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Spatially Selective Antenna For Very Close Proximity

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This multi-part article
provides background on
RFID antenna-transponder
interactions, and presents
an antenna technique that
achieves greater discrimination
when reading
multiple transponders

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Spatially Selective Antenna For Very Close Proximity

  1. 1. High Frequency Design From February 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC HF RFID Spatially Selective Antenna for Very Close Proximity HF RFID Applications—Part 1 By Boris Y. Tsirline, PhD Zebra Technologies Corporation T his article intro- variety of HF RFID applications exist today. This multi-part article duces a novel HF By 2006, most of the manufacturing and ser- provides background on antenna with high vice industries had adopted this technology, RFID antenna-transponder spatial selectivity—its including medication authentication in the interactions, and presents performance and func- pharmaceutical industry, patient identifica- an antenna technique that tional characteristics are tion in the health care industry, product iden- achieves greater discrimi- analyzed for two orthogo- tification and inventory tracking in the retail nation when reading nal alignments of the industry, access restriction for security sys- multiple transponders antenna and the trans- tems and tickets processing in transportation ponder using a simplified service. A wide application spectrum spreads mathematical model. This model describes the from commercial and military to home and relationship between the transponder interro- entertainment sectors. Recently introduced gation zone and the Reader RF power, anten- Near Field Communication (NFC) technology na and transponder geometries, and their is also based on the magnetic coupling tech- electrical components. nology [1] and is similar to Contactless Smart In order to describe the properties of a Card protocol. This technology opens new transponder in a non-uniform magnetic field applications for the RFID technology, such as an integral parameter is introduced in place of automatic payment using cellular phones in the commonly used transponder activation close proximity communication as a transac- magnetic field strength. A set of new measur- tion vehicle. able parameters for characterization of the Three key elements comprise every RFID antenna-transponder combination are intro- system: a transceiver (Reader), a transponder, duced. The interaction of a conventional reso- and an antenna. In order to satisfy the grow- nant loop antenna with nearby transponders ing demands for HF RFID equipment, many is examined and the antenna’s inability to dif- vendors offer two of the three—the HF ferentiate the targeted transponder from the Readers and the transponders [2]. The third adjacent transponders is demonstrated. element of the system, the antenna, is not often readily available. Although HF magnet- Introduction ic antennas are widely offered for radio broad- RFID (Radio Frequency Identification) casting, transmission sources location finders, technology, originally developed for automated and for EMI/RFI measurements, HF RFID identification of aircraft and ships as a sec- antenna selection is limited. While RFID and ondary radar application, has today become a non-RFID magnetic antennas share common powerful tool in business process automation features such as sensitivity to the magnetic in many industries. HF RFID (High component of RF wave and the ability to gen- Frequency RFID) is based on magnetic cou- erate one (in the near field), two principal dif- pling between the transponder and the anten- ferences exist. na and is highly immune to the interferences First, HF RFID antennas activate battery- typical for industrial environments. A great less transponders by transferring the magnet- 18 High Frequency Electronics
  2. 2. High Frequency Design HF RFID ic field energy to them. Second, antennas maintain the bi- Utilizing HF Transponders” directional data transfer between a Reader and transpon- • Implementation of Transponder Activation ders. The transponder data transmission is based on the Magnetic Flux parameter for a transponder in a “Load” modulation technique [3], which enables the non-uniform magnetic field, and its association with Reader to detect an antenna impedance modulation geometry and electrical properties—“HF Transpon- caused by a transponder. Further discussions will assume ders” that a Reader is sensitive enough to assure a reliable • Justification of new characterization parameters for transponder interrogation as soon as it gets energized. an antenna-transponder structure and qualitative RFID magnetic antennas for conventional applica- analysis of the interaction between a closely spaced tions are aimed at activation and identification of multi- conventional HF resonant loop antenna and ple transponders at the longest possible range. The goal of transponder—“Antenna-Transponder Characteri- the antenna design for such applications is to detect zation” transponders’ presence and provide a wide coverage area. • New SS antenna development and its mathematical Transponders are activated by uniform magnetic fields model correlating system performance parameters and the antenna interaction within the interrogation with a transponder and antenna geometry and their zone is largely independent of their parameters. mutual orientation—“Magnetic Flux through Antenna design methodology considerably changes for Transponder” RFID applications that demand encoding a single, target- • Quantitative analysis of spatially selective antenna- ed transponder surrounded by others. For these applica- transponder interaction for their two orthogonal tions the targeted transponder is positioned in very close alignments—“Antenna-Transponder Interaction” proximity to the antenna and a specified interrogation • Antenna circuit components justification based on region turns out to be comparable with the transponder the specified activation magnetic field, available dimensions. In this situation the antenna-transponder Reader RF power and transponder coupled distance is only a small fraction of their sizes—transpon- impedance—“Antenna Circuit.” ders operate in a principally heterogeneous magnetic field and their interaction with an antenna is heavily