Research Article Dual-Dipole UHF RFID Tag Antenna with Quasi-Isotropic Patterns Based on Four-Axis Reflection Symmetry

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1 Internationa Journa of Antennas and Propagation Voume 213, Artice ID , 9 pages Research Artice Dua-Dipoe UHF RFID Tag Antenna with Quasi-Isotropic Patterns Based on Four-Axis Refection Symmetry Chunfang Qin, 1,2 ingfei Mo, 3 Hongiang Zhou, 1 and Hongjian Zhang 1 1 State Key aboratory of Industria Contro Technoogy, Department of Contro Science and Engineering, Zhejiang University, Hangzhou 3127, China 2 Coege of Information Science and Engineering, Guangxi University for Nationaities, Nanning 536, China 3 Schoo of Instrument Science and Engineering, Southeast University, Nanjing 2196, China Correspondence shoud be addressed to Chunfang Qin; qinchf@gmai.com Received 3 January 213; Revised 3 Apri 213; Accepted 7 Apri 213 Academic Editor: Guo-Ping Gao Copyright 213 Chunfang Qin et a. This is an open access artice distributed under the Creative Commons Attribution icense, which permits unrestricted use, distribution, and reproduction in any medium, provided the origina work is propery cited. In many RFID practica appications, it is required that reader can effectivey read tags which are paced in radiation covering area randomy.inthispaper,apassiveuhfdua-dipoetagantennawith quasi-isotropic patterns is designed, which can reduce the sensibiity of tag read-orientation in a ong distance. Two dipoes with four-axis refection symmetric structure are used, and the two arms of the dipoe are bent to fi the space of the antenna. In this way, a quasi-isotropic tag is easier to be obtained. The test resuts show that the gain deviation of the proposed antenna was ess than 3.25 db, and the maximum reading range in different directions was from 6.9 m to 1. m, with better quasi-isotropic performance and reading range than other commercia tags. 1. Introduction Radio frequency identification (RFID) technoogy is a noncontact automatic identification technique that acquires reevantdataofanobjectwithradiofrequencytoidentifyit. Since the passive utra-high frequency (UHF) technoogy is characterized by great identification speed, mutitarget identification, and ong identification distance, it has been widey used in ogistics and suppy management, product manufacture and instaation, ibrary management, transportation, and other fieds. A UHF RFID system consists oftwounits,areaderandatag.sinceatagisnotaways oriented to a reader in some practica appications, such as management for uggage in airport or garment retai, it is required that reader can read tags which are at random paced within radiation coverage. Ordinary tag is usuay a passive singe-dipoe tag and has a nu reading zone aong its axis, which may cause missing a read. Isotropic antenna is a hypothetica antenna that radiates energy equay in a directions of the space. In reaity, an antenna whose gain deviation in any arbitrary direction is ess than 6 db can be considered as a quasi-isotropic antenna [1]. In recent years, some schoars have studied the quasiisotropic tag antennas. Khoodnyak et a. designed a threedimensiona antenna to eiminate the sensibiity of tag in reading orientation [2, 3]. However, for its three-dimensiona structure, the antenna is inconvenient for manufacture and canonybeusedinsomespeciacases.ahneta.introducedsomepanartagsthatareinsensitivetothereading orientations [1, 4 8].Thesetagsaredesignedonthebasis of singe-port chips which can be easiy obtained from the market with ow cost, whie the reading range of the tags is ony about two meters. But for ogistics or other appications, the tag with a arger reading range is more practica. There are three reasons: firsty, in some passages for arge cargo, a tag is required to be read from more than three meters; secondy, reading distance in the air wi be shortened when a number of tags are stacked; and thirdy, the tag with a arger reading range is more practica and suitabe for various appications. Therefore, it is important to design a quasi-isotropictagnotonywithargerreadingrangebut aso with ess directiona sensitivity. There are aso some commercia quasi-isotropic tags based on the dua-dipoe antennas, which have arge reading ranges, such as Invengo

