Electriclly Lrge Zero-Phse-Shift Metmteril-bsed Grid Arry Antenn for UHF Ner-Field RFID Reders Jin Shi, Xinming Qing, member IEEE, Zhi Ning Chen, Fellow, IEEE Abstrct A grid rry ntenn using zero-phse-shift (ZPS) metmteril-bsed lines is proposed to enlrge the interrogtion zone of reder ntenns used in ner-field ultr high frequency (UHF) rdio frequency identifiction (RFID) systems. The proposed grid rry ntenn is composed of number of grid cells nd double-sided prllel-strip line feeding network. Ech grid cell, nmely segmented loop constructed by the ZPS lines, hs uniform nd single direction flowing current itself. By configurtion of the cells shring common side with its djcent grid cells which feture reverse-direction flowing current, grid rry ntenn is formed to generte strong nd uniform mgnetic field distribution over lrge interrogtion zone even when the perimeter of the interrogtion zone reches up to 3 (where is the operting wvelength in free spce) or lrger. As n exmple, grid rry ntenn with segmented ZPS loop cells implemented onto piece of FR4 printed bord (PCB) is designed nd prototyped. The results show tht the grid rry ntenn chieves the impednce mtching over the frequency rnge from 79 to 4 MHz nd produces strong nd uniform mgnetic field distribution over n interrogtion zone of 38 mm 5 mm. Index Terms Antenn rry, grid rry ntenn, ner-field, RFID, segmented zero-phse-shift loop, UHF. U I. INTRODUCTION LTRA-HIGH frequency (UHF) ner-field rdio frequency identifiction (RFID) becomes more nd more ttrctive becuse of the promising opportunities in item-level RFID pplictions, such s sensitive products trcking, phrmceuticl logistics, trnsport, medicl products, nd bio-sensing pplictions [-5]. The inductive coupling between the reder ntenns nd tgs is preferred for the most UHF ner-field RFID pplictions becuse it is cpble of operting in close proximity to metls nd liquids [6-7]. On the other hnd, lrge interrogtion zone of the reder ntenn is ble to detect more tgs simultneously nd mke RFID system more efficient. The top chllenge of the UHF ner-field Mnuscript received November ##,. The work ws prtilly supported by Agency for Science, Technology nd Reserch (A*STAR), Singpore, A*STAR SERC Metmteril Progrm: Met-Antenns (9 54 97). J. Shi, X. Qing nd Z. N. Chen re with the Institute for Infocomm Reserch, A*STAR, Singpore. (e-mil: jshi@ir.-str.edu.sg; qingxm@ir.-str.edu.sg nd chenzn@ir.-str.edu.sg) Z. N. Chen is lso with Ntionl University of Singpore, Singpore. (e-mil: eleczn@nus.edu.sg). RFID reder ntenn design is to generte strong nd uniform mgnetic field distribution over n electriclly lrge interrogtion zone re. Some designs hve been reported to ddress the design of electriclly lrge single-loop ntenns for UHF ner-field RFID reders. The key design principle of such works is to ensure tht the current is of equl mgnitude nd in-phse long the loop. Dobkin et l. presented segmented loop ntenn loded with lumped cpcitors [8]. Oliver conceptully proposed three broken-loop ntenn ptents using triple lines, double lines nd single lines [9-]. Qing et l. proposed the segmented loops using distributed cpcitors [-3] or dsh lines [4-6]. In ddition, the phse shifter embedded solid-line loop [7] nd dul-printed dipoles [8] were utilized to get the in-phse current long the loops s well. The reported single segmented loop ntenns re with limited interrogtion zone, the perimeter of the interrogtion zone is less thn ( is the operting wvelength t 95 MHz in free spce). To further enlrge the interrogtion zone, the uthors introduced dul-loop segmented line ntenn [9], wherein the perimeter of the interrogtion zone is up to 3. In this pper, we propose the methodology of using grid