IJCSI International Journal of Computer Science Issues, Vol. 7, Issue 4, No 1, July 2010ISSN (Online): 1694-0784ISSN (Print): 1694-08147Magnetic-Field Coupling Characteristics of Ferrite-CoilAntennasfor Low-Frequency RFIDApplicationsAlain Tran, Miodrag Bolic, and Mustapha C.E. YagoubSchool of Information Technology & Engineering (SITE), University of Ottawa,Ottawa, Ontario, CanadaAbstractLow-frequency technology has been widely used for detectingobjects placed underground. Low-frequency radio-frequencyidentification (RFID) systems provide the advantage of betterpropagation in lossy materials such as rocks and soil. In thispaper, we assume that buried objects will be tagged with lowfrequency RFID passive transponders and that a reader with thelarge single-loop antenna will be used to detect the objects. Wepropose new orientation-insensitive transponder’s antenna.Simulated and measured results obtained from fabricatedantennas based on the new design show some advantage over thetraditional design. The new antenna offers a more uniformmagnetic field pattern.Keywords: Low-frequency RFID, double-rod, ferrite core, coils.1. IntroductionConventionally, low-frequency transponders have amultiple-turn coil wound around the longitudinal axis of acylindrical ferrite core. Therefore, this type of transponderis strongly directional, which means the transponder’sreception sensitivity is highest to the signals incident fromthe direction parallel to the longitudinal axis of the ferritecore and lowest to the signals incident from the directionorthogonal to the longitudinal axis of the ferrite core.RFID technology has been used in the detection ofunderground objects [1-3]. A simple RFID system consistsof two components which communicate wirelessly. Thefirst component, a reader, is connected to a relatively largeantenna. The second component, called the tag ortransponder has a small antenna. Low-Frequency (LF)band or Long-wave radio frequencies correspond to thosebelow 500 kHz. Most publications on the RFIDtechnology focused on transponders without a ferrite core.At LF, transponders with ferrite-core increase themagnetic field coupling between the reader’s and thetransponder’s antennas.It is possible to increase the overall reception sensitivity ofthis type of transponder’s antenna by arranging two orthree transponder-units that are mutually orthogonal to oneanother. However, it is not always practical to mount twoor three transponders on one object-to-be-identified due tolimited space. When two or three separate transponders aremounted in one object this way, the space occupied by theobject increases and thus the object itself must be largeenough to accommodate these transponders.This paper presents one novel design that alleviates above-discussed problems that conventional cylindrical ferrite-core antennas have. Compared to the scenario whereseveral transponder-antennas are to be used to provide thesame feature, the new design would be smaller.This type of RFID system is intended to be used to detectthe buried objects in the proximity of the reader single-loop antenna in media such as rocks, wet soil, cement, etc.Typically, the transponders would be attached to theunderground objects, while the reader’s large single-loopantenna is placed directly on the ground surface or verynear the ground surface.RFID systems need to be designed and tuned to operate inthese conditions. However, few published academicresearch has focused on these areas, especially for low-frequency RFID. Thus an investigation of some importantparameters affecting the low-frequency RFID performance,by means of numerical simulations of transponder-readercoupling supported by the measurements, which ispresented in this paper, is beneficial.Signal attenuation has been shown to be a major factorwhen the frequency is above LF range [4-6]. In thisresearch, we studied the performance of RFID systemsworking in the range of 125-150 kHz.Wireless communication can be grouped into two modes:near-field and far-field communications. In the near field