2 2 Internationa Journa of Antennas and Propagation Port1 Port2 Sum (a) Four-axis refection symmetry Port1 Port2 Sum (b) Non-four-axis refection symmetry Figure 1: Radiation patterns of tag antenna with four-axis refection symmetry and non-four-axis refection symmetry. TF823,ImpinjH47,andUPMfrog3D.However,some measurement resuts show that the gain deviations of the above quasi-isotropic dua-dipoe tags are aways greater than 5 db. Actuay, with appropriate design for the tag antenna, its performance of quasi-isotropic patterns can be further improved. In this paper, a passive UHF dua-dipoe tag antenna with quasi-isotropic patterns is designed. Two dipoes with fouraxis refection symmetric structure are adopted; namey, after being foded the tag antenna can competey overap not ony aong the axis of 18 or 9 27, but aso aong the axis of or Thetwoarmsofthedipoearefoded to fi the space of the antenna. If ony the resonant frequency of this antenna is adjusted in an appropriate range, its gain deviation in any arbitrary direction in the space can be hed to be about 3 db and a quasi-isotropic tag is easier to be impemented. The test resuts show that the gain deviation of thetagdesignedinthispapercoudachieveessthan3.25db and the maximum reading range in different directions was up to meters. The rest of this paper is organized as foows. In Section 2, the design of the antenna with four-axis refection symmetry is discussed. In Section 3, theproposedantennaandthree commercia tag antennas are simuated. Section 4 shows the measurement and anaysis of the antennas. Section 5 is the concusion. 2. Design of the Antenna with Four-Axis Refection Symmetry Ordinary dipoe tag is generay thin and ong and is a deformation of haf-wave dipoe. The antenna is fed from its center. Hence its eectric current is distributed aong the antenna as roughy sinusoida pattern and is zero at both ends. If the axis of an ordinary dipoe tag coincides with axis in coordinate, the radiation pattern of the dipoe tag in Z pane is uniform, but the shape of radiation pattern in Z pane and paneisikean 8. Areadercanhardyreada tag if it points at the nu reading zone for the tag aong its axis. The gain deviation of an ordinary dipoe in the space is generay 25 5 db. To weaken the infuence of nu reading zone of an ordinary dipoe and guarantee a arge reading range, a kind of dua-dipoe that consists of two mutuay perpendicuar dipoes can be designed. The two dipoes are axia symmetric to offset respective nu reading zone of a singe dipoe. For a haf-wave dipoe, its haf power beam width is about 8,so two mutuay perpendicuar dipoes can cover a haf power band width of about 32 in the whoe space. And in the rest space of about 4,thepowerradiatedbythetwodipoes wi be added, so the gain aong the axis of two antennas wi greaty increase to offset nu reading zone of a singe dipoe. If the dua-dipoe tag antenna designed in this paper competey overaps after being foded not ony aong the axis of 18 or 9 27, but aso aong the axis of or ; that is, the dua-dipoe antenna is four-axis refection symmetric, it wi radiate energy uniformy in the space. Theoreticay, gain deviation of a dua-dipoe tag antenna with four-axis refection symmetry is approximatey 3 db. Its schematic drawing is shown as Figure 1(a).Thecurvesofport 1 and port 2 indicate the gain of a singe port of the duadipoeandthecurveofsumistotagainofthetwoports.ifa dua-dipoe antenna is not four-axis refection symmetric, in its nu reading zone offset by each other, far-fied radiation wi be strengthened in some paces and in some other paces