rry ntenn to enlrge the interrogtion zone of the RFID reder ntenn. The perimeters of the interrogtion zone of the proposed ner-field grid rry ntenns cn be enlrged up to 3 or lrger. Comprisons between the ner-field grid rry ntenn nd other pproches show tht the proposed ntenn cn provide stronger nd more uniform mgnetic field distribution over the interrogtion zone. A grid rry ntenn prototype with segmented zero-phse-shift (ZPS) loop cells is investigted nd vlidted experimentlly. The procedure to implement this ntenn prototype is ddressed with prcticl guideline. II. TOPOLOGIES OF UHF NEAR-FIELD GRID ARRAY ANTENNAS The grid rry ntenn is proposed by Krus in 964 []. It fetures the dvntges of high gin, nrrow bem, nd low side lobes nd hs been used s fr-field ntenn for yers [-]. However, there is no report found in open litertures of such ntenns in ner-field pplictions. A. Configurtion of the rry ntenns
To cler the principle of the UHF ner-field grid rry ntenn, design exmple is studied first. Fig. shows the topologies of the proposed grid rry ntenn nd other rry ntenns. The cell should be loop with in-phse current, which cn keep the mgnetic field inside ech single loop strong nd uniform nd be esy to configure rry ntenns with more cells. In principle, the size of cell is not criticl while lrger loop is more preferble considering the complexity nd the loss of the feeding network. Therefore, the segmented ZPS loop is more suitble to be the cell of the ner-field rry ntenn. Antenns A nd B re of conventionl ntenn rry configurtion, the segmented ZPS loops re seprted by distnce, d. The currents long the two loops re in-direction nd reverse-direction, respectively. To relize such current distribution, the two output ports of the feeding network for Antenns A nd B should be in-phse nd 8 o out-of-phse, respectively. Antenn A Antenn B Antenn C Antenn D B B Loop Loop A A d () d B B Loop Loop y o A(A ) (c) A(A ) B(B ) Loop Loop (d) x A A B(B ) Loop Loop Fig.. Topologies of the ner-field rry ntenns with cells, () Antenn A: seprted two loops with in-direction current; Antenn B: seprted two loops with reverse-direction current; (c) Antenn C: grid rry ntenn with in-direction current; (d) Antenn D: proposed grid rry ntenn with reverse-direction current. Antenns C nd D re the grid rry ntenn configurtion, wherein the loops shre common side AB (or A B ). The currents long the two loops re in-direction nd reverse-direction, respectively. To relize such current flowing, the two output ports of the feeding network in Antenns C nd D should be in-phse nd 8 o out-of-phse, respectively. Antenn C is similr to lrge segmented ZPS loop with doubled re becuse the currents long the common side AB (or A B ) of ech loop re cncelled ech other so tht there is hrdly current on the middle line. Insted, the Antenn D hs current long the common side AB (or A B ). Mgnetic field distribution of rry ntenns The mgnetic field distribution of the rry ntenns re compred by simultion using IE3D softwre [3]. Fig. shows the cell utilized to configure the rry ntenns. The segmented ZPS loop is similr to tht in Ref. 6. The four rry ntenns re designed onto the FR4 substrte (thickness of.5 mm, reltive dielectric constnt of 4.4, nd loss tngent of.). The feeding networks re designed properly to chieve desired impednce mtching nd provide required phse t the ports. 