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UHF RFID Antennas for Printer-Encoders— Part 1: System ...
 

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    UHF RFID Antennas for Printer-Encoders— Part 1: System ... UHF RFID Antennas for Printer-Encoders— Part 1: System ... Document Transcript

    • High Frequency Design From September 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC RFID ANTENNAS UHF RFID Antennas for Printer-Encoders— Part 1: System Requirements By Boris Y. Tsirline Zebra Technologies Corporation T his series of arti- erated in almost all sectors of modern society. This three-part series pre- cles reviews UHF Manufacturing, pharmaceutical [1], health- sents a detailed overview transmission line care [2], air luggage and supply-chain man- of RFID encoder systems antennas developed for agement, item-level identification for a variety and the antenna solutions RFID Printer-Encoders. of industries is a small number of the applica- required for reliable printing It explains the basic oper- tion fields. (writing) to individual tags ating principles of anten- With the exception of a completely auto- nas, their effect on the mated system, HF or UHF passive transpon- printer’s encoding function as well as how the ders are rarely used by themselves. They are antennas influence the design of labels with usually laminated with paper or plastic layers embedded transponders (Smart Labels). The forming Smart Labels or Tags, which are able survey of antennas is preceded by the evalua- to communicate with RFID Readers. The tion of antenna-transponder mutual coupling name of the transponders, passive or battery- in reactive near-field and by the analysis of less, comes from the fact that the transponder the Printer-Encoder environment, which is powered by energy transmitted by the yields four comparison criteria of the anten- Reader antenna. This power supports the nas’ performance. Reader-transponder communication—the After discussing system requirements, the interrogation process—which includes writing article covers two novel ultra-compact UHF data to the transponder’s memory and retriev- antennas based on the tapered stripline trans- ing previously stored information and/or the mission line, developed for the mobile RFID unique transponder identification data. Printer-Encoders. These antennas enable the Most modern RFID applications require printers to encode short Smart Labels on a that the Smart Labels be readable by an opti- short pitch. The paper presents the develop- cal scanner and a human being, in addition to ment of the antennas, HFSS modeling, and an the Reader. Consequently the Smart Labels empirical study of their geometries, character- containing the transponders often have print- istic impedance and bandwidth. This type of ed bar codes and human readable text. The UHF antennas used for stationary and most convenient instrument to simultaneous- portable RFID Printer-Encoders may be uti- ly print text, bar codes and encode Smart lized by numerous item-level close proximity Labels is an RFID Printer-Encoder, which per- RFID applications. forms all three functions at the same time. Besides the labels, Smart plastic cards with 1. Introduction embedded transponders have also become The Radio Frequency Identification (RFID) popular. High interest from credit card organi- technology and its three major components zations in the Smart card technology [3] has (Readers, transponders and antennas) have driven the development of plastic card experienced huge progress in the past ten Printer-Encoders. Mandate-driven American years. Initially developed for aircraft identifi- and European markets and Asian manufac- cation in the 1940s, this technology has prolif- turing distribution centers require increasing 28 High Frequency Electronics
    • High Frequency Design RFID ANTENNAS quantities of HF and UHF RFID widely used in the automated valida- Accelerated in the recent years, Printer-Encoders, Print Engine- tion procedures for Smart Labels and the evolution of the RFID technology Encoders for applicators, and mobile cards, preventing their re-encoding and the hungry market for Printer- Printer-Encoders working with fork- and data corruption, continuously Encoders has fueled the development lifts in the warehouses. In addition to securing a smooth transition of many of specialized UHF antennas. Their the printing and the initial encoding RFID pilot programs and extending ability to work with transponders in purposes, this equipment is also the successes of existing applications. very close proximity and communi- cate selectively with only one target- ed transponder, tightly spaced with others, essentially distinguishes the specialized UHF antennas from the conventional ones. In contrast to the antennas designed for long range RFID applications, these specialized antennas are very similar