3 Internationa Journa of Antennas and Propagation 3 2 w 1 3 r 1 1 r 2 r 1 r 2 w Figure 2: Structure of the proposed dua-dipoe tag antenna. The substrate of the antenna is made of PTFE, with an area of and a thickness of h. Main dimensions of antenna radiator are determined by 1, 2, 3,andw, and the sizes of feed oop are determined by r 1 and r 2. Figure 3: Structure of the inear dua-dipoe tag antenna. The substrate of the antenna is made of PTFE, with an area of and a thickness of h. 2 wi be weakened. For this reason, the gain deviation in the entirespacewibepoorerthanthatofdua-dipoeantenna with four-axis refection symmetry. Figure 1(b) iustrates this situation. The structure of the dua-dipoe tag antenna designed in this paper is shown in Figure 2. Itscenterfrequencyis 915 MHz. The corresponding operation waveength is about 328mminfreespaceandisabout21mminthemedium fuy fied with poyfuortetraethyene (PTFE) of dieectric constant For a haf-wave dipoe, its printed dipoe antenna incudes both medium and free space, the actua arm ength of the proposed antenna shoud be mm, soaveragevaueof132mmwastakenasaninitiavaue of tota ength of two antenna arms, and then its actua vaue was given using simuation software. If the tag adopts aineardipoe,theareaofdua-dipoetagisabout mm, which is not convenient for most cases. As ong as the operation frequency is fufied, the tag shoud be as sma as possibe. The tag antenna is designed to be competey symmetric not ony aong the axis of 18 or 9 27,but aso aong the axis of or Thetwoarmsof thedipoearebenttofithesurroundingspaceasmuchas possibe. This design can, on one hand, ensure more uniform current distribution in antenna and improve its performance of quasi-isotropic patterns, on the other hand, miniaturize the tag antenna. 3. Simuations of the Antennas 3.1. Simuations of Dua-Dipoe Tag Antenna with Four-Axis Refection Symmetry. The proposed antenna was simuated using HFSS (High Frequency Simuation Structure) software ofansyscompanybasedonthefiniteeementmethod.the ine width w of the antenna was fixed to be 1 mm. 1 is a key 1 r 2 r 1 Figure 4: Structure of the one-fod dua-dipoe tag antenna. The substrate of the antenna is made of PTFE, with an area of and a thickness of h. factor which determines the dimensions of the tag. During the simuation, 1 was graduay reduced from 66 mm, 2 was increased, and the antenna was bent to fi the space between 2 and the feed oop as much as possibe. Finay its resonant frequency was adjusted to be about 915 MHz by fine tuning 2. First, a inear dua-dipoe tag antenna was simuated. Figure 3 shows the structure of the inear dua-dipoe tag antenna. When 1 was 55 mm, the resonant frequency of the inear dua-dipoe tag antenna can be adjusted to 914 MHz. The normaized radiation patterns of the inear dua-dipoe tag antenna are shown in Figure 5.Thesimuatedmaximum and minimum gain vaues of the tag antenna in three panes w

4 4 Internationa Journa of Antennas and Propagation inear antenna One-fod antenna inear antenna One-fod antenna (a) Z pane (b) Z pane inear antenna One-fod antenna (c) pane Figure 5: Simuated normaized radiation patterns of the three tag antennas. Tabe 1: Simuated gain vaues in three panes of the three tag antennas (Unit: db). Gain Z Z Gain deviation inear antenna max inear antenna min One-fod antenna max One-fod antenna min max min are isted in Tabe 1, with a gain deviation of 3.64 db in different directions. Then the segment 2 was added on the inear dua-dipoe tag antenna and the segment 1 was graduay shortened. The inear dua-dipoe tag antenna became a one-fod dua-dipoe tag antenna and the structure of the one-fod dua-dipoe tag antenna is shown as Figure 4. When 1 and 2 were 28 mm and4mm,respectivey,theresonantfrequencyoftheonefod dua-dipoe tag antenna can be adjusted to 916 MHz. The normaized radiation patterns of the one-fod dua-dipoe tag antenna are shown in Figure 5. Tabe 1 iustrates the maximum and minimum gain vaues of the tag antenna in three panes, with a gain deviation of 3.28 db in different directions. In order to obtain the uniform radiation and reduce the area of tag antenna, the antenna was bent to fi the space