54 5 55.65 55.65 54 55.3 5.5 RF in 55.5 55.5 5.85 5.85 55 5.7 Unit: mm Fig.. The configurtion nd detiled dimensions of the segmented ZPS loop cell. The mgnetic field distribution (H z ) of the two rry ntenns, Antenns A nd B, with different d re studied. The Crtesin coordinte system is oriented such tht the upper surfce of the FR4 PCB lies in the x-y plne nd the center of the re between the two loops is t the origin of the coordinte system. It cn be seen from Fig. 3 tht both Antenns A nd B suffer the problem tht the mgnetic field produced by currents long the lines AB nd A B re prtilly cncelled in some portion of the re due to the reverse directions of the mgnetic fields. The Antenn A genertes reltively wek mgnetic field in the portion ner the lines AB nd A B, nd the smller the d is, the weker the mgnetic field is. Moreover, the mgnetic field in the middle re of the Antenn A will be decresed with the incresing of the distnce d. For the Antenn B, the re between the two seprted segmented loops hs very wek mgnetic field distribution, nd the lger the d is, the lrger the
, A/m (db), A/m (db), A/m (db) 3 re with wek mgnetic field distribution is. d = mm d = mm d = 3 mm Low M field Type A -5-5 5 5 d = mm d = mm d = 3 mm X, mm () Low M field Type B -5-5 5 5 X, mm Fig.3. The mgnetic field distributions of the rry ntenns in Antenn A nd B with vrying d t 95 MHz (z =.5 mm) long the x-xis, () Antenn A, nd Antenn B. For the Antenns C nd D, the two segmented ZPS loops shre common side without ny seprtion between them. The -D mgnetic field distributions of the four kinds of rry ntenn t 95 MHz re exhibited in Fig. 4, where d is 3 mm for Antenns A nd B. The Antenns A nd B hve the res with reltively wek mgnetic field distribution becuse of the mgnetic field cncelltion. The Antenns C nd D don t hve such n re becuse there is no djcent current cusing reverse mgnetic field t the sme portion. However, the mgnetic field distribution of the Antenn C is weker thn tht of the Antenn D becuse the Antenn C is similr to single segmented ZPS loop with the sme re nd thus no mgnetic field compenstion from the current long the common side. Therefore, the proposed Antenn D cn generte the strongest mgnetic field distribution mong the four kinds of rry ntenns. From Fig. 5, it cn be seen tht the minimum mgnetic field inside the interrogtion zone of the proposed grid rry ntenn is t lest 3 db lrger thn those of the three other rry ntenns, s well s the single segmented loop ntenn with the sme size. Furthermore, the proposed grid rry ntenn exhibits the uniform mgnetic field distribution in the mjority of the interrogtion zone. The design procedure of the proposed grid rry ntenn will be given in Section III. () (c) Mx Hz.5 (A/m) db - db -5 db -8 db - db -4 db -7 db db -4 db Mx Hz.5 (A/m) db - db -5 db -8 db - db -4 db -7 db db -4 db Mx Hz.5 (A/m) db - db -5 db -8 db - db -4 db -7 db db -4 db Mx Hz.5 (A/m) db - db -5 db -8 db - db -4 db -7 db db -4 db (d) Fig.4. Simulted -D mgnetic field distribution t 95 MHz (z =.5 mm) of the UHF ner-field rry ntenns: () Antenn A with d = 3 mm, Antenn B with d = 3 mm, (c) Antenn C nd (d) Antenn D. Antenn A Antenn B Antenn C Antenn D Single segmented loop -5-5 5 5 X, mm Fig.5. The simulted mgnetic field distribution of the rrys s well s the single segmented loop ntenn with the sme size t 95 MHz (z =.5 mm) long the x-xis. B. Topologies of the Grid Arry Antenns with lrger interrogtion zones
4 The perimeter or re of the interrogtion zone of the grid rry ntenn is limited to 3 or.5 becuse the side length of squre segmented ZPS loop is limited to.5 when keeping the mgnetic field distribution strong nd uniform inside the interrogtion zone [9]. To get lrger interrogtion zone, where the mgnetic field is much stronger nd more uniform thn tht of the single segmented ZPS loop with the sme size, the grid rry ntenn with more segmented ZPS loops is demonstrted s follows. Fig. 6 shows the severl configurtions of the grid rry ntenn with more segmented ZPS loops nd the desired currents long ech loop. The