U H F R F I D Antennas For Printer Encoders


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This three-part series presents
a detailed overview
of RFID encoder systems
and the antenna solutions
required for reliable printing
(writing) to individual tags

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U H F R F I D Antennas For Printer Encoders

  1. 1. 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
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. 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
  8. 8. 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
  9. 9. High Frequency Design From October 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC RFID ANTENNAS UHF RFID Antennas for Printer-Encoders— Part 2: Antenna Types By Boris Y. Tsirline Zebra Technologies Corporation P art 2 of this article Understanding antenna continues the dis- performance is one key to cussion of RFID obtaining accurate and antennas with a look at reliable writing of RFID tags, the operating environ- especially in a high-volume ment. of antenna and its automated system associated RFID tag printer-encoder. Readers may wish to have Part 1 available to follow the author’s references to earlier comments and figures. 3. UHF Printer-Encoder Environment Smart Label design restricted by the antenna-transponder interaction in very close Figure 5 · Common architecture of thermal proximity is complicated further by the transfer UHF RFID Printer-Encoder. Printer-Encoder environment. A common architecture of a thermal transfer UHF RFID Printer-Encoder (Fig. 5) yields an arrange- length. This zone is relatively far away from ment of four key internal areas: the media the antenna and is inactive. In case of para- supply zone, the following adjacent transpon- sitic activation of the transponders in this ders zone, the targeted transponder zone and zone, the simplest solution regardless of the the encoded transponder zone. This represen- antenna and label dimensions is shielding. tation assumes that the antenna is positioned Following adjacent transponders zone has underneath the printhead and behind the a variable length. Typically the design goal is platen roller. During printer operation, the to make this inactive zone as long as possible transponders pass through the zones sequen- in order to be able to process densely spaced tially from the media roll to the printhead. short labels and achieve a short pitch. If an Each zone can be active or inactive in terms of antenna positioned right after the platen its ability to activate the transponders located roller has intensive radiation or an extensive in it. The zone lengths are correlated to the longitudinal length, the inactive following printer structure, the antenna construction adjacent transponders zone shrinks. In this and its dimensions. Consequently the zones case the printer can still process narrow labels impact the Smart Label design and impose but with an extended transponder placement constraints on the minimum Smart Label range. The printer requires an increased pitch pitch and on the two transponder placement on the liner (Fig. 1(b)). This approach leads to parameters: the placement starting distance a noticeable waste of liner material and limits and the transponder placement range (Fig. 1). the minimum label length. In order to protect Media supply zone usually has a fixed the transponders against activation in this 36 High Frequency Electronics
  10. 10. High Frequency Design RFID ANTENNAS zone shielding component may be used. The disadvantage of a shielding solution is that the geometries of the shield- ing components depend on the transponder dimensions and involve adjustment for every new transponder form- factor. Targeted transponder zone generally depends on the antenna and the transponder dimensions as well as the Reader’s RF power. This zone is active, of course. When the antenna occupies much of the space between the plat- en roller and the media supply roll, the targeted transpon- der zone is relatively long. An extended targeted transpon- der zone requires either a long label or an outsized pitch in order to avoid collisions or transponder re-encoding with the wrong data. A short antenna, closely positioned to the platen roller, affords a short placement starting dis- λ Figure 6 · Open Transmission Line antenna—λ/4 wave- tance for the transponders and their short transponder length microstrip patch. placement range. Relatively short labels often have a partition distance between them that is only a fraction of their width (Fig. Two criteria are proposed for the integral characteri- 1(d)). Consequently, transponders embedded into such zation of relations between the antennas construction, labels are grouped close to each other. In this dense the printer zone dimensions and the Smart Label design arrangement all transponders can be activated simultane- parameters. The first criterion is the antenna structural ously by a “low” resolution antenna. The long range RFID feasibility, which reflects the space required for the systems commonly employ an anti-collision technique for antenna installation and designates the interval occupied processing a group of transponders. This technique is by the antenna along a transponder’s path. The second impractical for Printer-Encoders because it is unable to criterion is the transponder placement boundaries, which identify single targeted transponder. Only an antenna characterizes