U H F R F I D Antennas For Printer Encoders

28 High Frequency Electronics

High Frequency Design

RFID ANTENNAS

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

By Boris Y. TsirlineZebra Technologies Corporation

This series of arti-cles reviews UHFtransmission line

antennas developed forRFID Printer-Encoders.It explains the basic oper-ating principles of anten-nas, their effect on the

printer’s encoding function as well as how theantennas influence the design of labels withembedded transponders (Smart Labels). Thesurvey of antennas is preceded by the evalua-tion of antenna-transponder mutual couplingin reactive near-field and by the analysis ofthe Printer-Encoder environment, whichyields four comparison criteria of the anten-nas’ performance.

After discussing system requirements, thearticle covers two novel ultra-compact UHFantennas based on the tapered stripline trans-mission line, developed for the mobile RFIDPrinter-Encoders. These antennas enable theprinters to encode short Smart Labels on ashort pitch. The paper presents the develop-ment of the antennas, HFSS modeling, and anempirical study of their geometries, character-istic impedance and bandwidth. This type ofUHF antennas used for stationary andportable RFID Printer-Encoders may be uti-lized by numerous item-level close proximityRFID applications.

1. IntroductionThe Radio Frequency Identification (RFID)

technology and its three major components(Readers, transponders and antennas) haveexperienced huge progress in the past tenyears. Initially developed for aircraft identifi-cation in the 1940s, this technology has prolif-

erated in almost all sectors of modern society.Manufacturing, pharmaceutical [1], health-care [2], air luggage and supply-chain man-agement, item-level identification for a varietyof industries is a small number of the applica-tion fields.

With the exception of a completely auto-mated system, HF or UHF passive transpon-ders are rarely used by themselves. They areusually laminated with paper or plastic layersforming Smart Labels or Tags, which are ableto communicate with RFID Readers. Thename of the transponders, passive or battery-less, comes from the fact that the transponderis powered by energy transmitted by theReader antenna. This power supports theReader-transponder communication—theinterrogation process—which includes writingdata to the transponder’s memory and retriev-ing previously stored information and/or theunique transponder identification data.

Most modern RFID applications requirethat the Smart Labels be readable by an opti-cal scanner and a human being, in addition tothe Reader. Consequently the Smart Labelscontaining the transponders often have print-ed bar codes and human readable text. Themost convenient instrument to simultaneous-ly print text, bar codes and encode SmartLabels is an RFID Printer-Encoder, which per-forms all three functions at the same time.Besides the labels, Smart plastic cards withembedded transponders have also becomepopular. High interest from credit card organi-zations in the Smart card technology [3] hasdriven the development of plastic cardPrinter-Encoders. Mandate-driven Americanand European markets and Asian manufac-turing distribution centers require increasing

This three-part series pre-sents a detailed overview

of RFID encoder systemsand the antenna solutions

required for reliable printing(writing) to individual tags

From September 2007 High Frequency ElectronicsCopyright © 2007 Summit Technical Media, LLC


RFID ANTENNAS

quantities of HF and UHF RFIDPrinter-Encoders, Print Engine-Encoders for applicators, and mobilePrinter-Encoders working with fork-lifts in the warehouses. In addition tothe printing and the initial encodingpurposes, this equipment is also

widely used in the automated valida-tion procedures for Smart Labels andcards, preventing their re-encodingand data corruption, continuouslysecuring a smooth transition of manyRFID pilot programs and extendingthe successes of existing applications.

Accelerated in the recent years,the evolution of the RFID technologyand the hungry market for Printer-Encoders has fueled the developmentof specialized UHF antennas. Theirability to work with transponders invery close proximity and communi-cate selectively with only one target-ed transponder, tightly spaced withothers, essentially distinguishes thespecialized UHF antennas from theconventional ones. In contrast to theantennas designed for long rangeRFID applications, these specializedantennas are very similar to RF bi-directional couplers based on electro-magnetically coupled transmissionlines [4] that are common in the RFand microwave realm. The differencefrom 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 antennadesign for long range RFID applica-tions, assume new meanings and def-initions. For example, beamwidthbecomes transponder encoding range,and antenna directivity becomes spa-tial selectivity. The antenna-transponder interaction occurs in acomplex printer environment, whichcan 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, thecomposite architecture of the Printer-Encoders creates an RF unfriendlyenvironment, affects the transpon-ders’ interrogation process, andimposes limitations on the antennadimensions and location. Most impor-tantly, the Printer-Encoder andantenna designs also dictate the min-imum acceptable size of the SmartLabels and their transponder place-ment. Because of these mechanical

constraints the transponders cannotbe placed arbitrarily in a SmartLabel—their placement must be sep-arately specified for every printerbrand and model.

The Smart Labels specification,which dictates a particular transpon-der placement, indirectly expressesthe RFID printer’s encoding capabili-ty. A list of parameters describingtransponders placement includes thetransponder placement range, theplacement starting distance, and theseparation distance between theadjacent labels on a liner, known asthe pitch (Fig. 1(a) and (b)).