perimeter or re of the interrogtion zone of the rry ntenns in Fig. (), nd (c) could be 4 or.5.5, 5 or.5, nd 4 or, respectively, if the side length of ech loop is.5. The topologies of the grid rry ntenns in Figs. (), nd re suitble for the interrogtion zone with long side, while the topology in Fig. (c) fits for the interrogtion zone with lrge re but short perimeter. Loop Loop () Loop 3 Loop Loop Loop 3 Loop 4 Loop 3 Loop 4 grid rry ntenn, the guidelines of configuring the grid rry ntenn re summrized s following. ) The segmented ZPS loop is preferred to be the grid cell. ) The currents long djcent loops should be in reverse direction. 3) The input ports of the djcent loops should be 8 o out-of-phse. 4) The feeding network should be esy to control the in-phse or out-of phse reltionship between the output ports of itself. In principle, more segmented ZPS loops cn configure grid rry ntenn with lrger perimeter or re of the interrogtion zone. But the loss cused by the feeding network should be considered becuse the lrge loss in complicted feeding network my degrde the strength of the mgnetic field. III. GRID ARRAY ANTENNA PROTOTYPE WITH SEGMENTED ZPS LOOPS As n exmple, the grid rry ntenn prototype is designed nd prototyped. The design method cn be extended to the ner-field grid rry ntenns with more segmented ZPS loops. A. Antenn configurtion 54 5.85 5 55.65 55.65 43.7 54 55.3 84.5 5.5 55 5.7 Feeding network Top lyer 8.3.3 8.3 Bottom lyer.3. 84.5 5.85 5.85 5.85 Vi.4 o y 7.8 55.5.3 55.5 RF in 55.65 55.65 5.7 33.7 55.3.3 55 54 5.6 39.8 33.7 54 Unit: mm 3 x Feeding Network () Loop Loop (c) Fig.6. Topologies of the UHF ner-field grid rry ntenns with () 3 segmented ZPS loops, 4 segmented ZPS loops nd (c) segmented ZPS loops. To generte s strong nd uniform s possible mgnetic field distribution inside the interrogtion zone of UHF ner-field Fig. 7. Configurtion of the UHF ner-field grid rry ntenn, () ntenn with detiled dimensions; photo of the ntenn prototype using FR4 substrte.
, A/m (db), A/m (db) 5 The proposed grid rry ntenn printed onto piece of FR4 printed circuit bord (PCB) (thickness of.5 mm, reltive dielectric constnt of 4.4, nd loss tngent of.) is shown in Fig. 7. A Crtesin coordinte system is oriented such tht the upper surfce of the FR4 PCB lies in the x-y plne nd the center between the two loops is t the origin of the coordinte system. The internl re (38 mm 5 mm) of the grid rry ntenn is indicted s the interrogtion zone with perimeter of 9 mm or bout.8 t 95 MHz. The grid rry ntenn is fed by the double-sided prllel-strip line (DSPSL) printed onto the opposite sides of the substrte. A feeding network comprising DSPSLs nd coplnr stripline stubs is dopted to feed nd mtch the ntenn to the 5-Ω feed. The strips on top/bottom sides re connected to the inner/outer conductors of SubMiniture version A (SMA) connector, respectively. B. Design procedure The design procedure of the grid rry ntenn is summrized s following three steps. Step Design segmented loop s the grid cell The first is to configure segmented ZPS loop with in-phse current long the loop, s shown in Fig.. The methodology to configure the segmented loop is similr to tht in Ref. 6. The excited line sections re moved outside the segmented loop to mke it esy to control the operting frequency. Step Configure the grid rry Delete Fill up o y x Move L Unit: mm () (c) Fig. 8. Chnge from two seprted segmented sides into one common segmented side of the grid rry ntenn. The second is to configure the grid rry with the segmented ZPS loops by combining the two seprted ZPS loop cells. The key is to configure the common side. The other three sides of ech segmented loop will not be chnged. Fig. 