RFID Applications Utilizing HF Transponders dependent as much on the distance between them as on HF RFID applications and their relevant antennas, their dimensions and mutual alignment. magnetically coupled with the transponders working at To be successful in performing an interrogation of only 13.56 MHz, can be at least divided into two industry inde- one transponder an antenna should have a feature we can pendent groups. The first group, as mentioned before, rep- call spatial selectivity (SS). Spatial selectivity is an resents the “spatially distributed items” application type. antenna’s ability to communicate with a single transpon- Antenna design for this group is aimed at achieving max- der, staying within the maximum available RF power imum operational range with the transponders, which are from a Reader, and not communicating with neighboring in the uniform magnetic field and located relatively far transponders. from an antenna or, in any case, weakly coupled with it. Previously proposed solutions to increase an antenna’s This group can be simply characterized by an inequality SS were based on shielding and suppression of magnetic field in adjacent areas. The disadvantage of such an SMAX << D (1) approach is that dimensions of shielding components are greatly dependent on transponder geometry and must be where SMAX = maximum size of an antenna or transpon- adjusted for every new transponder type. The shielding of der, and D = distance between an antenna and transpon- adjacent areas is only suitable for RFID applications der. which use one, exclusive form-factor transponder. For Although the numerous wide-ranging technical RFID applications working with a variety of transpon- papers, articles and surveys analyzing an antenna opti- ders, the shielding method inevitably complicates an mization for this group have been written [3, 4, 5] there RFID system, including antenna design. are no works known to the author that are devoted to The scope of this article is to demonstrate another development of antennas working in very close proximity strategy for achieving high SS. This approach is based on to transponders. an antenna magnetic flux forming technique and a spe- The distance-dimensions relation (1), meaning a cific antenna-transponder alignment. homogeneous magnetic field for transponders, signifi- The considerations will include: cantly simplifies calculations of antenna parameters. Under condition (1) an antenna magnetic flux density dis- • Classification of RFID applications and parametric tribution is calculated for a point in place of a transpon- analysis of HF transponders—“RFID Applications der. An antenna can be designed almost independently of 20 High Frequency Electronics
  3. 3. High Frequency Design HF RFID a transponder’s position because their presence does not practically influence antenna electrical properties. Among the huge variety of RFID applications, a sec- ond group can be distinguished. This group represents a “conveyor” type, or item-level RFID. The demand for an item-level identification can be encountered, for example, in PCB fabrication, automotive parts manufacturing and assembly, IC manufacturing, book sorting in libraries, ticket processing in transportation service, monetary value certificate handling, enhancement in gaming indus- try, home automation, pharmaceutical manufacturing, implantable medical devices, walking and reading assis- tance for visually impaired people, and smart packaging. The conveyor type of application is a scenario where Figure 1 · HF antenna for item-level RFID on conveyor. transponders (attached to the items) are arranged one after another and prepared for a sequential interrogation in a short distance to an antenna (Fig. 1). A Reader has to In close proximity to an antenna, the 3D magnetic flux identify only one targeted object that is surrounded by density is non-uniform, and the magnetic flux through a adjacent items (transponders). This group can be charac- transponder depends on its location and orientation with- terized by the inequality: regard to the antenna. Thus, for RFID applications described by eq. (2), an antenna should be developed with D << SMIN (2) consideration for transponder geometry and its electrical characteristics. where SMIN = the minimum size of an antenna or transponder. HF Transponders A few divisions of the second group of RFID applica- The dimensions of typical transponders (commonly tions can further include “static-object” and “dynamic- called tags) used for HF item-level RFID and other appli- object” sub-groups. A “static-object” is the item (a cations vary from approximately 20 by 35 mm, for a transponder) that is always positioned for an interroga- device made by Texas Instruments [6], up to 85 by 135 tion in close, but fixed distance to an antenna. The degree mm for one made by UPM Rafsec [7]. A transponder spec- of antenna-transponder coupling and their mutual align- ification usually includes the IC types, a resonant fre- ment remain unchanged for every conveyor stop-cycle. A quency with its tolerance, and the most important param- “dynamic-object” sub-group is the one where an aimed eter for an antenna design—maximum required activa- transponder is also surrounded by other adjacent tion magnetic field strength HA in the uniform field. The transponders. In difference from the “static-object” case a field strength ranges approximately from 98 to 120 longer interrogation range should be made available for a [dBµA/m] depending on ICs used, transponder inductors transponder as it travelings on a continuously moving and their dimensions and also on how well a transponder conveyor. And again, a Reader should selectively interro- is tuned to an operational frequency. In practical design it gate only one predefined transponder among others. is more convenient to use H value expressed in [A/m] Under the conditions of eq. (2), antenna-transponder units. The conversion [dBµA/m] to [A/m] unit gives coupling in relation to their mutual alignment changes significantly. An antenna with low SS will activate the H[ A / m] = 10{( H [ dBµA / m ]−120) / 20} targeted transponder and two (or more) closely-spaced adjacent transponders. Although a Reader’s anti-collision Consequently, the transponder activation magnetic function can manage an identification of many simulta- flux density BA [Vs/m2] for the uniform field can be neously activated transponders, it is unable to confine a obtained using targeted transponder. To discriminate a targeted transponder at a predefined location using such an anten- BA = µ0 H A ⎡Vs / m2 ⎤ ⎣ ⎦ (3) na, all transponders (items) must be appropriately spaced along their traveling path. Unfortunately, the extension of where µ0 = 4π*1E–07 [Vs/Am] (the free-space magnetic the separation interval substantially increases total permeability). interrogation time. In case of smart label encoding in The parameter HA is specified for the uniform mag- RFID printers. an increase of the transponder’s pitch netic field and can be directly used in calculations of the causes noticeable carrier material waste. antennas satisfying the inequality in (1). For applications 22 High Frequency Electronics