When the dimensions of labelsrequired for printing and encoding are4" × 6" or 4" × 4" and their length sig-nificantly exceeds the transponder’swidth, or the labels are relativelyshort but far apart from each other(Fig. 1 (b)), the antenna can easilycommunicate with the targetedtransponder without collision with theadjacent transponders. The complica-

tion and the challenge occur when thePrinter-Encoder must encode shortSmart Labels densely spaced on theliner (Fig. 1 (c) and (d)). In this case

encoding a small component labelrequires an antenna with high spatialresolution. This attribute is the so-called spatial selectivity that is the

Figure 1 · Smart Label structure and transponder placement. (a) SmartLabel design; (b) short labels with long pitch; (c) short Smart Labels; (d)small labels with short pitch.



RFID ANTENNAS

antenna’s capability to reliably interrogate the selectedtransponder without activating surrounding ones.

The ability of a printer to encode the transpondersplaced near the leading edge of the label defines thetransponder placement starting distance and directly cor-relates with the antenna dimensions and its positioninside the printer. The most challenging design goal is tomake a printer-encoder capable of working with shortSmart Labels, where the length of the Smart Label isnearly equal to the width of the embedded transponder(Fig. 1 (d)).

This review examines the capabilities and limitationsof the different planar transmission line (TL) UHF anten-nas, which are used for RFID Printer-Encoders requiringthe interrogation of a single transponder tightly spacedwith other transponders and in very close proximity tothe antenna. The review focuses on the low profile spa-tially selective mismatched stripline and double-conduc-tor stripline TL antennas designed for printers capable ofinterrogating densely spaced short Smart Labels. Themismatched resonant TL antennas typically have a nar-row bandwidth. To overcome this limitation a bandwidthimproving technique originally developed for impedancematching TL transformers is applied to microstrip andstripline TL antennas. This article also presents anempirical verification of the antenna geometries and elec-trical parameters that were initially derived by usingAnsoft High Frequency Structure Simulator (HFSS).

The ultra-compact stripline UHF antennas enable:

• Individual encoding of short Smart Labels withsmall pitch;

• Acceptance of transponders with broad deviations ofresonance frequency and activation power thresh-olds;

• Positioning of the transponder placement area nearthe leading edge of the label;

• Printer batch mode encoding without involvement ofthe anti-collision management;

• Space saving design of the mobile RFID printers;• Effortless installation and straightforward RFID

conversion of the existing bar code printers.

The next section, 2. Antenna-Transponder Coupling inClose Proximity examines magnetic and electric field dis-tribution along the antenna-transponder structure, ener-gy transfer and coupling mechanism between them. Thissection also identifies two criteria for comparison of fieldintensity and impedance bandwidth of the antennas.

Section 3. Printer-Encoder Environment classifies fourcritical printer zones, relates their lengths to the con-straints imposed by the antenna dimensions on theSmart Label design and transponder placement parame-ters, and establishes two geometrical criteria for antenna

comparison.Section 4. UHF Antennas for Stationary Printer-

Encoders presents a comprehensive review of several TLantennas developed for stationary UHF Printer-Encodersand qualitative analysis of their impact on the printer’sencoding performance.

Section 5. UHF Antennas for Mobile Printer-Encodersintroduces the ultra-compact novel UHF stripline TLantennas, their strengths for mobile RFID Printer-Encoders, and optimization of the antenna geometriesand electrical parameters using HFSS.

[Sections 3, 4 and 5 will be published in the Octoberand November issues of High Frequency Electronics.]

2. Antenna-Transponder Coupling in CloseProximity

Although the UHF passive transponders produced bythe leading vendors could have their antennas shapedsimilar to a meander-line [5], bow-tie [6], or cross dipole[7], the majority of them are half-wavelength dipole orfolded-dipole antennas [5], [8], [9]. The dipole antenna ismost popular in various RFID applications because of thenear-omnidirectional radiation pattern in the far-field[10] and a straightforward chip impedance matching pro-cedure [11]. The half-wavelength (λ/2), in free space, of anoperational frequency 915 MHz (ISM US RFID band) is164 mm. The physical length of the transponders mayrange from 120 to 20-25 mm depending on the permittiv-ity of their substrate materials and the antenna profiles.

There are three spherical spaces surrounding theReader and transponder antennas in the transmittingmode: reactive near-field, radiating near-field, and far-field [12, 13, 14, 15]. The radius of each sphere depends onthe operational frequency (or wavelength) and the largestlinear dimension (D) of the antennas. The interactionmechanism and the energy transfer between two anten-nas are determined by whether they are located withineach other’s near-field, radiating near-field, or far field.For the RFID 915 MHz frequency band the dimension Dof antennas is usually chosen as one-half wavelength.

The far-field, having a propagating wave, starts out-side the sphere with radius R1, which can only be approx-imated [12] because of the violated condition D > λ.

Therefore, for an antenna with the largest lineardimension D = 164 mm, the radius of the sphere for far-field is R1 > 164 mm and is smaller for shorter antennas.