to RF bi- directional couplers based on electro- magnetically coupled transmission lines [4] that are common in the RF and microwave realm. The difference from RF couplers is the variable dis- tance between the coupled devices, the variability of transponder shapes, and a single RF port of the antenna- transponder structure. Conventional antenna characteri- zation parameters such as gain, radi- ation pattern, radiation power effi- ciency, directivity and beamwidth, which are normally used in antenna design for long range RFID applica- tions, assume new meanings and def- initions. For example, beamwidth becomes transponder encoding range, and antenna directivity becomes spa- tial selectivity. The antenna- transponder interaction occurs in a complex printer environment, which can disturb the nearby electromag- netic field, the antenna characteriza- tion parameters turn out to be depen- dent on the surrounding objects, transponder electrical parameters, and dimensions. Furthermore, the composite architecture of the Printer- Encoders creates an RF unfriendly environment, affects the transpon- ders’ interrogation process, and imposes limitations on the antenna dimensions and location. Most impor- tantly, the Printer-Encoder and antenna designs also dictate the min- imum acceptable size of the Smart Labels and their transponder place- ment. Because of these mechanical
    • constraints the transponders cannot be placed arbitrarily in a Smart Label—their placement must be sep- arately specified for every printer brand and model. The Smart Labels specification, which dictates a particular transpon- der placement, indirectly expresses the RFID printer’s encoding capabili- ty. A list of parameters describing transponders placement includes the transponder placement range, the placement starting distance, and the separation distance between the adjacent labels on a liner, known as the pitch (Fig. 1(a) and (b)). When the dimensions of labels required for printing and encoding are Figure 1 · Smart Label structure and transponder placement. (a) Smart 4" × 6" or 4" × 4" and their length sig- Label design; (b) short labels with long pitch; (c) short Smart Labels; (d) nificantly exceeds the transponder’s small labels with short pitch. width, or the labels are relatively short but far apart from each other (Fig. 1 (b)), the antenna can easily tion and the challenge occur when the encoding a small component label communicate with the targeted Printer-Encoder must encode short requires an antenna with high spatial transponder without collision with the Smart Labels densely spaced on the resolution. This attribute is the so- adjacent transponders. The complica- liner (Fig. 1 (c) and (d)). In this case called spatial selectivity that is the
    • High Frequency Design RFID ANTENNAS antenna’s capability to reliably interrogate the selected comparison. transponder without activating surrounding ones. Section 4. UHF Antennas for Stationary Printer- The ability of a printer to encode the transponders Encoders presents a comprehensive review of several TL placed near the leading edge of the label defines the antennas developed for stationary UHF Printer-Encoders transponder placement starting distance and directly cor- and qualitative analysis of their impact on the printer’s relates with the antenna dimensions and its position encoding performance. inside the printer. The most challenging design goal is to Section 5. UHF Antennas for Mobile Printer-Encoders make a printer-encoder capable of working with short introduces the ultra-compact novel UHF stripline TL Smart Labels, where the length of the Smart Label is antennas, their strengths for mobile RFID Printer- nearly equal to the width of the embedded transponder Encoders, and optimization of the antenna geometries (Fig. 1 (d)). and electrical parameters using HFSS. This review examines the capabilities and limitations [Sections 3, 4 and 5 will be published in the October of the different planar transmission line (TL) UHF anten- and November issues of High Frequency Electronics.] nas, which are used for RFID Printer-Encoders requiring the interrogation of a single transponder tightly spaced 2. Antenna-Transponder Coupling in Close with other transponders and in very close proximity to Proximity the antenna. The review focuses on the low profile spa- Although the UHF passive transponders produced by tially selective mismatched stripline and double-conduc- the leading vendors could have their antennas shaped tor stripline TL antennas designed for printers capable of similar to a meander-line [5], bow-tie [6], or cross dipole interrogating densely spaced short Smart Labels. The [7], the majority of them are half-wavelength dipole or mismatched resonant TL antennas typically have a nar- folded-dipole antennas [5], [8], [9]. The dipole antenna is row bandwidth. To overcome this limitation