  4. 4. High Frequency Design HF RFID compliant with the inequality of (2) the transponders are in the heterogeneous magnetic field with the spatially depended flux density. Antenna calculations for such a case cannot utilize eq. (3) and a Transponder Activation Magnetic Flux (TAMF) ΦA should be engaged instead. TAMF for the transponder, which is perfectly tuned to an operational frequency, can be found as Φ A = BA A [ Vs] (4) where A = a transponder loop area (m2). The value A in (4) is the geometry mean dimensions (GMDm) must be used instead of the transponder coil physical dimensions [8]. Figure 2 · Transponder equivalent circuit. The time varying magnetic flux induces the voltage VC in the transponder coil tuned at resonance equal to the operational frequency f0 [Hz], thus tuning capacitor C (comprising an imaginary part of an IC’s impedance), a coil inductance L, a resistor RL pre- VC = 2πfOQΒΑΝ T senting an inductor losses and a resistor RP, which simu- lates a real part of an IC’s impedance. The Q-factor for the or using magnetic flux it gives LCR parallel circuit is determined as VC = 2πfOQΝ T Φ (5) 1 Q= (8) RL 2πf R L where Q is the transponder quality factor and NT = num- + 2πf R L RP ber of turns of transponder coil. If the transponder resonant frequency is fR and differ- ent than the frequency f0 then the transponder voltage Considering (7) and (8) the ΦA value given by (7) is amplification will depend on a degree of frequency devia- higher than the activation flux calculated in (4) for the tion (fR – f0) from the operational frequency. Linking the tuned transponder. voltage VC (5) of the parallel resonant circuit, which is In the non-uniform magnetic field a transponder gets typical for HF transponders, with the IC’s specified sup- activated when Magnetic Flux through Transponder ply voltage VA, gives (MFT) ΦT exceeds ΦA value. Then the activation flux ΦA can be used in an antenna-transponder evaluation and ⎡⎛ 2 ( f R − fO ) ⎞ ⎤ 2 analysis as a threshold that defines the boundary condi- VC = VA 1 + ⎢⎜ ⎟ Q⎥ (6) tions for a transponder activation interval. The integral ⎢ ⎣⎝ fR ⎠ ⎦⎥ parameter MFT characterizing an antenna-transponder structure is given by Equation (6) concludes the more the transponder is detuned the higher voltage VC is required to achieve the Φ T = ∫∫ BX ,Y , Z ∗ d A [ Vs] (9) voltage VA. Then the transponder activation flux ΦA can A be derived equating (6) and (5) and is given by where BX,Y,Z is the 3D distribution of the magnetic flux ⎡⎛ 2 ( f R − fO ) ⎞ ⎤ density (normal to a surface of a transponder coil) and lin- 2 VA 1 + ⎢⎜ ⎟ Q⎥ ear function of the current I circulating in antenna coil. ⎣⎝ fR ⎠ ⎦ (7) This current is defined by Reader RF power and ΦA = 2πfOQΝ T antenna equivalent impedance. The impedance of the loop antenna tuned to resonance can be presented by few com- From the formula (7) it follows that a detuned ponents. It consists of a radiation resistance, a resistance transponder with low Q-factor will have a higher activation that is equivalent to resistive losses of the coil, including magnetic flux compared with a tuned, high Q transponder. tuning and matching elements, and an impedance that is In general case a quality factor of the parallel reso- induced by a transponder via magnetic coupling. nance circuit can be found by considering a transponder Considering the fact that a total circumference of an equivalent schematic (Fig. 2), which includes a resonant antenna coil is much shorter than an operational wave- 24 High Frequency Electronics
  5. 5. length, the radiation resistance can acteristic of an antenna-transponder der activation in adjacent areas, the be neglected. To simplify further an combined structure. By definition, a SS parameter is obtained as initial analysis of an antenna- high SS implies that for an activation transponder interaction it is assumed of a targeted transponder, located in PTAA that the magnetic flux in an antenna an encoding interval, an antenna SS = 10 Log [dB] (10) coil that is produced by the current in requires much less power than maxi- PTAT a transponder is insignificant com- mum power available from the pared with the magnetic flux pro- Reader. Upon assigning PTAT for min- SS can also be defined by the mag- duced by an antenna itself. This imum RF power to activate a netic flux ratio using the value ΦA (4) assumption therefore implies that transponder in a targeted area and and the flux through an adjacent the impedance induced by a tuned PTAA for power required for transpon- transponder ΦΤAD transponder is much smaller than the tuned antenna resistive losses. Antenna-Transponder Characterization An HF antenna and a transponder working in immediate proximity to each other form a virtual device with one bi-directional RF port. Properties of this device are defined by both ele- ments—an antenna and a transpon- der, and the traditional antenna char- acteristics such as directivity or antenna gain become inappropriate for a description of such a combined structure. Being a one-port RF device further complicates its performance assessment. Only two characteristics of an antenna-transponder conglom- erate are practically available for testing. They are antenna impedance and an RF power level for which a Reader indicates whether a transpon- der interrogation process (including a completion of Write and Read com- mands) has been successful or not. With the aim of finding a proper way for characterizating an antenna- transponder combination, a set of new parameters was established and implemented. Among them are Spatial Selectivity (SS) introduced earlier, RF Power Margin, Relative Activation Power and transponder activation interval. These parameters are measurable and capable of describing the antenna-transponder properties and performance such as the transponder activation interval and the system robustness. Spatial Selectivity (as much as other parameters) is not an attribute of an antenna itself but rather a char-