5 Internationa Journa of Antennas and Propagation 5 Tabe 2: Parameter vaues of optimized antenna. Parameters w r 1 r 2 Vaues (mm) Resistance (Ohm) Frequency (MHz) Reactance (Ohm) Refection coefficient S Frequency (MHz) r 1 =9.3,r 2 = 8.3 r 1 = 9.8, r 2 = 8.8 r 1 = 1.3, r 2 =9.3 r 1 =9.3,r 2 = 8.3 r 1 = 9.8, r 2 = 8.8 r 1 = 1.3, r 2 =9.3 Target Figure 6: Simuated impedance of the proposed antenna with different sizes of the feed oop. between 2 and the feed oop whie the antenna became a muti-fod dua-dipoe tag antenna, as shown in Figure 2. If the main parameters of the tag antenna are set foowing Tabe 2, its resonant frequency can be adjusted to 914MHz. The normaized radiation patterns of the tag antenna are shown in Figure 5. The maximum and minimum gain vaues of the tag antenna in three panes are isted in Tabe 1,witha gain deviation of 2.64 db in different directions. Theabovesimuatedresutsshowthatthequasi-isotropic performanceoftagantennaiseasiertobeobtainedasongas the dua-dipoe tag antenna is four-axis refection symmetric. As the dipoe arms are bent to fi the antenna space graduay, the quasi-isotropic performance of tag antenna becomes better since the gain deviation of the antenna radiation pattern reduces graduay. Impedancematchoftheantennacanbeachievedmainy through the adjustment of sizes of feed oop if the four arms of dua-dipoe are basicay constant. The width of feed oopwasfixedtobe1mm,andbyadjustingitsinnerradius and outer radius, appropriate impedance can be obtained to match the impedance of chip. The Monza 4D chip from Impinj was used, with an impedance of 11-j143 Ω at 915 MHz [9]. Figure 6 iustrates the simuated impedance curves of the antenna with different sizes of the feed oop. Figure 7 shows the simuated refection coefficient curves of the antenna with different sizes of the feed oop. It can be seen from Figures 6 and 7 thatifouterradiusoffeedoopis 9.3 mm and its inner radius is 8.3 mm, the impedance of the antenna matches chip best and the refection coefficient S 11 is 3.7 db. Considering the resonant frequency, its outer radius and inner radius were eventuay seected to be 9.8 mm and 8.8 mm, respectivey. The thick soid ine parts of Figures 6 and 7 are the corresponding curves. Figure 7: Simuated refection coefficient of the proposed antenna with different sizes of the feed oop. Tabe 3: Dimensions of the three commercia tag antennas. Tags Invengo TF823 Impinj H47 UPM frog 3D Parameters Vaues (mm) Tabe 4: Simuated gain vaues of the three commercia tag antennas in three panes (Unit: db). Gain Z Z Invengo TF823 max Invengo TF823 min Impinj H47 max Impinj H47 min UPMfrog3Dmax UPM frog 3D min Gain deviation It can be seen in Figure 7 that the refection coefficient S 11 is 2.97 db at the resonance frequency of 914 MHz. A parameter vaues of optimized antenna are isted in Tabe Compare with Three Commercia Quasi-Isotropic Tag Antennas. For comparison, three representative commercia quasi-isotropic tags, Invengo TF823, Impinj H47, and UPM frog3d,wereasosimuated.thethreetagsareadesigned with Monza 4 of Impinj. Figure 8 shows the structures of the above three tag antennas. It can be seen from the figure that these antennas use non-four-axis refection symmetric structures. Main dimensions of the antennas are isted in Tabe 3. And the simuated maximum and minimum gain vauesofthetagantennasinthreepanesareistedintabe 4,

6 6 Internationa Journa of Antennas and Propagation 1 1 (a) Invengo TF823 (b) Impinj H47 1 (c) UPM frog 3D Figure 8: Structure of the three commercia tag antennas. The area of substrate is,themateriaofsubstrateispet,andtheareaof antenna radiator is 1 1. with a gain deviation of 4.2 db for Invengo TF823, 4.39 db forimpinjh47,and5.12dbforupmfrog3d. The simuation resuts show that the tag antenna with four-axis refection symmetric structure, due to its uniform far-fied radiation, has ess gain deviation than a tag antenna with non-four-axis refection symmetric structure. 4. Measurement and Anaysis The photograph of the fabricated dua-dipoe tag designed in this paper is shown in Figure 9. The PTFE substrate was used for the tag, with the parameters of dieectric constant 2.65, Figure 9: Photograph of the fabricated tag.