IJCSI International Journal of Computer Science Issues, Vol. 7, Issue 4, No 1, July 2010www.IJCSI.org8region, the electric and magnetic field components decayrapidly with the distance (i.e.: one over distance-cube) .In low-frequency RFID systems, as distance between thereader and the transponder is much shorter than the free-space wavelength, the magnetic coupling between thereader’s single-loop antenna and the transponder’s coilantenna is the dominant mode of operation.One of the research objectives was to study the near-fieldcoupling of the low frequency RFID system as a functionof various media where the transponder is buried.Experimentations were performed on a coil transponderplaced underneath a large container containing the mediato be studied. The experimentations were limited to smallrocks and open-air. Numerical data were obtained usingFEKO. FEKO is a full wave, method of moment (MoM)based simulation software for the analysis of variouselectromagnetic problems. FEKO calculates theelectromagnetic fields by first calculating the electricsurface currents on conducting surfaces and equivalentelectric and magnetic surface current on the surface of adielectric volume .We analyzed the effects of the single-loop antenna’s radiuson the received power at the transponder-transponder’santenna in Section-2. In addition, the effect of the relative-orientation between the reader and the transponder-antenna and the effect of using a ferrite core, to thecoupling, is presented. In Section 3, the proposedtransponder-antenna is described.2. Characterization of Single-Loop AntennaCurrent low-frequency RFID systems operate in the near-field region, where the communicating antennas aremultiple-turn antennas. In our research, the transponder-coil consists of approximately 500 turns. We used twocoils with inner diameter of 0.8 cm. One coil contained aferrite core, and another one was used without the core.The reader’s antenna is a large single-loop antenna ofvarious diameters.The use of the ferrite core helped increase the magneticcoupling between the reader’s single-loop and thetransponder’s coil, particularly for the LF range.An investigation of two coils (with ferrite-core andwithout ferrite-core) has been conducted to compare theeffect of the ferrite-core. Both have the same number ofwindings (500 turns). This number of turns stems from theoptimal number of turns (to maximize the radiated power),calculated from equations given in . The ferritepermeability was estimated to be 1300, based on typicalferrite values .2.1 Effect of the Single-Loop Size and Multiple-TurnCoil PositionThe single-loop’s radius influences the coupling betweenthe multiple-turn coil and the single-loop. The effect ofhorizontal positioning (with respect to the plane of theloop) is analyzed. Measured and simulated data are plottedin Figure 1. It shows the simulated near-field total powercoupled from a single-turn loop to a ferrite-coil, taken inthe air at one meter above the loop-horizontal plane. Thepower at the loop’s output terminals is 216 mW.Fig. 1 Coupling between a large coil and a small coil, simulated andmeasured (diamond-dot), as a function of lateral position.From our measurements and simulations, the coupledpower decreases as the loop-radius increases. At LF, thewave-length equals to 2.14 km; thus the single-turn loopand the coil effective electrical-size are extremely small atthis frequency, compared to the wave-length. In a two-dimensional representation, the measured results showed asimilar trend as the simulated data, i.e. the coupled powerpeaked when the coil’s position was at the center of thesingle-turn loop, when the loop-radius was at one meter orless, and the vertical-distance was at one meter. On theother hand, when the loop-radius was large, there were twomaxima, each located near the edge of the single-turn loop,similar to results published in .2.2 Effect of Ferrite and Coil-Orientation withrespect to the Z-axisThe rotation of the transponder’s coil can greatly affect thecoupling between it and the reader’s loop. Fig. 2 depictsthe θ-angle of this rotation.Position of the coil moving 1m above the loops plane [m]Simulatedpowerreceivedatthecoil[mW]-4 -3.2 -2.4 -1.6 -0.8 0 0.8 1.6 2.4 3.2 40.00080.0010.010.10.5radius=3mradius=2mradius=0.5mradius=4mradius=1mRadius=0.5m (measurement)