a bandwidth most popular in various RFID applications because of the improving technique originally developed for impedance near-omnidirectional radiation pattern in the far-field matching TL transformers is applied to microstrip and [10] and a straightforward chip impedance matching pro- stripline TL antennas. This article also presents an cedure [11]. The half-wavelength (λ/2), in free space, of an empirical verification of the antenna geometries and elec- operational frequency 915 MHz (ISM US RFID band) is trical parameters that were initially derived by using 164 mm. The physical length of the transponders may Ansoft High Frequency Structure Simulator (HFSS). range from 120 to 20-25 mm depending on the permittiv- The ultra-compact stripline UHF antennas enable: ity of their substrate materials and the antenna profiles. There are three spherical spaces surrounding the • Individual encoding of short Smart Labels with Reader and transponder antennas in the transmitting small pitch; mode: reactive near-field, radiating near-field, and far- • Acceptance of transponders with broad deviations of field [12, 13, 14, 15]. The radius of each sphere depends on resonance frequency and activation power thresh- the operational frequency (or wavelength) and the largest olds; linear dimension (D) of the antennas. The interaction • Positioning of the transponder placement area near mechanism and the energy transfer between two anten- the leading edge of the label; nas are determined by whether they are located within • Printer batch mode encoding without involvement of each other’s near-field, radiating near-field, or far field. the anti-collision management; For the RFID 915 MHz frequency band the dimension D • Space saving design of the mobile RFID printers; of antennas is usually chosen as one-half wavelength. • Effortless installation and straightforward RFID The far-field, having a propagating wave, starts out- conversion of the existing bar code printers. side the sphere with radius R1, which can only be approx- imated [12] because of the violated condition D > λ. The next section, 2. Antenna-Transponder Coupling in Close Proximity examines magnetic and electric field dis- D2 R1 > 2 tribution along the antenna-transponder structure, ener- λ gy transfer and coupling mechanism between them. This section also identifies two criteria for comparison of field Therefore, for an antenna with the largest linear intensity and impedance bandwidth of the antennas. dimension D = 164 mm, the radius of the sphere for far- Section 3. Printer-Encoder Environment classifies four field is R1 > 164 mm and is smaller for shorter antennas. critical printer zones, relates their lengths to the con- When a transponder is located at distance R1 or far- straints imposed by the antenna dimensions on the ther from the Reader’s antenna, the electromagnetic com- Smart Label design and transponder placement parame- ponents of the propagating wave and its impedance are ters, and establishes two geometrical criteria for antenna independent of Reader antenna’s geometry, and the field 32 High Frequency Electronics
    • High Frequency Design RFID ANTENNAS The electro-magnetic components of an antenna’s reactive near-field and its wave impedance vary signifi- cantly across the antenna’s physical structure. The anten- na and the transponder inside a Printer-Encoder operate in each other’s non-uniform reactive near-fields. The electric and magnetic field strength distributions for the half-wavelength transponder dipole antenna in the transmitting mode are depicted in Figure 2. In close proximity, the magnetic field is typically concentrated at the center of the dipole, where the current attains its maximum value, while the maximum electric field strength is at the edges of the dipole arms [17]. Applying the reciprocity theorem [12], which states that an anten- na’s transmitting performance is equal to its receiving performance, one can conclude that a Reader’s antenna should have a similar to dipole electro-magnetic field dis- tribution for the best coupling with a transponder. Figure 2 · Dipole current distribution and fields. The source of antenna’s field is electric charges flow- ing through the antenna. Charges slowly moving in space create the reactive near-field and fast moving charges cre- surrounding the transponder is uniform. In long range ate the far-field [18]. At the UHF band the charges also UHF RFID applications, where the transponders are sev- vary in time with the period of 0.5 or 1 nanosecond. The eral meters away from the Reader’s antenna, they are in temporal variation also contributes to the antenna’s far- each other’s uniform far-field. During data transmission field radiation. The antennas radiated near-field and far- the transponder varies the impedance of its antenna and field strengths significantly increase if charges are