7 Internationa Journa of Antennas and Propagation 7 oss tangent.2, and thickness.765 mm. And the tag chip was connected to the antenna by bonding. To verify the performance of the tag, its reading orientation, reading range, and antenna impedance were tested. For comparison, Invengo TF823, Impinj H47, and UPM frog 3D were aso tested. Speedway Revoution Reader of Impinj, with operation frequency of 92 MHz 928 MHz, output power of 1 dbm 3 dbm, and adjustment step of.25 dbm, was used to test reading orientation and reading range. CS-RHCP of CS Company was used as a reader antenna, with gain of 6dBi. The reading range and the reading orientation of the tags were measured in an anechoic chamber as iustrated in Figure 1. In the anechoic chamber, the tag is paced at d (d =1m) distance from the reader antenna. The minimum power P min required to communicate with the tag is obtained by varying the reader output power. The reading range r can be cacuated using Friis free-space deformationa formua as [1] r=d EIRP P min G t, (1) where EIRP is effective isotropic radiated power, is the oss of the connecting cabe, and G t is the gain of the reader antenna. According to the parameter test method of performance of EPC UHF RFID tag [11],thetagwastestedinthreedifferent panes with a measurement point every 1.Thegainvaues of the four tags in each direction were normaized with a maximum vaue of db. The measured radiation patterns are iustrated in Figure 11. The maximum and minimum vaues of power required for the test of four tag antennas in three panes are isted in Tabe 5. From the tabe, the tag antenna designed in this paper has a gain deviation of 3.25 db, whie gain deviations of tag antennas of Invengo TF823, Impinj H47, and UPM frog 3D are 5.25 db, 5.75 db, and 6.5 db, respectivey. The tag designed in this paper has a maximum reading range of meters in different directions at the frequency of 915 MHz, whie the vaue is meters for Invengo TF823, meters for Impinj H47, and meters for UPM frog 3D. The test resuts indicate that the tag antenna designed in this paper, due to its four-axis refection symmetry, has better quasi-isotropic performance than a tag antenna with nonfour-axis refection symmetric structure. MS4624B vector network anayzer of Anritsu Company was adopted to measure the impedance of the tag antenna. The dua-dipoe antenna designed in this paper can be considered as two dipoe antennas with singe port. Dipoe antenna is a kind of baanced antenna and cannot be directy measured with a vector network anayzer. According to anaysis of Tikhov et a., indirect measurement based on the method of images is feasibe [12]. Referring to the method introduced by Tikhov et a. [12], a setup was designed for the measurement of the antenna impedance. The structure of the measurement setup is shown in Figure 12.Itiscomposed of three parts: the meta pate, a haf of the antenna (the monopoe antenna), and an SMA connector. An infinite Figure 1: Measurement environment of the tag. Tabe 5: Minimum power required for the test of the four kinds of tag antennas in three panes. Power Z Z (dbm) (dbm) (dbm) max min Invengo TF823 max Invengo TF823 min Impinj H47 max Impinj H47 min UPMfrog3Dmax UPM frog 3D min Gain deviation pane is simuated by the meta pate which consists of a stainess stee part (7 7 mm) and a copper part (16 16 mm). The monopoe antenna is paced on the copper pate which can wed the antenna convenienty. The SMA connector is hidden under the copper pate. Whie measuring, the SMA connector of the vector network anayzer was connected to the SMA connector of the measurement setup. The vector network anayzer fed to the antenna and measured the impedance of the monopoe antenna. The input impedance of the dipoe tag antenna was twice of the measured impedance of the monopoe antenna. As the vector network anayzer can be ony precaibrated to the port output termina, increased phase change due to SMA connector was corrected through simuation and cacuation. The measured antenna impedance was obtained, as shown in Figure 13. It can be seen that the rea part and the imaginary part of the measured antenna impedance intersect with the target vaues at about 915 MHz. Using a compex impedance refection coefficient cacuation formua, the measured power refection coefficient curve was converted with the method described by Mo and Qin [13]. Figure 14 shows the measured power refection coefficient of the antenna. It can be seen from the figure that the measured resonant frequency of the antenna is 921 MHz and its refection coefficient S 11 is 25.3 db. 5. Concusion In this paper, a passive UHF dua-dipoe tag antenna with quasi-isotropic patterns is designed, which can be read

8 8 Internationa Journa of Antennas and Propagation Invengo TF823 Impinj H47 UPM frog 3D Invengo TF823 Impinj H47 UPM frog 3D (a) Z pane (b) Z pane Invengo TF823 Impinj H47 UPM frog 3D (c) pane Figure 11: Measured normaized radiation patterns of the four kinds of tag antennas. Figure 12: Measurement setup for the proposed antenna impedance. reiaby from a ong distance in different directions. Owing to the four-axis refection symmetric structure and the bent arms, its gain deviation in any arbitrary direction is approximatey 3 db and it coud be reiaby read within a range of more than 6 meters in different directions. Three representative commercia dua-dipoe quasi-isotropic tag antennas were compared in this paper. The resuts show that the proposed four-axis refection symmetric tag antenna is with better quasi-isotropic performance and reading range than other commercia tags. Therefore, to effectivey reduce nu reading zone of ordinary singe dipoe and reaize a arger reading range, four-axis refection symmetric structure shoud be used for dua-dipoe tag antenna with quasiisotropic patterns. Confict of Interests The authors do not have any confict of interests with the contentofthepaper.