the antenna spatial selectivity and the with spatial selectivity can work with a single closest associated transponder placement parameters of the transponder without activating the adjacent ones. The Smart Label. higher the spatial selectivity of an antenna, the shorter These four criteria established above are intended to the transponder placement range. In the best case the be utilized for the comprehensive study and comparison transponder placement range can equal the transponder’s of the existing UHF antennas for stationary and mobile width. The shielding components can also be used to form RFID Printer-Encoders and also to determine the corre- the targeted transponder zone or to limit its longitudinal lation between the printer encoding function and the length with the same disadvantages as for the following Smart Label parameters limitations. adjacent transponders zone application described above. Encoded transponder zone length mainly depends on 4. UHF Antennas for Stationary Printer-Encoders the antenna field strength and the printer components Microstrip, stripline and others PCB transmission surrounding this area. The encoded transponder zone lines developed primarily for RF energy transfer have should be inactive. The antennas with highly intensive become accepted as antennas by UHF Printer-Encoders electro-magnetic field and some printer components with and by other RFID close proximity applications. Their the wave re-radiating ability can inadvertently make this planar structure, ability to handle relatively high RF zone active. In this case every encoded and printed label power and inexpensive, precise fabrication process enable must be either peeled or torn off in order to prevent the easy integration. Any transmission line antenna signifi- transponders collision or an incorrect re-encoding. The cantly changes its behavior and electrical properties need to take off the encoded transponders prohibits print- depending on the line length and its terminating status. er operation in the batch processing mode. Alternatively There are two TL antenna types: Open TL type based on the label pitch may be increased such that the encoded the open TL and Terminated TL type—antennas based on transponder leaves this zone when the next transponder the loaded TL. arrives for an encoding. The application of the shielding components suppressing electro-magnetic field in the Antenna Based on Open TL encoded transponder zone imposes functional limitations An Open TL antenna type is represented by a quarter- on the printer. For example, the shielding elements mount- wavelength microstrip patch antenna (Fig. 6) [21], [22]. ed in this zone conflict with the use of an external cutter. The patch antenna for close proximity applications differs 38 High Frequency Electronics
  11. 11. High Frequency Design RFID ANTENNAS from a conventional patch antenna in having shielding of the patch (Fig. 6). Bandwidth of patch antennas with- components along the non-radiating patch sides and a out a shield is narrow, approximately 30 to 50 MHz. In much narrower radiating edge in order to decrease field order to tolerate parameter deviations the geometry of strength in radiating near- and far-field zones. every antenna must be adjusted for frequency tuning and impedance matching. Antenna Structural Feasibility This antenna is based on PCB and enclosed in shield- Antennas Based on Terminated TL ing case with one open side. The antenna is arranged in In contrast to Open TL, antennas based on parallel with a transponder in an encoding area and Terminated TL could be resonant or not-resonant. They resides in interval of 20-25 mm (including its mounting may have wide or narrow bandwidths depending on the components) behind the platen roller. TL length and the terminating load value. In the most common case a terminated TL exhibits three specific fea- Transponder Placement Boundaries tures that have defined three trends in UHF antenna The antenna is positioned close to the platen roller development for very close proximity RFID applications. and provides a short transponder placement range and These features are related to the TL input impedance. placement starting distance, which allows the processing The input impedance ZIN of any loss-free TL having of short Smart Labels with a short pitch. characteristic impedance ZC, a length l and terminated by The antenna shielding elements are engaged to limit a load ZL in general is described [24] as the transponder interrogation interval. Electro-magnetic shielding is probably the oldest method of insulating the ⎛ Z + jZC tan β ⎞ transponder designated encoding area. In RFID technolo- ZIN = ZC ⎜ L ⎟ (4) gy shielding was initially employed for selective single ⎝ ZC + jZL tan β ⎠ transponder testing in the presence of others [23]. The shielding disadvantage appears when transponder form- where ß is the phase constant, which for a uniform, factors change frequently, for example, for different label loss-free TL is inversely proportional to a wavelength λ sizes, and so do the geometries of the shielding elements. and is given by 2π Encoding Field Intensity β= (5) λ Parallel alignment of the antenna with a transponder in the encoding area ensures improved coupling. Substituting (5) in (4) we obtain: However, because electrical charges are highly accelerat- ⎛ 2π ⎞ ed at the open edge of the antenna, it has very strong ⎜ ZL + jZC tan λ ⎟ reactive and radiating near-field