spa- changes its field, but these field disturbances do not affect tially accelerated. Whenever a charge abruptly changes the current distribution of the antenna in the transmit- direction or vanishes, for example, because of the anten- ting mode or its electrical parameters. The antenna- na’s structure, the electrical energy applied to an antenna transponder bi-directional communication is provided by is efficiently converted into radiation [19]. The antenna’s the propagating wave, there is no coupling between them, radiation efficiency increases when its length approaches and therefore no mutual influence. the half-wavelength of its operational frequency. This Inside the sphere with the radius R1 is the radiating explains why the radiation of the half-wavelength dipole near-field. The inner boundary of this region is approxi- antenna having zero current value at the ends of its arms mated by the radius R2 is very efficient. High intensity radiation in the far-field is desirable for the long range RFID systems, which are D3 based on propagating waves. But to be appropriate for R2 > 0.62 λ very close proximity applications, an antenna should have a strong reactive near-field and a weak far-field to comply For the frequency of 915 MHz and the half-wavelength with the EMI/RFI regulations. antenna dimension D = 164 mm, the radius R2 = 72 mm. In very close proximity, the transponder activation This field is partly a product of the continuous electric energy is mostly delivered through the quasi-static elec- and magnetic field energy exchange with the antenna, tromagnetic coupling with the antenna. The coupling but predominantly is a radiation wave. The antenna is grade of two closely spaced devices depends significantly loosely coupled with the transponder and they have a on their separation distance, geometrical profiles, and weak mutual influence. mutual alignment. Magnetic coupling is provided by The region of the pure reactive near-field is within the mutual inductance [16] and electric coupling through estimated radius R2. This field is the result of the contin- static capacitances [4]. Mutual inductance is an attribute uous electric and magnetic field energy exchange with of closely spaced wires carrying current. The current antenna electrical energy. The field strength is propor- through each antenna creates the corresponding magnet- tional to the antenna’s Q-factor and the current flowing ic flux that induces voltage and current in the other through it. For Printer-Encoders and other very close antenna. Static capacitance is an attribute of closely proximity applications the antenna-transponder separa- spaced conductive plates or areas having opposite tion distance is 5-10 mm and is much shorter than the lin- charges. The antenna or transponder dipole’s arms are ear dimensions of the Reader’s or transponder’s antennas. these plates. The arms charges cause electric field 34 High Frequency Electronics
    • High Frequency Design RFID ANTENNAS strength variation and develop voltage across a nearby antenna and transponder constructing elements, which are in that field. Transponders use a backscatter data transfer mecha- nism, re-radiating received signals back to the interroga- tor by their own antennas. For data transfer the transponder modulates the impedance of its own antenna and changes the surrounding antenna field distribution. This modulation influences the field of the Reader’s antenna and in turn changes its current distribution, antenna impedance, and frequency tuning. An increase in separation distance reduces the grade of antenna- transponder electro-magnetic coupling. The energy delivered to a transponder is used by the transponder’s IC to support its interrogation. The RF power (PT) delivered to a transponder is a product of its Figure 3 · Power delivered to a transponder and RF coupling grade with an antenna and the strength of the power margin. reactive near-field of the antenna powered by a Reader. Regardless of the antenna type the power PT can be expressed by the equation: Equation (1) demonstrates that the coefficient K, power PA or both can be decreased for power reduction. PT = K * PA (1) For some types of antennas the coefficient K is indepen- dent of the antenna-transponder alignment and can be where K is a power transfer coefficient and PA is Reader decreased to low power PT. The drawback of lowering the power applied to an antenna. coefficient K or power PA is the loss of the RF power mar- In the RFID systems with spatially independent and gin over the transponder activation power threshold for high value coefficient K, the power delivered to a its placement range, as shown for Antenna #1 in Figure 3. transponder can considerably exceed its activation power The RF power margin is defined as the maximum sup- threshold. The activation power threshold is the Reader’s pression (in dB) of the