8 exhibits how to chnge the two seprted segmented ZPS lines into the common segmented side of the grid rry ntenn. As shown in Fig. 8(), to configure the common segmented side, the seprted segmented ZPS lines of the two segmented ZPS loops re moved close to ech other. Then the two segmented lines in the middle re deleted, nd one lower/upper gp on one of the two outside segmented lines (here, the lower gp on the left side segmented line re selected) is filled up to get the common side s shown in Fig. 8. However, the mgnetic field distribution of the grid rry ntenn would not be the best becuse two pirs of gps on the common side re o y x too close to ech other to keep the in-phse current long the common side. Therefore, the two gps on the left segmented line of the common side should be moved wy from the two gps on the right segmented line while keeping the distnce ( mm) between the two gps unchnged s shown s in Fig. 8(c). Fig. 9 shows mgnetic field distribution of the grid rry ntenn t 95 MHz long y = -45,, nd 45 mm when the moving distnce L is chnged. As shown in Fig. 9, the grid rry ntenn hs the strongest mgnetic field distribution (H z ) when L is 8 mm, which is exctly hlf of the distnce ( mm) of the segmented line sections between the two gps. The length of mm is lso the min length of the single segmented ZPS loop to keep the current long the loop in-phse t 95 MHz. Step 3 Design the feeding network The lst is to design feeding network to mke the two djcent input ports of the grid rry 8 o out-of-phse nd chieve the impednce mtching. To mke the feeding network meet the phse requirement, especilly for the grid rry ntenn with more grid cells, the DSPSL is selected to configure the feeding network becuse the DSPSL is esy to reverse the phse [4] by reversely connecting the two strips to the excited lines of the segmented loop. -5-35 y = -45 mm y = mm -35 L = 4 mm y = 45 mm -5 L = mm y = -45 mm -35 y = mm L = 8 mm y = 45 mm -5-5 5 5 X, mm y = -45 mm y = mm y = 45 mm -5 Fig. 9. The simulted mgnetic field distributions of the grid rry ntenn with segmented ZPS loops with vrying L t 95 MHz (z =.5 mm) long y = -45,, nd 45 mm. The feeding network is designed with two steps. First, the prllel line shorted stubs re used to mtch the grid rry with two input ports to 5 in wide frequency rnge. Then, multistge DSPSL sections nd DSPSL T-junction re
long x-xis, A/m (db) long x = -78 nd 78 mm, A/m (db) S, S, nd S (db) S, S, nd S (db) long x = -78 nd 78 mm, A/m (db) long x-xis, A/m (db) 6 utilized to complete the feeding network nd mtch the grid rry ntenn to 5 in wide frequency rnge. The simulted S-prmeters of ech step re shown s in Fig.. C. Results The ntenn is optimized nd prototyped with n overll size of 38 mm 8 mm.5 mm nd offers n interrogtion zone of 38 mm 5 mm. The ntenn prototype fbricted onto n FR4 PCB is shown in Fig. 7b. nd mgnetic field distribution Fig. exhibits the simulted current distribution of the proposed grid rry ntenn operting t 95 MHz. From Fig., it cn be seen tht the currents long ech loop re still in-phse, but the two currents re reverse-direction, one is clockwise, nd the other is counterclockwise. The current long the common side is stronger thn those currents long other sides except the sides with excited line sections becuse the currents long the two loops re superposed ech other when the currents from the two loops flow into the common side. Such current distribution cn meet the current requirement for the grid rry ntenn topology in Fig. (d). S S One port with completed feeding network Two ports without feeding network S S S S S Two ports with only prllel line stubs 7 8 9 Frequency (MHz) Fig.. Simulted S-prmeters of the grid rry