intensity. The antenna ZIN = ZC ⎜ ⎟ (6) energy efficiency is very high and a transponder encoding ⎜ ZC + jZL tan 2π ⎟ ⎝ λ ⎠ at 5-10 mm from the antenna requires a few milliwatts of the Reader RF power. Shielding elements create losses in There are three conclusions of interest from equation (6). the antenna near-field and change its distribution around the antenna. Shielding reduces energy in the area of adja- 1. If TL characteristic impedance ZC meets the con- cent transponders but works inefficiently for radiating dition: near- field. A strong antenna electric field can potentially activate the transponders in encoded transponder zone or ZC = ZL (7) in following adjacent transponders zone (Fig. 5) and thus this antenna requires RF power control to reduce this Then substitution of (7) in equation (6) gives: field. The collision risk drives the Reader operational RF power down to the level that is insufficient to activate ZIN = ZL (8) transponders in the adjacent zones and significantly decreases system power margin as illustrated by Antenna In reference to (8) the impedance ZIN is theoreti- #1 in Figure 3. Magnetic field mostly concentrated near cally independent of TL length and equal to the the grounded edge partly contributes to the transponder terminating load for any frequency. Although in activating power. reality the bandwidth is limited by parasitic effects associated with non-ideal TL components, Impedance Bandwidth it can easily reach 5 to 6 GHz. In this case voltage Antenna feeding port impedance match is achieved by standing wave ratio (VSWR) of the TL is about 1; finding the appropriate point close to the grounded edge voltage along the whole TL length is equal to the 40 High Frequency Electronics
  12. 12. input voltage. The electric field strength distribu- tion around the TL is also homogeneous. 2. If the TL length is a quarter-wavelength: l =λ/4 (9) Substituting ßl = π/2, from (5) and (9) in (6) obtain: ZC 2 ZIN ( f0 ) = (10) ZL The ability of TL to transform load impedance (10) is widely used for impedance matching in the vicinity of one particular operational frequency (ƒ0). 3. If the TL length satisfies the condition: l =λ/2 (11) Substituting ßl = π, from (5) and (11) in (6) obtain: ZIN ( f0 ) = ZL (12) Equation (12) is valid for any impedance value ZC for Figure 7 · Antennas based on Terminated Non- one particular frequency ƒ0. TL experiences a standing Resonant TL. (a) “Two-Wire” transmission line; (b) “Dual wave with SWR ≥ 1 depending on how much impedance Microstrip” transmission line. ZC differs from impedance ZL. In an extreme case for a huge mismatch SWR >> 1, the voltage amplitudes near the edges of a λ/2 wavelength TL are in anti-phase and mounting elements) from the platen roller back to the can attain almost a double the input voltage value. This media roll. The antennas are very convenient for imple- voltage amplification increases the electric field strength menting a transponder interrogation method known as immediately adjacent to the TL and for SWR >> 1 is “encoding on the run” along the media feed direction. The almost equivalent to the input power increase of up to 4 distance between the “wires” on the dielectric substrate is times for the matched TL. 20-40 mm (Fig. 7(a)). The combined structure, “Dual Microstrip” transmission line, is 45-60 mm in length with Antennas Based on Terminated Non-Resonant TL two microstrips 20-40 mm apart (Fig. 7(b)). The so-called Terminated Non-Resonant TL antennas are presented by “Two-Wire” TL [25] (Fig. 7(a)) formed by Transponder Placement Boundaries two PCB traces and by a combined arrangement of two Both antennas have an excellent selectivity outside of microstrip transmission lines [26] (Fig. 7(b)). This group the targeted transponder zone (Fig. 5) to prevent commu- of antennas utilizes the TL phenomena (8). For all anten- nications with adjacent transponders; however, the zone nas based on terminated TL, their electrical charges slow- itself is much wider than a transponder width. Antennas ly accelerate at the edges. Therefore, the antennas have a must be field upgradeable for different transponder form- weak radiating near- and far-field intensity, while high factors and redesigned to adjust the transponder place- current provides relatively strong reactive near-field. ment range (Fig. 1(b)). With these antennas a long pitch is required to encode short Smart Labels. Antenna Structural Feasibility Both antennas, based on a highly technological PCB Encoding Field Intensity fabrication process, have an orthogonal alignment of their Depending on permittivity of the dielectric substrate, traces with the targeted transponder. Their structures antennas can have a width of traces approximately 1.5-3 take up to 45-60 mm in longitudinal length (including the mm either for the “Two-Wire” TL or for the “Dual- October 2007 41
