,8], nd solid-line loop emedded phse shifter [9] re constructed to keep the current long the loop equl nd in-phse to get strong nd uniform This work ws supported in prt y the Ntionl Nturl Science Foundtion of Chin under Grnt Yonghui To is with the Deprtment of Engineering Science, University of Science nd Technology of Chin, Hefei 2327, Chin. Erfu Yng is with Spce Mechtronic Systems Technology Lortory (SMeSTech), Strthclyde Spce Institute, Deprtment of Design, Mnufcture nd Engineering Mngement, University of Strthclyde, 74 Jmes Weir Building, 75 Montrose Street, Glsgow G1 1XJ, United Kingdom. Ynxin Dong nd Gng Wng re with Key Lortory of Electromgnetic Spce Informtion nd Deprtment of Electronic Engineering nd Informtion Science, University of Science nd Technology of Chin, Hefei 2327, Chin. (e-mil: gwng1@ustc.edu.cn). mgnetic field over n electriclly-lrge interrogtion zone. However, it s hrd to relize the sme mgnetic field intensity t the centre of the loop s the mgnetic field in the regions close to the loop lines, which will cuse shrp gp. Menwhile, it s shown tht designing electriclly-lrge ner-field UHF RFID reder ntenn is ig chllenge, especilly when the perimeter of interrogtion zone reches three wvelengths or more [1]. The mjority of the ultrhigh frequency ner-field RFID reder ntenn designs re sed on the theory tht try to ensure the current long with the loop shring sme phse nd direction y the mens of dding vrious cpcitors. For exmple, in 27, Dokin et l proposed segmented loop ntenn, with dimeter of 1 cm demonstrted desirle performnce t 915 MHz, consists of series of metl line sections nd numers of cpcitors shping loop[3]. In[6], doule-c shped distriuted cpcitors. In 213, Shi et l presented dul loop ntenn composed of min nd prsitic loop which oth re constructed using segmented lines with fork-shped cpcitors [11]. And then, they proposed grid-rry ntenn composed of two segment loops to enlrge the interrogtion zone [1]. The mjor chllenge with design of ner-field UHF ntenn is how to cover n electriclly-lrge ut still ner-field region with strong nd uniform mgnetic field distriution. In this pper, we propose ner-field UHF RFID reder ntenn sed on Koch frctl structure. By optimizing the frctl structure, reding nulls should e voided in the entire covered ner-field region. Outside the restricted ner-field region, the field strength or the red re should decrese s fst s possile so tht more precise position of the RFID tg cn e locted. II KOCH FRACTAL CONFIGURATION Bsic Koch frctl structure is depicted in Fig. 1. For the need of designing ner-field UHF RFID reder ntenn, we modified the Koch frctl structure properly. As depicted in Fig. 1, the modified Koch frctl cn e defined y four vrile nmely (mm),(mm),n( recursive totl numer) nd. And the direction of one frctl structure of ech line segment is chnged, which cn mke the Koch curve more flexile nd homogeneous to fill the spce. At the sme time, to lessen the chnce of structure overlpping in the ntenn optimiztion process, s the recursive totl numer (N) increses ech time, only the lines generted in lst frctl process will e replced y frctl curves. The dvntges of using Koch frctl structure for UHF RFID reder ntenn lie in its self-similrity nd filling-ility.

2 Return Loss (db) 2 Due to the self-similrity, the four prmeters cn uild model. When the Koch frctl is used s UHF ntenn due to the trvelling-wve current on the frctl, rdition will occur t ech turn of frctl structure. Therefore, y properly designing the frctl geometry, uniform ner-field coverge cn e cquired over the re covered y the Koch frctl. As n ntenn, Koch frctl cn e fed either from the center s shown in Fig. 2, or from the side s shown in Fig. 2. For the side-fed structure it is quite convenient to enlrge the ner-field coverge re y forming networks. KOCH N= KOCH N= where S mens return loss t 915MHz, 11 H represents z z-component of mgnetic field to e designed for uniform coverge, represents the re to e covered in ner-field region. A. Center-fed Koch-Frctl Antenn Firstly, we consider center-fed Koch-Frctl Antenn covering ner-field squre. The model of the ntenn is shown in Fig /2 N=1 α N=1 N=2 N=2 y N=3 Fig. 1 Bsic Koch frctl structure, modified Koch frctl structure. N=3 Feed point Fig. 3 Optimized