ntenn when there is no feeding network, only prllel line stubs, or completed feeding network. Mx E-.5 (A/m) db - db -5 db -8 db - db -4 db -7 db db -4 db Fig.. Simulted current distribution of the grid rry ntenn t 95MHz. -5 x = -78 mm 84 MHz 95 MHz 9 MHz x = 78 mm 84 MHz 95 MHz 9 MHz - 84 MHz -7 95 MHz 9 MHz -8-5 -5 5 5-5 X or Y, mm Fig.. Simulted mgnetic field distribution of the grid rry ntenn t different frequencies (z =.5 mm) long x-xis nd x = -78 nd 78 mm. Fig. illustrtes the simulted mgnetic field distribution of the proposed grid rry ntenn t 84, 95, nd 9 MHz long the x-xis nd the lines x = -78 nd 78 mm, respectively. It is found tht the proposed ntenn chieves the uniform mgnetic field distribution with mximum vrition of 5 db t the intervl of -43 mm x -3 mm nd 3 mm x 43 mm long the x-xis nd.5 db over the rnge of -65 mm y 65 mm long the lines x = -78 nd 78 mm cross the frequency rnge of 84-9 MHz. x = -78 mm Simulted Mesured Simulted Mesured x = 78 mm Simulted Mesured - - -8 4 8 - X or Y, mm Fig.3. Simulted nd mesured mgnetic field distribution of the c grid rry ntenn prototype t 95 MHz (z =.5 mm) long x-xis nd x = -78 nd 78 mm. -5 Fig. 3 compres the simulted nd mesured mgnetic field distribution t 95 MHz. The mesurement method is the sme s tht in Ref. 9. The ner-field mgnetic field probe ws
S (db) Reding Rte (%) 7 plced on the surfce of the ntenn prototype nd the intervl of detection points is 5mm. Impednce mtching The impednce mtching mesurement of the proposed grid rry ntenn ws crried out using Agilent E53A vector network nlyzer. Fig. 4 shows the simulted nd mesured S of the proposed grid rry ntenn. The chieved frequency bnd of -db S rnges from 79 to 4 MHz or 7.3%. The mesured result well grees with the simultion. The slight shift of S is believed to be cused by the fbriction error of the gps nd the thickness of the substrte. -5-5 -5-35 Simulted Mesured 7 75 8 85 9 95 5 Frequency (MHz) Fig. 4. Simulted nd mesured S of the grid rry ntenn prototype. Reding rnge To further verify the performnce of the proposed grid rry ntenn, the ntenn prototype ws used s the reder ntenn in the UHF ner-field RFID system to detect UHF ner-field tgs. In the set-up shown in Fig. 5, the ntenn prototype ws connected to the Impinj Speedwy reder operting t 9-98 MHz with 3-dBm output [5] to detect 96 button type tgs (J, 5 8mm ) [6], where the tgs re positioned symmetriclly on rectngulr Styrofom with the sme re of the interrogtion zone. To ensure the relibility of the results, the tgs ttched on the Styrofom were rndomly plced on the ntenn, nd n verge of five tests ws recorded. The mesured reding rte ginst the reding rnge is exhibited in Fig. 6. The proposed grid rry ntenn offers the bi-directionl detection long the ±z xis. A % reding rte is chieved within mximum distnce of 3.5 mm, while the reding rte of the single segmented loop ntenn with the identicl size is gretly reduced t the sme distnce. Fig. 5. Ner-field RFID mesurement set-up using the Impinj Speedwy reder nd the grid rry ntenn prototype. 9 8 7 5 4 3 Tg Antenn Tg Proposed side-shred ntenn rry Single segmented loop ntenn 5 5 75 5 5 75 d, mm Fig. 6. Mesured reding rte ginst reding rnge of the grid rry ntenn prototype. d IV. CONCLUSION Designing electriclly-lrge UHF ner-field RFID reder ntenns is big chllenge, especilly when the perimeter of interrogtion zone reches three wvelengths or more. The proposed grid rry ntenns cn produce strong nd uniform mgnetic field distributions in the ner-field region of the ntenn with the perimeter of the interrogtion zone up to three operting wvelengths or more. As n exmple, the grid rry ntenn prototype with the segmented loops hs demonstrted the much enhnced mgnetic