  13. 13. High Frequency Design RFID ANTENNAS Microstrip” TL to attain characteristic impedance of 100 Encoding Field Intensity ohms. Because of the antenna-transponder orthogonal The microstrip TL base element for these antennas orientation, the antennas form a small mutual static has a lower characteristic impedance ZC than the load capacitance and have a loose coupling with transponders. impedance ZL and therefore a wider than non-resonant The areas of the electric field strength for the “Two-Wire” TL conductive strip, which increases static capacitance TL are not quite close to transponder’s most sensitive and a coupling with a transponder. The impedance mis- edges. Both antennas have comparatively low power effi- match causes a wave reflection with standing wave ratio ciency but could have a high RF power margin. The areas SWR >1 along the line and increases the electric field of intensive electric field of the “Dual-Microstrip” struc- strength above the line. The reflection coefficient Γ is a ture are positioned closer to the sensitive transponder complex voltage (current) ratio, which may be expressed edges but the mutual overlapping area is small and the in terms of the antenna characteristic impedance and coupling grade is still low. The electric field strength is load impedance (ZC and ZL) correspondingly: homogeneous along both transmission lines and ampli- fied by transformer usage. Magnetic field surrounding ZL − ZC Γ= (13) every TL is practically not contributing to transponders ZL + ZC activation power. Substituting (13) in (3) we obtain Impedance Bandwidth ZL Both transmission lines are terminated by loads SWR = (14) ZC matching their characteristic impedances. They have SWR ~1 over a frequency band that is much wider than 1 The equation (14) shows that an increase in ratio GHz. The “Two-Wire” TL width W1 defines its character- between the load impedance ZL and the microstrip istic impedance that is about 300 ohms. To satisfy the con- impedance ZC causes an amplification of SWR and makes dition (7) TL is loaded by a 300 ohm resistor. An RF trans- stronger electric field above the TL. The impedance ZC is former with impedance ratio equal to 6 is used to provide inversely proportional to the conductive strip width W2 or the 50 ohm antenna port impedance match and anti- W3 (Fig. 8(a) and (b)). For both antennas the conductive phase voltages. The combined structure—“Dual strip widths can be made comparable to the transponder Microstrip” transmission line (Fig. 7(b)), loaded by two width and RF power margin can attain 3-6 dB level with- 100-ohm resistors R1, makes the characteristic out a significant expansion of the encoding range. impedance of the antenna independent of the distance D. The quarter-wave TL antenna contributes to transpon- It also uses an RF transformer with the impedance ratio der power delivery by electric field at one side of the TL and of 2 for impedance matching and phase shifting. by magnetic field at the transponder’s center. The half- wave TL antenna is twice as long, has double the mutual Antennas Based on Terminated Uniform Resonant TL static capacitance with a transponder, and therefore main- The second type of antennas is based on terminated tains an enriched coupling and encoding field intensity. but mismatched TL. The so-called Terminated Uniform Resonant TL antennas are demonstrated by the λ/4 (Fig. Impedance Bandwidth 8(a)) and the λ/2 (Fig. 8(b)) length of the uniform The geometries of λ/4 and λ/2 TL antennas, terminat- microstrip TL. This group of antennas realizes TL phe- ed by mismatched loads, define their resonant frequency nomena (10) and (12) respectively. Antenna port and consequently their bandwidth. The bandwidth ∆ƒ of impedance is matched to the system impedance without the quarter-wave TL antenna can be obtained from [27], additional matching network. ⎧ ⎪ 4 ⎡ Γ 2 Z0 ZL ⎤⎫ ⎪ Antenna Structural Feasibility ∆f = f0 ⎨2 − arccos ⎢ m ∗ ⎥⎬ (15) ⎪ π ⎢ 1 − Γ m ZL − Z0 2 ⎥⎪ Both antennas are in parallel alignment with the tar- ⎩ ⎣ ⎦⎭ geted transponder and occupy a 20-30 mm interval behind the printer’s platen roller. Applying equations for microstrip characteristic impedance and strip width from [28], the bandwidth of Transponder Placement Boundaries the quarter-wave TL antenna is calculated using equation These antennas allow a printer to achieve a short (15) for the frequency 915 MHz as a function of the strip transponder placement starting distance 10-15 mm and width for impedance ZL in the range of 2 to 8 ohms (Fig. placement range 20-25 mm for transponders with dimen- 8(c)). The plot shows that the strip width W2 can be sions 8 × 95 mm or 10 × 95 mm [11, 13]. The pitch for the increased up to 35 mm without violating the justified labels is in the range of 40-50 mm. antenna bandwidth of 150 MHz. 42 High Frequency Electronics
  14. 14. Figure 8 · Antennas based on Terminated Uniform Resonant microstrip TL for 915 MHz band. (a) λ/4 TL antenna; (b) λ/2 TL antenna; (c) λ/4 TL band- width vs. width; (d) λ/2 TL antenna bandwidth vs. width. Substituting the antenna length l = λ/2 in (6) for port impedance ZIN, for the half-wave TL antenna the reflection coefficient Γ is: (Z − ZL ) tan 4 θ + 4 ( Z0 ZL ) ( Z0 − ZL ) tan 2 θ 4 4 2 2 2 2 2 Γ= 0 ⎢4 ( Z Z )2 + ( Z 2 + Z 2 )2 tan 2 θ ⎥ (16) ⎢ ⎣ 0 L 0 L ⎥ ⎦ where f θ = βl; θ = π f0 For the maximum reflection coefficient Γm = 0.333 in (16) that corresponds to SWR = 2, θm and the bandwidth ∆ƒ can be obtained, ⎛ θ ⎞ ∆f = 2 f0 ⎜ 1 − m ⎟ (17) ⎝ π ⎠ where fm θm = π f0 and ƒm corresponds to Γm. Using equations for microstrip characteristic impedance and strip width from [28], the bandwidth ∆ƒ from (17) of λ/2 wavelength TL antenna is plot- ted versus strip width W3 (Fig. 8(d)) for ZL = 50 ohm and the frequency 915 MHz. In order to comply with the requirement of ∆ƒ = 150 MHz, the strip width W3 of the half-wavelength TL antenna should not exceed 14 mm. This bandwidth restriction limits the transponder placement range in the case when printer design requires a wide transponder encoding area.