center-fed Koch frctl ntenn structure nd its current distriution. x ~ y (mm) H(A/m,dB) +3 ~ 8 +2 Fig. 2 Koch frctl ntenn fed from the center, or from the side. III KOCH FRACTAL NEAR-FIELD ANTENNA DESIGN Filling ility is nother fether tht help fcilitting the structure uniformity of the interrogtion zone. In this study, to cquire squre interrogtion zone, the modified Koch frctl trnsformtion is pplied to every side of squre with perimeter of pproximtely 2 t 915 MHz (viz., the size of the ntenn is 16mm16mm). The comintion of model code nd 4NEC2 s source code contriutes to the rpid optimiztion nd good results [12]. In this simultion, only one prmeter is kept unchnged, while three prmeters re vried t time. In the code, rnges of the three prmeters hve een set. When the process is on, the figures of these rnges could e optimized. It is illustrted tht the good results will e found s long s the oundry conditions prolems eing settled. The ojective functions for impednce mtching (f 1 ) nd mgnetic field distriution (f 2 ) re defined s 1 f 1 S ( db) (1) f 2 11 min H z( db) mx H z( db) min H z( db) mx(, ),(2) min x (mm) Fig. 4 Dynmic rnge of distriution of mgnetic field intensity on plne z = 3 mm Fig. 5 Return loss of the center-fed Koch frctl ntenn. Fig. 3 shows the lyout of optimised center-fed Koch-frctl ntenn (α=68.45, =53.6mm, =8mm, rtio=/=.67) nd the current distriution. Fig. 4 shows the distriution of mplitude of z-component of mgnetic field, Hz (A/m), in db on the xy-plne of z = 3 mm. And the dynmic rnge of Hz is mesured to e less thn 3.5dB in the whole interrogtion

3 Return Loss (db) 3 zone(x16mm, y16mm). Dynmic rnge of 3.5dB indictes quite stisfctory uniform coverge, which is superior to the results cquired in the present UHF ner-field RFID reder ntenns. Fig. 5 shows the simulted return loss t 915 MHz. It is oserved tht the optimised center-fed Koch-frctl ntenn hs stisfctory input impednce mtching over frequency nd MHz. B. Side-fed Koch-Frctl Antenn However, in prcticl ppliction, multiple RFID reder ntenns my work together to enlrge the interrogtion zone or relize hierrchicl nd regionl position. In these cses, side-feeding should e dopted to fcilitte plcing nd feeding ntenns. Therefore, we consider side-fed Koch-frctl ntenn s shown in Fig. 2. y Feed point Fig. 6 Optimized side-fed Koch frctl ntenn structure nd its current distriution. y (mm) x Hz( A/m,dB) min x (mm) Fig. 7 Dynmic rnge of distriution of mgnetic field intensity on plne z = 3 mm. Fig. 6 shows the lyout of optimised side-fed Koch-frctl ntenn (=54.28,=32.8mm,=8mm,rtio=/=.41)nd the current distriution. The Fig. 7 shows the distriution of mplitude of z-component of mgnetic field, Hz (A/m), in db on the xy-plne of z = 3 mm. From Fig. 7, it is mesured tht the dynmic rnge of Hz is within 3dB in the whole interrogtion zone(x16mm, y16mm). Fig. 8 illustrtes the simulted return loss t 915 MHz. It is found tht the optimised Koch-frctl ntenn hs stisfctory input impednce mtching over frequency nd MHz Fig. 8 Return loss of the side-fed Koch frctl ntenn. In conclusion, y pplying modified Koch-frctl structures, the UHF RFID ntenn cn provide perfect impednce mtching, s generting mgnetic field distriution eing more uniform thn tht of trditionl ner-field UHF loop ntenns [8-12]. Tht s ecuse the filling ility of modified Koch frctl structure cn help ntenn improve the structure uniformity. And owing to the Koch frctl structure s self-similrity, model of the ntenn is uilt to optimize Koch frctl structures to cquire optimized current distriution, which cn generte mgnetic field in interrogtion zone s uniform s possile. IV FABRICATION AND TEST Prototype of the designed side-fed Koch-frctl ntenn is fricted y using copper wires on pper ord, s shown in Fig. 9. The pper ord hs dimensions of mm, thickness of 3.5 mm, nd dielectric constnt of ɛ r = 2.5. At the feeding port, n SMA connector of 5Ω input impednce is directly connected to the two ends of the feeding copper wire of the ntenn, s shown in the zoomed picture in Fig. 9. The impednce mtching of the prototype ws mesured using Agilent E523A vector network nlyzer. The simulted nd mesured return loss of the ntenn plotted in Fig. 1, show good greement. Fig.9 Photo of the side-fed Koch-frctl ntenn prototype.