  6. 6. High Frequency Design HF RFID found using (13) when Ξ = 0. Practically, an interaction ΦA SS = 20 Log [dB] (11) interval is measured by registering two transponder posi- Φ TAD tions where the Reader starts and stops its communica- tions while supplying an antenna with maximum RF RF power margin Ψ is another important parameter power. The parameter Ξ inside an interaction interval for directly related to a transponder activation interval or an any transponder position is measured by attenuating operational range. As was mentioned above, a one-port maximum available RF power from the Reader up to the device allows a practical measurement of antenna mini- point where its communication with a transponder fails. mum RF power when the Reader indicates it’s establish- An attenuation value expressed in dB corresponds to Ξ . ing a communication with the transponder for its differ- A collection of test results allows reconstruction of a ent positions inside an activation interval. Obviously, as transponder performance map for its activation interval. lower power is applied to an antenna, the shorter this This map assures a detection of any inconsistencies an interval is. By attenuating the maximum available interrogation region might have. Together with the mea- Reader RF power P0 to the level PMIN when a specified surement of RF power margin Ψ (12), Ξ test data enable activation interval is achieved, the power margin Ψ can system robustness analysis and antenna design verifica- be defined as tion. These actions are necessary because antennas and transponders parameters have natural deviations from their nominal values. Transponder activation flux, as an P0 Ψ = 10 Log [dB] (12) integral characteristic of antenna-transponder coupling, PMIN is sensitive to these deviations and so is Relative Activation Power. An antenna tuning frequency shift, RF Applying the same power suppression method as was port impedance mismatch, transponder resonant frequen- used for (12), a Relative Activation Power Ξ can be cy detuning and its excessive losses, and IC impedance defined as the ratio between the Reader RF power P0 and variations, just to name few, can occur in the manufac- the power PA applied to an antenna that changes a turing process or be caused by the influence of operational transponder status from non-activated to activated (and environmental conditions. High power margin compen- vice versa) for any position inside an activation interval sates for an increase of TAMF in RFID systems and thus makes an interrogation process more reliable. Introducing characterization parameters is instru- P0 Ξ = 10 Log [dB] (13) mental in an analysis of an antenna-transponder interac- PA tion and will be demonstrated first on an example of a conventional electrically small HF resonant rectangular The RF power margin (12) and the Relative Activation loop antenna. This antenna for the conveyor scenario is Power (13) are two versatile parameters describing an located in a parallel plane apart from a transponder but antenna-transponder energy transfer regardless of the in very close proximity to it. The total magnetic flux impact their individual characteristics might have on through transponder, which is contributed by a few spa- environmental conditions. tially distributed antenna elements, changes along a High SS might be achieved by changing antenna prop- transponder traveling above an antenna (Fig. 3) depend- erties to decrease the magnetic field strength for adjacent ing on an antenna-transponder’s alignments at a given areas and shrink an activation interval for a targeted instant. When the transponder approaches the antenna it transponder. A properly designed antenna with high SS makes use of the flux primarily from the element 3 (Fig. does not activate adjacent transponders even at maxi- 3a). The elements 1, 2, and 4 make an insignificant flux mum available RF power. For applications working with contribution. The flux via transponder attains first maxi- multi-dimensional transponders, a short activation inter- mum value when the transponder leading edge is right val is the most preferable. Ideally, this interval should be above the element 3 (Fig. 3b). While the center of the equal or less than the length of the shortest transponder transponder is approximately above the element 3 (Fig. type engaged. 3c), the antenna element 1 also supplies the transponder Likewise (13) the Relative Activation Flux Θ can be with magnetic flux, which has a direction in opposition to acquired as flux from element 3, thus dropping the total magnetic flux through the transponder to zero. As soon as the transpon- ΦT Θ = 20Log [dB] der becomes co-centered with the antenna coil (Fig. 3d) all ΦA 4 elements supply the transponder with the unidirection- al magnetic flux. A further transponder movement caus- Thus boundary points of an interaction interval can be es the same interaction (Fig. 3e and 3f) as was described 26 High Frequency Electronics
  7. 7. High Frequency Design HF RFID published in the next issue (March 2007). References 1. P. Albert, D. Ruffieux, O. Zhuk, “Understanding the Role of NFC-Based RFID Devices in the Consumer/ Mobile Market,” Emerging Wireless Technology/A Supplement to RF Design, pp. 2-4, August, 2005. 2. “Find a Vendor,” RFID Journal, 2006. www.rfidjour- nal.com/article/findvendor. 3. K. Finkenzeller, RFID Handbook–Fundamentals and Applications in Contactless Smart Cards and Identification, Second Edition, John Wiley & Sons, 2003. ISBN: 0-470-84402-7. 