  15. 15. High Frequency Design RFID ANTENNAS Antennas Based on Terminated Tapered Resonant TL Another sub-group of the second type of antennas is the so-called Terminated Tapered Resonant TL antennas. The design goal is to achieve for microstrip TL antennas a relatively wide bandwidth and an increased grade of coupling with transponders. This goal is accomplished by implementing a method previously developed for band- width enhancement of impedance matching TL trans- formers. This method is based on the theory of small reflections [24] applied to a tapered (non-linear) profile of characteristic impedance for any TL. Antennas are pre- sented by the λ/4 wave and the λ/2 wave non-uniform microstrip TL. Antenna Structural Feasibility The width of the quarter-wave non-uniform microstrip TL is tapered from W4 to W5 (Fig. 9(a)). The edge widths of the half-wave non-uniform microstrip TL antenna are W6 (Fig. 9(b)). Both antennas can be made wider than the widths of uniform microstrip TL anten- nas. The corresponding lengths of non-uniform microstrip TL antennas are shorter than lengths of uni- form ones because of the extension of the sides of the tapered microstrip TL. The considered example is the half-wave microstrip linear width (non-linear character- istic impedance) taper TL (Fig. 9(b)). The width of the TL varies linearly from 18 to 4.5 and back to 18 mm, the dielectric constant of the substrate is 4.25, and the height of the substrate and the length of the strip are 1.6 mm and 65 mm respectively. Transponder Placement Boundaries Terminated Tapered Resonant TL antennas can pro- vide the same placement starting distance and placement range compared to the Terminated Uniform Resonant TL antennas with equally wide conductive strip. For an extended transponder placement range the tapered con- ductive strip can be made wider without sacrificing the antenna bandwidth. Encoding Field Intensity Field distribution above the quarter-wave terminated tapered TL antenna (Fig. 9(a)) covers only a part of the targeted transponder thus delivering half the power of Figure 9 · Antennas based on Terminated Tapered the half-wave TL antenna. Electric and magnetic field Resonant microstrip TL: (a) λ/4 TL antenna; (b) λ/2 TL distribution of the half-wave terminated tapered TL antenna; (c) S11 parameter for λ/2 TL antenna dimen- antenna (Fig. 9(b)) is concentrated at the most field sen- sions 4.5 × 18 × 65 mm. sitive transponder areas. The antenna with linearly vari- able width at the input end W6 = 18 mm maintains a greater mutual static capacitance with the transponder Impedance Bandwidth and provides a higher spatial selectivity than the uniform Like other terminated resonant TL antennas, the TL antenna with the narrower conductive strip. The RF tapered TL antennas have their port impedance of 50 power margin can achieve 6 dB without a significant ohm without an additional matching network. In contrast increase in the transponder encoding range. to the uniform TL, the λ/2 wavelength linearly tapered 44 High Frequency Electronics
  16. 16. width microstrip TL antenna has a widened bandwidth. Measured reflection loss (S11) of an antenna with a conductive strip at the input end width W6 = 18 mm (Fig. 9 (c)) shows that its bandwidth exceeds 150 MHz. The taper implementation for the λ/4 wave microstrip TL is not necessary for the band- width enhancement unless dictated by other design reasons. The uniform λ/4 wave microstrip TL with a strip width W4 of up to 30 mm already has BW in the range of 150 MHz (Fig. 8(c)). The importance of tapered λ/4 wave TL sec- tions was shown by Young [29, 30] for a bandpass filter design. He demon- strated that every second impedance step quarter-wave transformer replaced with an opposite impedance step provides the equal input and output impedances. It implies that the two parts of λ/4 wave tapered TL can be used as building blocks for tapered λ/2 wave TL antennas. It was shown by Collin [24] that reflection coefficient of tapered TL is: ( ) L 1 −2 jβz d Γ IN ( f ) = 2∫ e ln Z dz (18) 0 dz where z is the position along the taper, L is the taper length, Z is the taper variation, Z0 represents the reference impedance at the input end of the taper. There are numerous solutions for (18) available for several characteristic impedance profiles (not strip width profiles) including exponential, linear, tri- angular [27], Klopfenstein [31], and Hecken [32] in order to increase the band- width. For example, for the exponential taper the input reflection coefficient can be obtained [27]: 1 Z sin βL Γ IN = Ln L e− jβL (19) 2 Z0 βL This simplified solution (19) assumes TEM propagation mode for TL and both its characteristic impedance and propagation coefficient are distance- independent. Practically these parameters are changes along a line and prop- agation wave is not quite TEM. The actual two-section combined TL length then is shorter than λ/2 wavelength. For a maximum allowed reflection coef- ficient in the pass band the taper profile introduced by Klopfenstein has the shortest total length. The reflection coefficient along a non-uniform TL can be described by a non-linear Riccati-type differential equation [33], which does not have a gen- eral analytical solution. The analysis can be based on numerical methods [34] or performed using electromagnetic analysis software, such as HFSS from Ansoft Corporation [35]. [This final part of this article series will appear in the nextissue of High Frequency Electronics. All references will be listed at the end of Part 3.] Author Information Boris Y. Tsirline is the Principal Engineer at Zebra Technologies Corporation in Vernon Hills, IL. He received a BS and MS degrees in RF & Microwave Engineering from Moscow Aviation University, Russia in 1973 and a PhD in EE from Moscow State University in 1986. He has been in the Automatic Identification and Data Capture industry since 1995; first as an RF Engineer involved in LF RFID design at TRW, then at Zebra Technologies Corporation, where he has been since 1998. He can be reached by e-mail at BTsirline@zebra.com.