IJCSI International Journal of Computer Science Issues, Vol. 7, Issue 4, No 1, July 2010www.IJCSI.org9Fig. 2 Angle of rotation of the transponder.Simulation of a coil with and without ferrite-core, whenthe angle of rotation is between 0 and 360o, has shown thata coil with ferrite-core (solid-line) has a magnetic couplingabout 125 times greater than a coil without a ferrite-core(dashed-line), as shown in Figure 3, where the values onthe left-y-axis were normalized to the maximum non-ferrite value.Fig. 3 Received magnetic field at the transponder’s coil as a function ofits orientation’s angle.This increase of the received magnetic field is due to thelongitudinal shape and the permeability of the cylindricalferrite-core.3. Transponder-Antenna for Low-FrequencyRFIDThe cylindrical (rod) coil antenna is a highly directionalantenna when it operates in the axial radiation mode. Twoof the main parameters of the antenna are the high numberof turns and the coil’s circumference which is required tobe much smaller than the axial length of the coil-cylinder.Because of the high directivity of the cylindrical coilantenna with a very small diameter, good transmission andreception can only be guaranteed when the incomingelectromagnetic waves are on the axis of the cylindricalcoil. The maximum transponder’s sensitivity can beimproved with increased diameter, number of coil-turns, orinsertion of a ferrite core . However, this will increasethe size and the weight of the antenna, which when used asRFID transponders, would not be desirable.Instead, we propose the design of a dual-coil ferrite-coreantenna that has a more uniform performance and is lessdependent on the orientation of the cylindrical coil.Shown in Figure 4 are two fabricated antennas: 14cmferrite-rod length and 7cm ferrite-rod length.Fig. 4 Fabricated double-rod antennas (14cm version and 7cm version).The two ferrite-cores are symmetric; one ferrite-core is cutinto two equal parts, which are then attached at the centerof the other ferrite-core.The operation of a coil-antenna with a single ferrite-corehas long been employed to receive magnetic field. Theseantennas normally consist of a coil wound on a singlecylindrical core of high permeability of which the length-to-diameter ratio is very high. Provided that the magneticflux concentration in the ferrite core is uniform, a long-cylindrical ferrite coil has an effective inductanceestimated to be :(1)where μ is the effective permeability of the coil, l is thelength of the wounded ferrite-core, N is the number ofcoil-turns, and a is the radius of one wire-loop.Electric field equations in the far-field of a cylindrical-coilantenna have been derived in .L fer r ite a 2N 2 lfffff
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IJCSI International Journal of Computer Science Issues, Vol. 7, Issue 4, No 1, July 2010www.IJCSI.org11Stewart and William Hudson for their constructivecomments.References D. J. Hind, “Radio frequency identification and trackingsystems in hazardous areas”, Electrical Safety in HazardousEnvironments, 1994, Fifth International Conference on, pp.215-27. R. Davis, K. Shubert, T. Barnum, “Buried ordnancedetection: electromagnetic modeling of munition-mountedradio frequency identification transponders”, Magnetics,IEEE Transactions on, 2006, 42(7), pp. 1883-91. T. Ruff, D. Hession-Kunz, “Application of radio-frequencyidentification systems to collisionavoidance inmetal/nonmetal mines”, Industry Applications, IEEETransactions on, 2001, 37(1), pp. 112-6. E. Pettinelli, P. Burghignoli, A.R. Pisani, “ElectromagneticPropagation of GPR Signals in Martian Subsurface ScenariosIncluding Material Losses and Scattering”, Geoscience andRemote Sensing, IEEE Transactions on, 2007, 45(5), pp.1271-81. K.K. Williams, R. Greeley, “Radar attenuation by sand:laboratory measurements of radar transmission”, Geoscienceand Remote Sensing, IEEE Transactions on, 2001, 39(11),pp. 2521-2526. D.M. McCann, P.D. Jackson, P.J. Fenning, “Comparison ofthe seismic and ground probing radar methods in geologicalsurveying”, Radar and Signal Processing, IEE Proceedings,1988, 135(4), pp. 380-90. EM Software & Systems-S.A. (Pty) Ltd, “FEKO ProductOverview”, Web site: www.feko.info. J.D. Brunett, V.V. Liepa, D.L. Sengupta, “Extrapolatingnear-field emissions of low-frequency loop transmitters”,Electromagnetic Compatibility, IEEE Transactions on, 2005,47(3), pp. 635-41. Microchip Technology Inc, “125 kHz RFID System DesignGuide”, 1998, Web site: www.microchip.com. National Magnetics Group, “Soft Ferrites – A User’sGuide”, Web site: www.magneticsgroup.com. J. August, “Low frequency transponder and system”, UnitedStates Patent Application Publication No.: US 2007/0063895,Mar. 22, 2007. J.D. Kraus, “Antennas”, McGraw-Hill Book Company,1988. C.A. Balanis, “Antenna Theory Analysis and Design”, 3rded., John Wiley & Sons, Inc., 2005. J.D. Kraus and K.R. Carver, Electromagnetics, 2nd ed.,pp.155–160, McGraw-Hill, New York, 1973.