4. Y. Lee, “Antenna Circuit Design for RFID Applications,” AN710, Microchip Technology Inc., 2003. ww1.microchip.com/downloads/en/appnotes/00710c.pdf. Figure 3 · Transponder interaction with a conventional 5. “Tutorial Overview of Inductively Coupled RFID antenna. Systems,” UPM Rafsec, May 2003. www.rafsec.com/ rfidsystems.pdf. 6. “Tag-it HF-I Standard Transponder Inlays,” above. It can be concluded that, during its travel, the Reference Guide. Lit number: 11-09-21-062, Texas transponder encounters three distinct intervals 1, 2, and Instruments Incorporated, December 2005. www.ti.com/ 3 where its flux ΦT1 exceeds a Transponder Activation rfid/docs/manuals/refmanuals/HF-IStandardInlays- Magnetic Flux ΦA (Fig. 3g). For the conveyor type of RFID RefGuide.pdf. applications these three intervals exactly cover the posi- 7. “HF tags and inlays, standard products,” UPM tions of the targeted and two adjacent transponders. In Raflatac, 2006. www.rafsec.com/hf_availability.htm. accordance with eq. (11) the antenna is not spatially 8. G. Zhong, C-K. Koh, “Exact Closed Form Formula for selective and it creates a collision situation. Analyzing the Partial Mutual Inductances of On-Chip Interconnects,” total flux through the transponder at different positions IEEE 20th International Conference on Computer (Fig. 3g) one can suggest to attenuate RF power from the Design, Freiburg, Germany, 2002. www.iccd-conference. Reader in order to reduce the magnetic flux (ΦT2) and org/proceedings/2002/17000428.pdf. achieve a single interrogation interval thus improving the antenna SS. While this suggestion is valid it works only Author Information under one condition—an RFID system always using sin- Boris Y. Tsirline is the senior RFID research engineer gle form-factor transponders with zero parameter toler- at Zebra Technologies Corporation in Vernon Hills, IL. He ances. In reality an RFID system must be capable of received a BS and MS degrees in RF & Microwave working with different transponder dimensions. Engineering from Moscow Aviation University, Russia in Moreover, the transponders from the same group have 1973 and a PhD in EE from Moscow State University in normally distributed parameters around their specified 1986. Before moving to the US in 1992, he served as a values and the flux ΦA becomes a zone (Fig. 3g). The Director of R&D at Automotive Electronics and expansion of the line ΦA is related, for example, to the Equipment Corp., Russia, developing military and transponder’s resonance frequency (7) and Q-factor (8) aerospace electronic systems. He has been in the deviation effects. Decreasing an antenna’s magnetic flux Automatic Identification and Data Capture industry worsens flux margin and could cause a low encoding yield since 1995; first as an RF Engineer involved in LF RFID of transponders because of a low RF power margin (12). design of automotive immobilizers at TRW (Automotive By following a good design rule of “3 dB” it can be con- Electronic Group) and then at Zebra Technologies cluded that in order to achieve high antenna perfor- Corporation since 1998. He managed the development of mance, the SS for the intervals 1 and 2, and the power Zebra’s first HF RFID printer-encoder and established margin Ψ for the interval 1 (Fig. 3g), should be equal to or the design methodology for HF and UHF spatially selec- exceed 3 dB. tive antenna-transponder structures used throughout the corporation divisions for RFID labels and cards printers. Coming in Part 2 Dr. Tsirline holds three non-classified Russian and two Part 2 will continue the discussion with the topic US patents and has numerous pending patents for RFID “Magnetic Flux Through the Transponder,” and will be enhancements. 28 High Frequency Electronics
  8. 8. High Frequency Design From March 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC HF RFID Spatially Selective Antenna for Very Close Proximity HF RFID Applications—Part 2 By Boris Y. Tsirline, PhD Zebra Technologies Corporation T his article contin- Part 2 of this article ues the description concludes the author’s and analysis of a description and analysis of novel HF antenna with RFID antennas that are high spatial selectivity. capable of communicating Its performance and with a single transponder functional characteristics (tags) while in close are analyzed for two proximity to other tags orthogonal alignments of the antenna and the trans-ponder using a simplified mathematical model. Part 1 provided background on RFID tech- nology, identified the problem to be solved, and Figure 1 (detail) · HF antenna for item-level began a discussion of the interaction of the RFID on conveyer. antenna and the transponder. Part 2 contin- ues the discussion, presenting an analysis of coupling mechanism between antenna and patch amplifies the magnetic flux generated transponder with respect to their orientation by the antenna and also concentrates the and relative movement. magnetic flux, increasing the coil inductance and it’s Q-factor. Magnetic Flux through Transponder To analyze the performance of this original One possible way to overcome multi-inter- antenna design and estimate its SS, a mathe- val interaction of an antenna-transponder and matical model was developed. This model, by improve antenna performance is to use a establishing a relationship between mechani- finite conductive element (or elements cal and electrical characteristics of an anten- grouped together) for the magnetic flux gener- na-transponder structure, enables a compari- ation. The element can be a short straight son of the parameters for two orthogonal wire carrying the time varying current. With alignments. the aim of achieving high spatial selectivity As was declared above, Magnetic Flux (SS), a straight short wire type HF antenna through Transponder ΦT is the most powerful was proposed and implemented [9] for the parameter used for an antenna-transponder smart label encoding process in RFID Printer- quality and performance characterization. It Encoders. This antenna is based on the con- can be found using Equation (9). In the gener- ventional resonant rectangular loop antenna al case, this formula includes a spatial distri- fabricated on PCB, where the traces provide bution of the non-uniform magnetic flux densi- the antenna conductors. Three sides of an ty generated by a current through a finite antenna are covered by a flexible ferrite patch length rod, with a diameter much smaller then and one side is left open (Fig. 1). The ferrite an antenna-transponder separation distance. 16 High Frequency Electronics