  17. 17. High Frequency Design From November 2007 High Frequency Electronics Copyright © 2007 Summit Technical Media, LLC RFID ANTENNAS UHF RFID Antennas for Printer-Encoders— Part 3: Mobile Equipment By Boris Y. Tsirline Zebra Technologies Corporation A ntennas for RFID The final installment of applications have this series looks at antennas unique require- for mobile or portable RFID ments, particularly for printer-encoder equipment the small spaces inside portable or mobile equip- ment. This final installment of this series of articles looks at antennas for these types of RFID printer-encoders, followed by summary comments for the entire series and an exten- sive list of references. UHF Antennas for Mobile Printer-Encoders Space saving for mobile RFID printer- encoders is the biggest concern. Printers require UHF antennas to be slim, because the space available for their installation is very limited. In addition to the geometric con- strains, the antennas must enable the encod- ing of short labels on a short pitch. Terminated tapered resonant stripline TL antennas are most qualified to meet these stringent require- ments of the portable printers. The stripline TL antennas are ultra-compact and conformal. They fit in the space near the printhead and can provide a short transponder placement range. These antennas have received the high- Figure 10 · Structure of terminated tapered est acceptance for transportable and station- stripline TL antennas: (a) single conductor TL ary RFID printer-encoders. Antennas are pre- antenna; (b) dual-conductor stripline TL sented by the half-wave stripline (Fig. 10(a)) antenna. and a double-conductor stripline (Fig. 10(b)) linear taper width TL. ders on the liner without activation of adja- Antenna Structural Feasibility cent transponders. Examples of a stripline Stripline TL antennas, which are arranged and double-conductor stripline TL antennas in parallel with the transponder in the encod- are built on PCB substrate and have dimen- ing area, occupy a very small space behind the sions of 3.5 × 18 × 100 mm and 6 × 14 × 100 platen roller (Fig. 11). These antennas allow mm, respectively. The internal conductor strip selective encoding of densely spaced transpon- (strips for a double-conductor stripline) is 18 High Frequency Electronics
  18. 18. High Frequency Design RFID ANTENNAS Figure 11 · Printer zones with stripline TL antenna. enclosed by two ground planes, stitched by vias along the other three sides of the antennas to organize electric walls and reduce parasitic radiation. The inner layer pro- file (Fig. 10 (a)) is a modified bow-tie shape with the width linearly varied from 9 to 4.5 and back to 9 mm for the stripline and from 10 to 3 to 10 mm for two strips of the double-conductor TL antenna. The dielectric constant of both substrates is 4.25 and their height is 3.5 and 6 mm accordingly. The length of the single stripline TL is 64 mm and for double-conductor line is 57 mm. The narrow cen- ter part of the inner layer is positioned close to the active edge of the TL in order to concentrate magnetic field at the center of this edge. This position of the maximum magnetic field usually corresponds to the center of a tar- geted for encoding transponder and supports an optimal energy transfer for the symmetrical antenna-transponder alignment. Transponder Placement Boundaries The single stripline TL antenna with a thickness of only 3.5 mm improves printer’s performance by providing a short transponder placement starting distance from the label’s leading edge. It enables individual encoding of short Smart Labels with a short pitch comparable to the transponders width (Fig. 1 (d)). The double-conductor stripline TL antenna with a thickness of only 6 mm was Figure 12 · HFSS simulation of tapered stripline TL. (a) developed for specific Smart Labels requiring a longer single conductor TL antenna—E field; (b) dual-conduc- transponder placement range and higher antenna energy tor stripline TL antenna—E field; (c) S11 for dual-con- efficiency than the single stripline TL antenna. ductor stripline TL antenna. Encoding Field Intensity Both antennas are in parallel alignment with target- coupling with a dipole type transponder antenna (Fig. 2). ed transponders and are coupled with them by one open The capacitive coupling maintained by the stripline TL long side edge. The electric field strength distribution antenna is relatively weak and permits very close posi- simulated using Ansoft HFSS for the single stripline TL tioning to transponders. The stripline TL antenna is less antenna (Fig. 12 (a)) and for the double-conductor spatially selective than the microstrip TL antenna but its stripline TL antenna (Fig. 12 (b)) shows optimal shape for RF power margin is still about 3 dB without a significant 20 High Frequency Electronics