field distribution nd reding rnge compring the single segmented loop with the sme size. The proposed grid rry ntenn is promising for UHF ner-field RFID reder pplictions. REFERENCES [] P. Hrrop, New field UHF vs. HF for item level tgging [Online]. Avilble: http://www.eurotg.org/?articles_nd_publictions [] D. Desmons, UHF Gen for item-level tgging, presented t the RFID World 6, [Online]. Avilble: www.impinj.com/files/impinj_ilt_rfid_world.pdf [3] UHF Gen for Item-level Tgging Impinj RFID technology series pper [Online]. Avilble: http://www.impinj.com/files/mr_gp_ed_3_ilt.pdf [4] C. Ajluni, Item-level RFID tkes off, RF Design Mg., Sep. 6. [5] Item-level visibility in the phrmceuticl supply chin: A comprison of HF nd UHF RFID technologies, Philips, TAGSYS, nd Texs Instruments [Online]. Avilble: http://www.tgsysrfid.com/moduls/tgsys/uplod/news/tagsysti-ph ilips White-Pper.pdf [6] P. V. Nikitin, K. V. S. Ro, nd S. Lzr, An overview of ner field UHF RFID, in Proc. IEEE Int. Conf. on RFID, Mr. 7, pp. 67 74. [7] S. Chen nd V. Thoms, Optimiztion of inductive RFID technology, in Proc. IEEE Int. Symp. On Electronics nd the Environment, My, pp. 8 87. [8] D. M. Dobkin, S. M. Weignd, nd N. Iyec, Segmented mgnetic ntenns for ner-field UHF RFID, Microw. J., vol. 5, no. 6, Jun. 7. [9] 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. 3, 8. [] R. A. Oliver, Broken-loop RFID reder ntenn for ner field nd fr field UHF RFID tgs, U.S. design ptent D574, 369 S, Aug. 5, 8. [] R. A. Oliver, Broken-loop RFID reder ntenn for ner field nd fr field UHF RFID tgs, U.S. design ptent D574, 37 S, Aug. 5, 8. [] Y. S. Ong, X. Qing, C. K. Goh, Z. N. Chen, A segmented loop ntenn for UHF ner-field RFID, Antenns nd Propgtion Society Interntionl Symposium, IEEE, pp. 4. [3] X. Qing, Z. N. Chen, nd C. K. Goh, UHF ner-field RFID reder ntenn with cpcitive couplers, IET Electron. Lett., Dec., vol. 46, no. 4, pp. 59-59. d
[4] X. Qing, C. K. Goh, nd Z. N. Chen, Segmented loop ntenn for UHF ner-field RFID pplictios, IET Electron. Lett., Aug. 9, vol. 45, no. 7, pp. 87 873. [5] X. Li, J. Lio, Y. Yun, nd D. Yu, Segmented coupling eye-shpe UHF bnd ner field ntenn design, IEEE Microwve Asi Pcific Conference, Singpore, Dec. 9, pp. 4 44. [6] X. Qing, C. K. Goh, nd Z. N. Chen, A brodbnd ner-field UHF RFID ntenn, IEEE Trns Antenns nd Propgtion, Dec., vol. 58, no., pp. 389 3838. [7] Z. N. Chen, C. K. Goh, nd X. Qing, Loop ntenn for UHF ner-field RFID reder, 4 th Europen Conference of Antenn nd Propgtion, Brcelon, Spin, April, pp. 4. [8] X. Li, nd Z. Yng, Dul-Printed-Dipoles Reder Antenn for UHF Ner-Field RFID Applictions, IEEE Antenn Wireless Propg. Lett., vol., pp. 39 4,. [9] J. Shi, X. Qing, Z. N. Chen nd C. K. Goh, Electriclly lrge dul-loop ntenn for UHF ner-field RFID reder, ccepted by IEEE Trns Antenns nd Propgtion, November. [] J. D. Krus, A bckwrd ngle-fire rry ntenn, IEEE Trns Antenns nd Propgtion, Jn. 964, vol., no., pp. 48 5. [] M. Sun, Y. P. Zhng, D. Liu, K. M. Chu, nd L. L. Wi, A bll grid rry pckge with microstrip grid rry ntenn for single-chip -GHz receiver, IEEE Trns Antenns nd Propgtion, Jun., vol. 59, no. 6, pp. 34 4. [] X. Chen, G. Wng, nd K. Hung, A novel widebnd nd compct microstrip grid rry ntenn, IEEE Trns Antenns nd Propgtion, Feb., vol. 58, no., pp. 596-599. [3] IE3D user s mnul relese, Zelnd Softwre Inc., Fremont, CA, Oct. 6 [4] J. X. Chen, C. H. K. Chin, nd Q. Xue, Double-sided prllel-strip line with n inserted conductor plne nd its ppliction, IEEE Trns. Microw. Theory Tech., vol. 55, no. 9, pp. 899 94, Sep. 7. [5] [Online]. Avilble: http://www.impini.com/products/rfid-reder.spx. [6] [Online]. Avilble: Tg ntenn designs, t http://www.rfidinfo.jp/whitepper/379.pdf 8