  9. 9. High Frequency Design HF RFID The the magnetic flux density at any field point nor- Integration area of the transponder (Fig. 4) is limited mal to a transponder plane is produced by a short wire of by its width ∆W and length ∆R. The total MFT ΦT (9) for length 2L carrying a current I along the (-x) direction the interval Y ≥ 0 can be obtained then by summing over (Fig. 4) in accordance with Biot-Savart Law, which is writ- the contributions from all transponder area differential ten for a general case [10] as elements dA = dxdy. This is eq. (15) y µ0 NI R + ∆R rdr W + ∆W ( x + L ) dx Cosθ = (14) ΦT = ∫ ( y + z02 ) ∫ 4π ( x + L ) + ( y2 + z02 ) 2 y + z0 2 2 R W 2 R + ∆R rdr W + ∆W ( x + L ) dx The equation (14) includes a normalization of a vector − ∫ (y + z0 2 ) ∫ ( x − L ) + ( y2 + z02 ) 2 2 B to the transponder plane by accepting an angle θ R W y Cosθ = Carrying out the integration (see Appendix) gives eq. y + z0 2 2 (16), shown below: Eq. (16) 18 High Frequency Electronics
  10. 10. High Frequency Design HF RFID Figure 4 · Diagram showing the transponder magnet- Figure 5 · Transponder crosswise movement—inter- ic flux integration setup. section. Equation (16) was derived under an assumption that regarding transponder movement direction. These two the magnetic flux created by a transponder itself has lit- movements named “Crosswise” and “Lateral” describe a tle effect on the magnetic field of the antenna. transponder orientation in regards to an antenna plane. The transponder magnetic flux for the interval where To facilitate an antenna-transponder interaction analysis and a performance comparison, the simple, but quite ade- ∆R quate for both cases, mathematical model described in ≤Y ≤0 (17) 2 (16) will be used. To estimate an antenna SS for the transponder was reconstructed from (16) representing the transpon- Crosswise movement (Fig. 4 and Fig. 6) the total der by two parts (Fig. 5) having a total length ∆R with the transponder flux from an antenna carrying 80 mA cur- limits for the first one Yi–∆R by ∆W and the second one Yi rent with 3 turns coil was calculated and plotted (Fig. 7) by ∆W. By setting R=0 and making ∆Ri the variable in for the antenna location at Y = 0. There are four antennas (16) the sum of the magnetic flux Φ1 and Φ2 on the inter- having a length that ranges from 10 to 40 mm analyzed. val (17) can be calculated considering their opposite direc- The distance (the geometry mean distance) between an tions. antenna wire and a transponder plane is Z0 = 5 mm. The Equation (16) for the magnetic flux through a magnetic flux was calculated for the transponder 40 by 40 transponder includes and relates the antenna length, dis- mm, which has 1 nWb the activation flux. For a 40 mm tance to a transponder, its coil dimensions and the num- length antenna, the flux curve changes sharply, and for ber of turns, and also indirectly considers an RF power the second adjacent transponder the antenna has SS of available from the Reader by including the current I. approximately 8 dB, the interaction interval has two sep- Antenna-Transponder Interaction The proposed new antenna allows two alignments Figure 7 · Total magnetic flux through transponder for Figure 6 · Transponder crosswise movement. crosswise movement. 20 High Frequency Electronics
  11. 11. High Frequency Design HF RFID Figure 8 · Transponder lateral movement. Figure 9 · Total magnetic flux through transponder for arated parts. For the Second Adjacent Transponder (Fig. lateral movement. 7) that is ~40 mm apart from the targeted transponder, SS of this antenna is negative (–8.5 dB). The total mag- netic flux trough the transponder exceeds its activation level thus presenting a collision situation. While the acti- vation interval for the targeted transponder is only 30 mm, the antenna for such alignment can be used success- fully for a small item-level RFID only if the First Adjacent Transponder (Fig. 6 and 7) is moving out from a conveyor belt after being encoded. For a transponder Lateral movement and correspond- ing alignment shown in Fig. 4 and Fig. 8, the curve of the total flux trough the transponder is not as sharp as for the previously considered alignment but instead there is the single, relatively wide, interaction interval (Fig. 9). The same four antennas with the length ranging from 10 to 40 mm were analyzed for the position co-centered at X = 0. The distance between the antenna wires and the Figure 10 · Total magnetic flux through four transpon- transponder edge (the geometry mean distance) is R = 5 ders for lateral movement. mm and the separation is Z0 = 0. The magnetic flux was calculated for the similar transponder. For 40 mm anten- na length, its SS is ~4.5 dB for both adjacent transpon- antenna demonstrates high spatial selectivity for all ders and the activation interval for the targeted transponders in wide dimensional range (see Table 1). transponder is ~65 mm. The flux or power margin exceeds Maximum Relative Activation Flux was calculated for 9 dB, which makes an interrogation process very reliable. every transponder co-centered with the antenna. For the antennas centered at X = 0 with length decreas- ing from 40 to 30 and 20 mm the activation intervals Antenna Circuit shrink from 65 mm to 55 and 38 mm accordingly, and the The magnetic flux produced by an antenna will inter- relative activation flux or power changes proportionally sect the wires in the transponder coil and create current from 9 dB to 7 and 4 dB. The antenna with 10 mm length flow. The induced current flow in the transponder will is unable to activate the transponder. The biggest advan- have its own magnetic flux, which will interact with the tages of such alignment are the wide activation interval, magnetic flux of an antenna. At some point, current high spatial selectivity and RF power margin. induced in an antenna circuit by transponder flux can Figure 10 depicts the total magnetic flux curves for become comparable with an antenna current and, conse- four transponders (Lateral movement) spaced 5 mm quently, change its impedance and magnetic flux. Thus a apart from the antenna having 3 wires, 200 mA current current flowing in the antenna coil is defined by RF power and 30 mm length and centered at X = 0. With average from the Reader and an impedance that depends on prop- transponder activation flux of approximately 2 nWb, the erties of both magnetically coupled resonant circuits. 22 High Frequency Electronics