  19. 19. High Frequency Design RFID ANTENNAS bandwidth. They are shorter than λ/2. A solution for reflection loss S11 and geometry calculations for the dou- ble-conductor TL antenna are obtained by HFSS simula- tion (Fig. 12 (c)) and verified empirically. For the above samples the single stripline (Fig. 13 (a)) and double-con- ductor stripline (Fig. 13 (b)) TL antenna S11 parameters demonstrate bandwidths in excess of 150 MHz. By vary- ing individual strip lengths the multi-conductor stripline TL antenna enables further increase in bandwidth, antenna sensitivity, spatial selectivity, power efficiency and transponder placement range. Conclusions The article provided a thorough consideration of UHF antennas for stationary and mobile printer-encoders. Terminated TL antennas, while maintaining a consider- able system power margin, can selectively interrogate transponders without RF power suppression. Increased available power delivered by the terminated resonant TL antennas to the encoding interval tolerates usage of transponders with large variation of their resonance fre- quency and activation power threshold. Moreover, enlarged bandwidth of terminated tapered resonant TL antennas allowed using inexpensive RoHS PCB dielectric materials with fairly wide deviations of permittivity, thickness of a substrate and copper cladding. The proposed miniature stripline TL antennas, with their compressed encoding range, permit portable print- er-encoders to work with short, densely spaced Smart Labels. The stripline antennas geometry, their conductive strip dimensions, and bandwidth obtained from Ansoft HFSS modeling for RFID 915 MHz band, have been veri- Figure 13 · Reflection loss S11 for stripline TL antenna fied empirically and found to be in a good agreement. samples. (a) single conductor TL antenna: 4.5 × 9 × 64 Antenna analysis, mostly concentrated on microstrip and mm; (b) dual-conductor TL antenna: 2× (3 × 10 × 57 stripline terminated TL, imposed no restrictions on the mm). type of TL. Other TL structures, for example, the coplanar waveguide or the slotline, may also be considered as change in the encoding range. The double-conductor building blocks of antennas for close proximity RFID stripline TL antenna in comparison with a single strip TL applications. Conclusively the stripline TL antenna is has improved field intensity due to a higher SWR gener- judged as a vital component for RFID applications involv- ated by an increased load. Its power efficiency, spatial ing equipment miniaturization or having spatial con- selectivity and coupling grade with a transponder are also straints for an antenna installation. increased due to a larger effective edge area. The double Besides RFID printer-encoders, there are many more stripline TL antenna has an RF power margin in excess applications of compact UHF antennas, including access of 6 dB. control (Homeland Security market), item-level RFID for conveyors, testing small transponders during their high Impedance Bandwidth volume manufacturing, quality validation in the Smart The port impedance of a single conductor stripline TL Labels conversion process (Industrial market), and scan- antenna is 50 ohms. For the double-conductor TL anten- ners of RFID Smart credit cards (Financial market). It is na the port impedance of 50 ohms is realized without an believed that presented information on UHF antennas additional matching network by connecting in parallel will be helpful in selection of UHF Printer-Encoder and as two strips, each loaded by a 100 ohm resistor. Both anten- well as a tutorial guide for RFID newcomers. Although nas utilize the same principles for bandwidth improve- the terminated TL antennas have low far-field radiation, ment as other tapered TL antennas and have a widened they are still a source of UHF electromagnetic energy. 22 High Frequency Electronics
  20. 20. High Frequency Design RFID ANTENNAS Antenna mounting elements and nearby metal-plastic 9. D.M. Dobkin, S.M. Weigand, “UHF RFID and Tag components can easily create a parasitic wave-guiding Antenna Scattering, Part II: Theory,” Microwave Journal, structure for this energy transmission, causing excessive Vol. 49, No. 6, pp. 86-96, June 2006. unintentional RF radiation that can interfere with the 10. (284) T. Breahna, D. Johns, “Simulation Spices transponder encoding process. UHF terminated TL RFID Read Rates,” Microwaves and RF, pp.66-76, March antennas have relatively low RF power efficiency in 2006. exchange for their spatial selectivity and thus, represent 11. P.V. Nikitin, K.V.S. Rao, S.F. Lam, V. Pillai, R. an improvement of energy conversion, and can be consid- Martinez, H. Heinrich, “Power Reflection Coefficient ered as a subject for further research. Analysis for Complex Impedances in RFID Tag Design,” Parts 1 and 2 of this series are available as PDF down- IEEE Transactions on Microwave Theory and Techniques, loads from the Archives section of this