Reader’s operational RF power RF power level at which the transponder becomes ener- achieved in the middle of the transponder placement gized and starts responding to the Reader’s commands. range, when the power falls to the transponder activation This power threshold level is a complex function that threshold level and the transponder stops communicat- depends on the antenna parameters, transponder IC ing. With a low power margin the interrogation process impedance matching and its activation voltage threshold. becomes unreliable because of the system’s susceptibility Most importantly, the activation power threshold may to the deviations of the transponder’s and the antenna’s depend on the transponder’s location inside a printer. The electrical parameters and their precise tuning. Although excessive activation power causes a relatively extensive some transponders with less than ideal parameters may communication interval and substantially increases the have acceptable performance for long range applications, transponder placement range (Fig. 1 (a)). the power delivered in the near-field may become insuffi- At disproportionate energy levels the reactive field will cient and the transponder will be missed. To stabilize the cover not only the targeted interrogation area but also the encoding process and make it robust, the RFID system surrounding areas, which is not a problem when dealing should have the highest possible RF power margin. with a single transponder, or when the transponders are In the RFID systems with spatially dependent coeffi- spaced far apart. Although this spatial separation local- cient K, its value changes depending on the antenna- izes the encoding interval, it also limits the minimum transponder coupling grade, which correlates to the ori- achievable label length. With closely spaced transponders, entation and proximity to each other. For such a system, the interrogation range must be controlled so as to prevent the coefficient K and the activation power achieve their accidental communications with the neighboring maximum values only for the transponder which is clos- transponders. One way to prevent this collision is to est to the antenna; they are noticeably lower for the adja- reduce the Reader’s power and consequently the length of cent transponders. the transponder placement range. In this case the deliv- A tightly spaced antenna and transponder behave as ered power is higher than the transponder’s activation an air-dielectric variable capacitor, which plates are power threshold only for the encoding range and is lower formed by the transponder and the antenna. Its capaci- than the activation power threshold outside of this range. tance is proportional to the area of the overlapping sur- 36 High Frequency Electronics
    • High Frequency Design RFID ANTENNAS faces of the antenna and the transponder. When the transponder moves closer to the antenna their mutual surface grows and the static capacitance increases. This increase in static capacitance improves the antenna- transponder coupling and raises the power delivered to the transponder as well. Similarly the magnetic coupling increases when the two current carrying “wires” move closer to each other. The power transfer coefficient varies along the communication range and the power delivered to the transponder throughout the encoding interval sig- nificantly exceeds the transponder’s activation power threshold. In this case the RFID system has a high RF power margin, which is illustrated by the power curve for Antenna #2 in Figure 3. For a selected antenna-transponder separation dis- tance, an RFID system achieves the maximum RF power margin when their mutual overlapping area is compara- ble with the transponders’ width. The limiting factor for the maximum grade of coupling is the impedance induced by the transponder in the antenna circuit. In very close proximity this induced complex impedance could cause a severe impedance mismatch between the Reader’s and the antenna’s ports leading to a drastic reduction of the transponder’s activation power. To characterize the RF system power margin and the antenna-transponder cou- pling grade the encoding field intensity is introduced. The coefficient K, in addition to being dependent on the antenna-transponder geometries and their alignment in the general case, is also a function of the antenna tun- ing frequency and the antenna impedance bandwidth Figure 4 · Antenna reflection coefficient and (BW). To justify the antenna bandwidth, at least two impedance bandwidth (BW). (a) BW = 90 MHz; (b) BW = aspects of the RFID system should be taken into account. 