  12. 12. High Frequency Design HF RFID Transponder SS for an “encoded” SS for a “following” Maximum Relative dimensions (mm) transponder (dB) transponder (dB) Activation Flux (dB) 23 × 38 15.4 12 6.3 45 × 45 14.2 8.9 8.4 34 × 65 17.4 15 9.7 45 × 76 16.8 14.7 10.6 Table 1 · Summary of SS and magnetic activation flux for four different transponder sizes. To find a complete description of the current in an and LT is the inductance of a transponder coil. antenna coil it is necessary to know the parameters that The magnetic flux through a transponder coil is characterize a relationship between the geometric struc- increasing when it comes close to an antenna and so does ture of an antenna-transponder and their electrical com- the coupling coefficient. At some separation distance an ponents. The first parameter called the mutual induc- antenna-transponder coupling attains critical level at tance M relates two nearby coils of magnetically coupled which power transfer efficiency from antenna to devices. The mutual inductance depends on the geometri- transponder achieves its maximum and RAL becomes cal arrangement of both circuits. The parameter M can be equal to RAT. Then the third parameter called the critical obtained from (16) and expressed as coupling coefficient Kc linking Q-factors of both circuits for this case ΦT Μ= [H] (18) 1 I Kc = (22) QAQT Then impedance ZAT induced in the antenna circuit by the transponder circuit [11] can be written as As required for the critical coupling, an apparent inductance LA of an antenna coil can be obtained by com- bining (18), (21) and (22) ( 2πf Μ ) ( 2πf Μ ) X T 2 2 RTS Z AT = − j (19) 2 RTS + XT2 RTS + X T 2 2 ΦT LA = L2 T where RTS is equivalent resistive component of transpon- I QAQT der impedance, and XT is equivalent reactive component of transponder impedance. If both antenna and transponder circuits are tuned at In order to maintain the same transponder magnetic resonance with XT = 0, Equation (19) becomes flux for the critical coupling as for the loosely coupled case, RF power from a Reader must be doubled and ( 2πf Μ ) antenna impedance matched to 2RAL. 2 RAT = [ohm ] (20) Matching an antenna combined impedance when the RT critical coupling takes place for a transponder positioned at the center of an activation interval further increases where RAT is a resistive component induced in an anten- an antenna SS. Improved SS is achieved because the na by a transponder. transponder movement to any of two positions corre- Thus antenna impedance at the resonance consists of sponding to the edges of the activation interval causes an its equivalent resistance RAL associated with circuit loss- antenna impedance mismatch. The impedance mismatch es in series with resistance RAT (20). in turn decreases the power transfer efficiency and as a The second parameter called the coupling coefficient K result lowers the magnetic flux available for two adjacent is the ratio showing a grade of coupling between two transponders. If a design goal is to enlarge an activation devices and defined as interval then an adaptive impedance matching might be implemented. The antenna impedance adaptive matching Μ uses adjustable matching components, which parameters K= (21) LA LT changes depending on an antenna coupling grade with a transponder to keep antenna port impedance equal to an where LA is the apparent inductance of an antenna coil, impedance of RF power source. 24 High Frequency Electronics
  13. 13. The antenna coil practical design vation power, power margin and rela- take into consideration the current presented here is a multi-interactive tive activation power have shown increase in a transponder coil when process, but regardless of the type of encouraging results. the magnetic field exceeds transpon- a fabrication technology, an apparent A complete analytical model for der activation flux and the IC voltage inductance can be measured and con- magnetic flux analysis in crowded regulator is engaged. firmed after covering three sides of a environments, and antenna- rectangular coil by the flexible ferrite transponder coupling may be further Acknowledgements with permeability in the range of 20 explored using HF structure simula- The author would like to thank to 40 [12, 13]. Utilization of ferrite tor software. This model should also Dr. Hohberger, C.P., Torchalski, K., materials with higher permeability is limited by the minimum value of antenna resonating capacitive ele- ments, which should be no less than 50 pF to prevent circuit detuning in the operating environment. Conclusions A single element antenna allows for precise identification of closely spaced miniature objects with a wide range of transponder geometries. This spatially selective antenna, and the mathematical model developed for its analysis, are not restricted in size and may be successfully applied to widespread RFID applications involving large-scale structures. One limitation of a highly spatial- ly selective antenna is that, despite its proximity to the transponder, the antenna requires the same RF power as a conventional antenna for long range RFID applications. This limita- tion applies only to HF transponders activated by the antenna’s magnetic field, whereas battery powered transponders require significantly less power for the antenna. For con- veyor-type RFID applications in crowded environments with sur- rounding metal and plastic parts, the magnetic field becomes distorted. Such environments can increase power losses, detune both the anten- na and the transponder, and require higher power for transponder activa- tion compared to open environments. Nonetheless, the antenna-transpon- der coupling was analyzed and a working system implemented for the HF RFID printer-encoders using the mathematical model described in this article. Empirically obtained correc- tive coefficients for transponder acti-
  14. 14. High Frequency Design HF RFID and Schwan, M., at Zebra Technologies Corporation for and Method for Selective Communication with RFID helpful and productive discussions regarding HF RFID Transponders,” U.S. Patent 6,848,616, B2, February 2005. spatially selective antennas development and Gawelczyk, 10. M.I. Grivich, D.P. Jackson, “The magnetic field of R., for his assistance in antennas fabrication and testing. current-carrying polygons: An application of vector field rotations,” American Association of Physics Teachers, Appendix American Journal of Physics, Vol. 68, No. 5, pp. 469-474, An Appendix that includes the derivation of Equation May 2000. http://physics.dickinson.edu/~dept_web/activi- 16 is included with the online version of this article at ties/papers/polygonarticle.pdf. www.highfrequencyelectronics.com. 11. D.A. Bell, Fundamentals of Electric Circuits/With Computer Program Manual, 4th Edition, Prentice Hall, References 1988. ISBN: 0-13-336645-6. 