magazine’s Web site: Vol. 53, No. 9, pp. 2721-2725, September 2005. www.highfrequencyelectronics.com 12. C.A. Balanis, Antenna Theory: Analysis and Design, 2nd Edition, John Wiley & Sons, 1996. Acknowledgements 13. C. Capps, “Near field or far field?,” EDN Magazine, The author would like to thank Zebra Technologies pp. 95-102, August 16, 2001. Corporation and its associates K. Torchalski, Director of 14. I. Straus, “Loops and Whips, Oh My!,” Conformity, RFID, and M. Schwan, System Manager for their helpful pp. 22-28, August 2002. and productive discussions regarding UHF RFID Printer- 15. I. Straus, “Near and Far Fields—From Statics to Encoders development, M. Fein, RF Engineer for his Radiation,” Conformity, pp. 18-23, February 2001. HFSS counseling, and R. Gawelczyk, Engineering 16. B.Y. Tsirline, “Spatially Selective Antenna for Very Technician for his outstanding support and assistance in Close Proximity HF RFID Applications-Part 1,” High antenna fabrication, testing and evaluation. The author Frequency Electronics, Vol. 6, No. 2, pp. 18-28, February, also would like to thank S. Kovanko, EE Engineer for 2007. carefully reading parts of the manuscript. 17. J.D. Griffin, “A Radio Assay for the Study of Radio Frequency Tag Antenna Performance,” MSEE Thesis, References Georgia Institute of Technology, August 2005. 1. “Item-Level Visibility in the Pharmaceutical Supply http://etd.gatech.edu/theses/available/etd-05022005- Chain: a Comparison of HF and UHF RFID Technologies,” 142356/unrestricted/griffin_joshua_d_200508_mast.pdf White Paper, Philips Semiconductors, TAGSYS, Texas 18. S.G. Downs, “Why Antennas Radiate,” QEX Instruments Inc., July 2004. http://www.tagsysrfid.com/ Magazine, pp. 38-42, January/February 2005. modules/tagsys/upload/news/TAGSYS-TI-Philips-White- 19. R. Schmitt, “Understanding Electromagnetic Paper.pdf Fields and Antenna Radiation Takes (Almost) No Math,” 2. M.C. O’Connor, “Study Shows Big Growth for RFID EDN, pp. 77-88, March 2, 2000. Printer-Encoders,” RFID Journal, Inc., July 25, 2006. 20. G. Kumar, K. P. Ray, Broadband Microstrip http://www.rfidjournal.com/article/articleprint/2515/-1/1/ Antennas, Artech House, 2003. 3. M.C. O’Connor, “RFID Changing Buying Behavior,” 21. J.F. Feltz, J.A. McCurdy, L.D. Neuhard, “RFID RFID Journal, Inc., July 21, 2006. http://www.rfidjournal. printer and antennas,” U.S. Patent Application com/article/articleprint/2508/-1/1/ 20050280537, December 22, 2005. 4. L.G. Maloratsky, Passive RF & Microwave 22. T.A. Chapman, R.E. Schumaker, A. W. Edwards, Integrated Circuits, Newnes, 2003. S.S. Morris, J.P. 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  21. 21. 16, 2005. at Automotive Electronics and Equip- spatially selective transponder 27. D.M. Pozar, Microwave ment Corp., Russia, developing mili- encoding modules used throughout Engineering, 2nd Edition, John Wiley tary and aerospace electronic sys- the corporation divisions for RFID & Sons, 1998. tems. He has been in the Automatic labels and cards printers. Dr. Tsirline 28. K.C. Gupta, R. Garg, I. Bahl, P. Identification and Data Capture holds three non-classified Russian Bhartia, Microstrip Lines and industry since 1995. He managed the and two US patents and has numer- Slotlines, Artech House, 1996. development of Zebra’s first HF RFID ous pending patents for RFID 29. L. Young, “The Quarter-Wave printer-encoder and established the enhancements. He can be reached by Transformer Prototype Circuit,” IRE design methodology for HF and UHF e-mail at BTsirline@zebra.com. Transactions on Microwave Theory and Techniques, pp. 483-489, September 1960. 30. G. Matthaei, L. Young, E.M.T. Jones, Microwave Filters, Impedance- Matching Networks, and Coupling Structures, Artech House, pp. 255- 354, 1980. 31. R. W. Klopfenstein, “A Transmission Line Taper of Improved Design,” Proceedings of the IRE, Vol. 44, pp. 31-35, January 1956. 32. R.P. Hecken, “A Near- Optimum Matching Section Without Discontinuities,” IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-20, No. 11, pp. 734-739, November 1972. 33. J.-T. Kuo, “Riccati Matrix Differential Equation Formulation for the Analysis of Nonuniform Multiple Coupled Microstrip Lines,” IEEE Transactions on Microwave Theory and Techniques, Vol. 44, No. 6, pp. 880-886, June 1996. 34. K. Lu, “An Efficient Method for Analysis of Arbitrary Nonuniform Transmission Lines,” IEEE Transac- tions on Microwave Theory and Techniques, Vol. 45, No. 1, pp. 9-14, January 1997. 35. “3D Electromagnetic-Field Simulation for High-Performance Electronic Design,” Ansoft Corp. Author Information Boris Y. Tsirline is the Principal Engineer at Zebra Technologies Corporation. He received a BS and MS degrees in RF & Microwave Engineering from Moscow Aviation University, Russia in 1973 and a PhD in EE from Moscow State University in 1986. Before moving to the US in 1992, he served as a Director of R&D