4 Reding Rte (%) Return Loss (db) 4 Simultion -1 Mesurement -2 eder Fig. 1 Simulted nd mesured S 11 of the side-fed Koch-frctl ntenn. Styrofom Antenn Reference Tg PC Reder Fig. 11 Ner-field RFID mesurement setup nd reference tg. Ner-field tg Tg d(mm) Fr-field tg Fig. 12 Mesured reding rte ginst reding distnce (d) of the side-fed Koch-frctl ntenn prototype. d Reder ntenn To verify the ner-field coverge performnce, the prototype of designed side-fed Koch-frctl ntenn is pplied to detect single reference tg. In the detection setup shown in Fig. 11, the ntenn prototype is connected to Alien ALR 99+ RFID reder, which my operte over MHz RFID nd with n output power of 24 dbm. Impinj utton type ner-field UHF tg, J41, with loop-like ntenn of dimeter of 12 mm, is tken s reference tg. (c) Fig. 13 Distriution of ctive grids on styrofom plte t severl typicl reding distnce d d=35mm, d=4mm, (c)d=45mm, (d)d=5mm. As shown in Fig. 11, single J41 tg is supported y squre Styrofom plte with dimension of mm nd ɛ r = On the Styrofom plte, locl coordintes is set nd the Styrofom plte is gridded for esy positioning of the tg on the xy-plne. By rising the plte to different height, nd deploying the reference tg t different coordintes in turn on the squre Styrofom, fce-to-fce to the Koch-frctl reder ntenn, the coordintes(ctive grids) t which the reference tg cn e interrogted y the RFID reder cn e mesured. And then y further nlyzing the distriution of ctive grids, the ner-field coverge performnce of ntenn prototype cn e evluted. Fig. 12 depicts the mesured reding rte ginst reding distnce (d) of the side-fed Koch-frctl ntenn prototype. It is oserved tht 1% reding rte is chieved within mximum distnce of 35 mm. Furthermore, s the reding distnce is ove 35mm, the reding rte rpidly declines from 1% of 35mm to % of 55mm. Such shrp decrese is eneficil for tg orienttion nd could e hrd to otin for conventionl Ner-field UHF loop ntenns [8-12]. The distriution of the ctive grids mrked y lck squres of dimensions of 1 1mm t severl typicl reding distnce (d=35mm, d=4mm, (c)d=45mm, (d)d=5mm) is shown is Fig. 13. It is oserved tht there is no reding null t the reding distnce of 35mm. Furthermore, it is lso found tht there is no reding null in the middle of the Styrofom plte t different reding distnce. This is coincided with the simultion results depicted in Fig. 7, tht with the designed side-fed Koch-frctl ntenn lrger mgnetic field intensity cn e cquired in the region round the centre of the interrogtion zone, rther thn round the outside frme. V CONCLUSION The mjor chllenge with design of ner-field UHF ntenn is how to cover n electriclly-lrge ut still ner-field region with strong nd uniform mgnetic field distriution. Frctl structures crete lot of possiilities for the ntenn designs. In this pper, design of ner-field UHF RFID ntenns sed on modified Koch frctl structure is presented. The simultion nd experiment results indicte tht the optimised Koch-frctl ntenns fed from the center or side oth cn generte (d)

5 5 loclized coverge re with dimension of.5.5 within reding distnce of 35cm ove the ntenn. As providing perfect impednce mtching, they generte mgnetic field distriution eing more uniform thn tht generted y conventionl loop ntenns. Koch frctl structure is very promising for ner-field UHF RFID pplictions. Firstly, the filling ility of frctl structures cn help fcilitting the structure uniformity of the interrogtion zone. And then due to the self-similrity, frctl structures prmeters cn e used to uild model, which comined with 4NEC2 s source code to otin excellent results rpidly. ACKNOWLEDGMENT This work ws supported in prt y the Ntionl Nturl Science Foundtion of Chin under Grnt REFERENCES 1. P. V. Nikitin, K. V. S. Ro, nd S. Lzr, An overview of ner field UHF RFID, in Proc. IEEE Int. Conf. RFID, pp , X. Qing nd Z. N. Chen, UHF ner-field RFID ntenns, in Proc. Int. Workshop Antenn Technol., pp. 1 4, M. Dokin, S. M. Weignd, nd N. Iyec, Segmented mgnetic ntenns for ner-field UHF RFID, Microw. J., vol. 5, no. 6, R. A. Oliver, Broken-loop RFID reder ntenn for ner field nd fr field UHF RFID tgs, U.S. design ptent D57, 337 S, Jun, Y. S. Ong, X. Qing, C. K. Goh, Z. N. Chen, A segmented loop ntenn for UHF ner-field RFID, in Proc. IEEE Antenns Propg. Soc. Int. Symp., pp. 1 4, X. Qing, Z. N. Chen, nd C. K. Goh, UHF ner-field RFID reder ntenn with cpcitive couplers, IET Electron. Lett., vol. 46, no. 24, pp , Dec X. Qing, C. K. Goh, nd Z. N. Chen, Segmented loop ntenn for UHF ner-field RFID pplictions, IET Electron. Lett., vol. 45, no. 17, pp , X. Qing, C. K. Goh, nd Z. N. Chen, A rodnd ner-field UHF RFID ntenn, IEEE Trns. Antenns Propg., vol. 58, no. 12, pp , Z. N. Chen, C. K. Goh, nd X. Qing, Loop ntenn for UHF ner-field RFID reder, in Proc. 4th Eur. Conf. Antenn Propg., Brcelon, Spin, pp. 1 4, J. Shi, X. Qing, nd Z. N. Chen, Electriclly lrge zero-phse-shift line grid-rry UHF ner-field RFID reder ntenn, IEEE Trns. Antenns Propg., vol. 62, no. 4, pp , J. Shi, X. Qing, Z. N. Chen, nd C. K. Goh, Electriclly lrge dul-loop ntenn for UHF ner-field RFID reder, IEEE Trns. Antenns Propg., vol. 61, no. 3, pp , D. Ding, Y. H. To, G. Wng, Design of ner-field ultr-high frequency rdio-frequency identifiction ntenn with frgmented wires for electriclly-lrge coverge, IET Microw. Antenns Propg., vol. 1, pp , 216.