1. P. Albert, D. Ruffieux, O. Zhuk, “Understanding the 12. “RFID Absorber Shielding,” FerriShield, Inc., 2006. Role of NFC-Based RFID Devices in the Consumer/Mobile http://www.ferrishield.com. Market,” Emerging Wireless Technology/A Supplement to 13. “Flex-Suppressor,” NEC TOKIN America Inc., RF Design, pp. 2-4, August, 2005. Vol.08, 2006. http://www.nec-tokin.com/english/product/ 2. “Find a Vendor,” RFID Journal, 2006. http://www. pdf_dl/FLEX.pdf. rfidjournal.com/article/findvendor. 14. K. Gieck, R. Gieck, Engineering Formulas, 6th edi- 3. K. Finkenzeller, RFID Handbook-Fundamentals tion, McGraw-Hill Inc., 1990. ISBN: 0-07-023455-8. and Applications in Contactless Smart Cards and Identification, Second Edition, John Wiley & Sons, 2003. Author Information ISBN: 0-470-84402-7. Boris Y. Tsirline is the senior RFID research engineer 4. Y. Lee, “Antenna Circuit Design for RFID at Zebra Technologies Corporation in Vernon Hills, IL. He Applications,” AN710, Microchip Technology Inc., 2003. received a BS and MS degrees in RF & Microwave http://ww1.microchip.com/downloads/en/appnotes/00710c Engineering from Moscow Aviation University, Russia in .pdf. 1973 and a PhD in EE from Moscow State University in 5. “Tutorial Overview of Inductively Coupled RFID 1986. Before moving to the US in 1992, he served as a Systems,” UPM Rafsec, May 2003. http://www.rafsec. Director of R&D at Automotive Electronics and com/rfidsystems.pdf. Equipment Corp., Russia, developing military and 6. “Tag-it HF-I Standard Transponder Inlays,” aerospace electronic systems. He has been in the Reference Guide. Lit number: 11-09-21-062, Texas Automatic Identification and Data Capture industry Instruments Incorporated, Dec. 2005. http://www.ti. since 1995; first as an RF Engineer involved in LF RFID com/rfid/docs/manuals/refmanuals/HF-IStandardInlays- design of automotive immobilizers at TRW (Automotive RefGuide.pdf. Electronic Group) and then at Zebra Technologies 7. “HF tags and inlays, standard products,” UPM Corporation since 1998. He managed the development of Raflatac, 2006. http://www.rafsec.com/hf_availability.htm. Zebra’s first HF RFID printer-encoder and established 8. G. Zhong, C-K. Koh, “Exact Closed Form Formula for the design methodology for HF and UHF spatially selec- Partial Mutual Inductances of On-Chip Interconnects,” tive antenna-transponder structures used throughout the IEEE 20th International Conference on Computer Design, corporation divisions for RFID labels and cards printers. Freiburg, Germany, 2002. http://www.iccd-conference. Dr. Tsirline holds three non-classified Russian and two org/proceedings/2002/17000428.pdf. US patents and has numerous pending patents for RFID 9. B.Y. Tsirline, C.P. Hohberger, R. Gawelczyk, “System enhancements. Information on the new High Frequency Electronics online edition: Each issue of High Frequency Electronics is now available online, appearing just like it does in printed form. The online edition is in PDF format, with a typical 7 to 10MB file size that is readily downloadable via any high- speed Internet connection. Although the online edition is currently available to anyone, we will soon implement a subscription policy that allows online subscribers to obtain the issue well in advance of its public release. We will maintain our technical article archives in the same manner we have done since the founding of the magazine. You can download, read and save interesting articles from the online edition, and you can also obtain past articles individually from the archives. The individual articles from each online edition will appear in the archives after the following month’s online edition has been published. The new online edition can be found at: www.highfrequencyelectronics.com 26 High Frequency Electronics
  15. 15. High Frequency Design HF RFID Appendix µ0 NI R + ∆R ydy W + ∆W ( x + L ) dx R + ∆R ydy W + ∆W ( x + L ) dx ΦT = ∫ ( y + z02 ) ∫ − ∫ ∫ (1) 4π ( x + L ) + ( y2 + z02 ) R ( y + z0 ) W ( x − L ) + ( y2 + z02 ) 2 2 2 2 2 R W W + ∆W ( x + L ) dx ∫ (2) ( x + L ) + ( y2 + z02 ) 2 W To do this integral we’ll use the following substitution: x + L = ρ and y2 + z02 = ε2 then dρ = dx and an integral from formula “i88” [14] ρ ∫ dρ = ρ2 + ε 2 (3) ( ρ + ε2 ) 2 Integral (2) can be written as: W + ∆W (y 2 + z0 2 ) + ( x + L ) 2 W = (y 2 + z0 2 ) + ( W + ∆W + L ) − 2 (y 2 + z0 2 ) + ( W + L ) 2 (4) or equation (4) can be presented in a form of y2 + ⎡ z0 2 + ( W + ∆W + L ) ⎤ − y2 + ⎡ z0 2 + ( W + L ) ⎤ 2 2 = (5) ⎣ ⎦ ⎣ ⎦ substituting first component in (1) ⎛ y y2 + ⎡ z 2 + W + ∆W + L 2 ⎤ y y2 + ⎡ z 2 + W + L 2 ⎤ ⎞ ⎣ 0 ( )⎦ ⎣ 0 ( ) ⎦⎟ R + ∆R ⎜ ∫ ⎜ ( y + z0 ) 2 2 − ( y + z0 ) 2 2 ⎟ dy (6) R ⎜ ⎟ ⎝ ⎠ To do integral (6) we’ll use the following substitution for the first component: y2 + ⎡ z0 2 + ( W + ∆W + L ) ⎤ 2 λ= ⎣ ⎦ then λ 2 = y2 + ⎡ z0 2 + ( W + ∆W + L ) ⎤ 2 ⎣ ⎦ y2 = λ 2 − ⎡ z0 2 + ( W + ∆W + L ) ⎣ and 2 High Frequency Electronics
  16. 16. y = λ 2 − ⎡ z0 2 + ( W + ∆W + L ) ⎤ 2 ⎣ ⎦ then λ dy = dλ λ 2 − ⎡ z0 2 + ( W + ∆W + L ) ⎤ 2 ⎣ ⎦ and use the following substitution for the second component: y2 + ⎡ z0 2 + ( W + L ) ⎤ 2 σ= ⎣ ⎦ then σ2 = y2 + ⎡ z0 2 + ( W + L ) ⎤ 2 ⎣ ⎦ y2 = σ2 − ⎡ z0 2 + ( W + L ) ⎤ 2 ⎣ ⎦ and y = σ2 − ⎡ z0 2 + ( W + L ) ⎤ 2 ⎣ ⎦ then σ dy = dσ σ − ⎡ z0 + ( W + L ) ⎤ 2 2 2 ⎣ ⎦ Integral (6) can be written as: R + ∆R R + ∆R λ2 σ2 ∫ R λ 2 − ( W + ∆W + L ) 2 dλ − ∫ R σ2 − ( W + L ) 2 dσ substituting (W + ∆W + L)2 = ξ2 and (W + L)2 = a2 λ2 λ ξ+λ ∫ λ2 − ξ2 dλ = λ − 2 Ln ξ − λ and σ2 a a+σ ∫ σ2 − a2 dσ = σ − 2 Ln a − σ [formula “i61” 14, ] Then two components of integral (6) can be rewritten as ( W + ∆W + L ) Ln ( W + ∆W + L ) + y2 + z0 2 + ( W + ∆W + L ) 2 R + ∆R R + ∆R y + ⎡ z0 2 + ( W + ∆W + L ) ⎤ 2 2 − (7) ⎣ ⎦ 2 ( W + ∆W + L ) − y + z0 + ( W + ∆W + L ) R 2 2 2 R and ( W + L ) Ln ( W + L ) + y2 + z0 2 + ( W + L ) 2 R + ∆R R + ∆R y2 + ⎡ z0 2 + ( W + L ) ⎤ 2 − (8) ⎣ ⎦ 2 (W + L) − y2 + z0 2 + ( W + L ) R 2 R March 2007 1
  17. 17. High Frequency Design HF RFID using (7) and (8) integral (6) is then (9) By repeating procedure above for: W + ∆W ( x − L ) dx − ∫ ( x − L ) + ( y2 + z02 ) 2 W and substituting a second component in (1) R + ∆R R + ∆R γ2 ν2 − ∫ R γ 2 − ( W + ∆W − L ) 2 dγ + ∫ R ν2 − (W − L ) 2 dν where y2 + ⎡ z0 2 + ( W − L ) ⎤ 2 γ= ⎣ ⎦ and y2 + ⎡ z0 2 + ( W − L ) ⎤ 2 ν= ⎣ ⎦ we can compute 2 High Frequency Electronics
  18. 18. (10) Adding (9) and (10), we will obtain a final result (Equation 16). March 2007 1

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