9 Internationa Journa of Antennas and Propagation 9 Resistance (Ohm) Refection coefficient S Frequency (MHz) Measured Target Figure13:Measuredimpedanceoftheproposedantenna Measured Haf power Reactance (Ohm) Frequency (MHz) the 8th Internationa Symposium on Antennas, Propagation and EM Theory (ISAPE 8), vo. 1 3, pp , Kunming, China, November 28. [4] J.Ahn,H.M.Jang,H.S.Moon,J.W.ee,andB.ee, Inductivey couped compact RFID tag antenna at 91 MHz with nearisotropic radar cross-section (RCS) patterns, IEEE Antennas and Wireess Propagation etters, vo. 6, pp , 27. [5]C.Cho,H.Choo,andI.Park, Printedsymmetricinvertedf antenna with a quasi-isotropic radiation pattern, Microwave and Optica Technoogy etters,vo.5,no.4,pp ,28. [6] S.. Chen, K. H. in, and R. Mittra, Miniature and near-3d omnidirectiona radiation pattern RFID tag antenna design, Eectronics etters,vo.45,no.18,pp ,29. [7] H.W.SonandC.S.Pyo, DesignofRFIDtagantennasusing an inductivey couped feed, Eectronics etters, vo.41,no.18, pp ,25. [8]C.F.Qin,.F.Mo,H..Zhou,andH.J.Zhang, Asinge port dipoe for UHF RFID tag antennas with eiminated readorientation sensitivity, in Proceedings of the Internationa Conference on Consumer Eectronics, Communications and Networks (CECNet 11), pp , ianning, China, Apri 211. [9] Monza 4 Tag Datasheet, [1] K. V. S. Rao, P. V. Nikitin, and S. F. am, Antenna design for UHF RFID tags: a review and a practica appication, IEEE Transactions on Antennas and Propagation, vo.53,no.12,pp , 25. [11] EPCgoba Inc., Tag Performance Parameters and Test Methods, 28. [12]. Tikhov,. Kim, and. H. Min, A nove sma antenna for passive RFID transponder, in Proceedings of the 35th European Microwave Conference, vo.1 3,pp ,Paris,France, October 25. [13]. F. Mo and C. F. Qin, Tunabe compact UHF RFID meta tag based on CPW open stub feed PIFA antenna, Internationa Journa of Antennas and Propagation, vo.212,articeid , 8 pages, 212. Figure 14: Measured refection coefficient of the proposed antenna. Acknowedgments This work was supported by the Nationa Key Technoogy R&D Program of China (29BADB9B9) and the Nationa Water Poution Contro and Management Technoogy Major Projects (28Z742-4). References [1] C. Cho, H. Choo, and I. Park, Broadband RFID tag antenna with quasi-isotropic radiation pattern, Eectronics etters, vo. 41, no. 2, pp , 25. [2] D. V. Khoodnyak, P. A. Turachuk, A. B. Mikhaiov, S.. Dudnikov, and I. B. Vendik, 3D antenna for UHF RFID tags with eiminated read-orientation sensitivity, in Proceedings of the 36th European Microwave Conference (EuMC 6), vo.1 4, pp , Manchester, UK, September 26. [3]H.W.Wang,..Wang,.iu,andW.S.u, 3Dantenna for UHF RFID tags with near omni-direction, in Proceedings of

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