150 MHz. The first one is the spectrum of modulated signals that are used by the Readers for transponders interrogation. For 915 MHz U.S. RFID band, the allocated spectrum is 26 MHz ranging from 902 to 928 MHz. RFID uses fre- BW1 ( SWR1 − 1) SWR2 quency hopping modulation around the central frequency = × (2) of 915 MHz. Although the Readers from different vendors BW2 SWR1 ( SWR2 − 1) operate at the same frequency band, they differ in their ability to handle hopping frequency phase shifts associat- Substituting BW2 = 26 MHz at SWR2 = 1.4 and SWR1 ed with phase difference of signals reflected from the = 2 in Equation (2), we can find BW1 = 54.4 MHz. This BW antenna port for different channels. The Readers, which is derived for a precisely tuned 915 MHz antenna. are based on I-Q synchronous detection of the transpon- The second aspect of the RFID antenna’s BW selection der’s re-radiated signals, typically require at their RF is associated with the deviations of antenna’s electrical port a Standing Wave Ratio (SWR) of 1.4 or less in the and mechanical parameters. The antenna fabrication pro- operational band in order to perform reliable interroga- cess typically utilizes non-ideal materials and non-ideal tion. The BW definition for conventional antennas is a fre- operational procedures, which impact the antenna center quency band over which an antenna has a SWR = 2 or has frequency, port impedance, and consequently the input its reflection loss or S11 parameter that is less than –9.5 reflection coefficient (Γ). The reflection coefficient is relat- dB. The tuning frequency is the center of the antenna’s ed to SWR by the well-known formula: bandwidth. To obtain a standard BW value of an antenna, the BW at SWR = 2 is calculated from the antenna BW at SWR − 1 SWR = 1.4. For example, for a microstrip antenna, the fol- Γ= (3) lowing equation from [20] can be used: SWR + 1 38 High Frequency Electronics
    • Using Equation (3) and SWR = The chosen impedance bandwidth 1.4, we obtain coefficient Γ = 0.166. criterion thus represents a character- This value can be used as the maxi- istic of the antenna in terms of the mum acceptable level of reflection for technological stability and the the bandwidth. If the resonant fre- EMI/RFI immunity. quency of a narrowband antenna changes noticeably, the antenna [This article will continue in the input reflection loss at the opera- next two issues of High Frequency tional frequency increases, and the Electronics, beginning with with sec- power transfer coefficient K drops. tion 3. Printer-Encoder Environment. For example, a microstrip antenna All references will be listed at the end based on the substrate material of the final installment.] IS410 (ISOLA) with the dielectric constant ε = 4.25 ±0.15 and BW = 90 Author Information MHz can have a resonant frequency Boris Y. Tsirline is the Principal from 900 to 930 MHz (Fig. 4 (a)) and Engineer at Zebra Technologies maintain an unacceptably high Corporation in Vernon Hills, IL. He reflection coefficient |Γ| below 0.24 received a BS and MS degrees in RF (SWR = 1.63) for the 902-928 MHz & Microwave Engineering from frequency span instead of the Γ = Moscow Aviation University, Russia 0.166 required for SWR = 1.4. If an in 1973 and a PhD in EE from antenna based on the same dielectric Moscow State University in 1986. material has BW = 150 MHz, its Before moving to the US in 1992, he reflection coefficient magnitude is served as a Director of R&D at |Γ| <0.14 for the 902-928 MHz band Automotive Electronics and Equip- (Fig. 4 (b)) and it complies with the ment Corp., Russia, developing mili- Reader SWR requirements. tary and aerospace electronic sys- Deviations of other antenna tems. He has been in the Automatic parameters including the thickness Identification and Data Capture of the substrate and the copper industry since 1995; first as an RF cladding have less influence on the Engineer involved in LF RFID design antenna resonant frequency than the at TRW, and then at Zebra dielectric constant and the BW of 150 Technologies Corporation since 1998. MHz can be considered as a conser- He managed the development of vative estimate for the desirable Zebra’s first HF RFID printer- antenna bandwidth in order to toler- encoder and established the design ate technological deviations of the methodology for HF and UHF spa- antenna’s electrical and mechanical tially selective transponder encoding parameters. On the other hand, an modules used throughout the corpo- excessive antenna bandwidth is not ration divisions for RFID labels and advantageous. Antennas with sub- cards printers. Dr. Tsirline holds stantially wider than necessary three non-classified Russian and two bandwidth could potentially be sus- US patents and has numerous pend- ceptible to the electro-magnetic inter- ing patents for RFID enhancements. ferences caused by the printer’s near- He can be reached by e-mail at by electrical and electronic devices. BTsirline@zebra.com. DO YOU HAVE A GREAT IDEA OR INTERESTING PROJECT TO SHARE? Proposals for articles may be sent to Gary Breed, Editorial Director: gary@highfrequencyelectronics.com