When a transponder is located at distance R1 or far-ther from the Reader’s antenna, the electromagnetic com-ponents of the propagating wave and its impedance areindependent of Reader antenna’s geometry, and the field

RD

1

2

2>λ



RFID ANTENNAS

surrounding the transponder is uniform. In long rangeUHF RFID applications, where the transponders are sev-eral meters away from the Reader’s antenna, they are ineach other’s uniform far-field. During data transmissionthe transponder varies the impedance of its antenna andchanges its field, but these field disturbances do not affectthe current distribution of the antenna in the transmit-ting mode or its electrical parameters. The antenna-transponder bi-directional communication is provided bythe propagating wave, there is no coupling between them,and therefore no mutual influence.

Inside the sphere with the radius R1 is the radiatingnear-field. The inner boundary of this region is approxi-mated by the radius R2

For the frequency of 915 MHz and the half-wavelengthantenna dimension D = 164 mm, the radius R2 = 72 mm.This field is partly a product of the continuous electricand magnetic field energy exchange with the antenna,but predominantly is a radiation wave. The antenna isloosely coupled with the transponder and they have aweak mutual influence.

The region of the pure reactive near-field is within theestimated radius R2. This field is the result of the contin-uous electric and magnetic field energy exchange withantenna electrical energy. The field strength is propor-tional to the antenna’s Q-factor and the current flowingthrough it. For Printer-Encoders and other very closeproximity applications the antenna-transponder separa-tion distance is 5-10 mm and is much shorter than the lin-ear dimensions of the Reader’s or transponder’s antennas.

The electro-magnetic components of an antenna’sreactive 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 operatein each other’s non-uniform reactive near-fields.

The electric and magnetic field strength distributionsfor the half-wavelength transponder dipole antenna inthe transmitting mode are depicted in Figure 2. In closeproximity, the magnetic field is typically concentrated atthe center of the dipole, where the current attains itsmaximum value, while the maximum electric fieldstrength is at the edges of the dipole arms [17]. Applyingthe reciprocity theorem [12], which states that an anten-na’s transmitting performance is equal to its receivingperformance, one can conclude that a Reader’s antennashould have a similar to dipole electro-magnetic field dis-tribution for the best coupling with a transponder.

The source of antenna’s field is electric charges flow-ing through the antenna. Charges slowly moving in spacecreate the reactive near-field and fast moving charges cre-ate the far-field [18]. At the UHF band the charges alsovary in time with the period of 0.5 or 1 nanosecond. Thetemporal variation also contributes to the antenna’s far-field radiation. The antennas radiated near-field and far-field strengths significantly increase if charges are spa-tially accelerated. Whenever a charge abruptly changesdirection or vanishes, for example, because of the anten-na’s structure, the electrical energy applied to an antennais efficiently converted into radiation [19]. The antenna’sradiation efficiency increases when its length approachesthe half-wavelength of its operational frequency. Thisexplains why the radiation of the half-wavelength dipoleantenna having zero current value at the ends of its armsis very efficient. High intensity radiation in the far-fieldis desirable for the long range RFID systems, which arebased on propagating waves. But to be appropriate forvery close proximity applications, an antenna should havea strong reactive near-field and a weak far-field to complywith the EMI/RFI regulations.

In very close proximity, the transponder activationenergy is mostly delivered through the quasi-static elec-tromagnetic coupling with the antenna. The couplinggrade of two closely spaced devices depends significantlyon their separation distance, geometrical profiles, andmutual alignment. Magnetic coupling is provided bymutual inductance [16] and electric coupling throughstatic capacitances [4]. Mutual inductance is an attributeof closely spaced wires carrying current. The currentthrough each antenna creates the corresponding magnet-ic flux that induces voltage and current in the otherantenna. Static capacitance is an attribute of closelyspaced conductive plates or areas having oppositecharges. The antenna or transponder dipole’s arms arethese plates. The arms charges cause electric field

RD

2

3

0 62> .λ

Figure 2 · Dipole current distribution and fields.



RFID ANTENNAS

strength variation and develop voltage across a nearbyantenna and transponder constructing elements, whichare 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 thetransponder modulates the impedance of its own antennaand changes the surrounding antenna field distribution.This modulation influences the field of the Reader’santenna and in turn changes its current distribution,antenna impedance, and frequency tuning. An increase inseparation distance reduces the grade of antenna-transponder electro-magnetic coupling.

The energy delivered to a transponder is used by thetransponder’s IC to support its interrogation. The RFpower (PT) delivered to a transponder is a product of itscoupling grade with an antenna and the strength of thereactive near-field of the antenna powered by a Reader.Regardless of the antenna type the power PT can beexpressed by the equation:

(1)

where K is a power transfer coefficient and PA is Readerpower applied to an antenna.

In the RFID systems with spatially independent andhigh value coefficient K, the power delivered to atransponder can considerably exceed its activation powerthreshold. The activation power threshold is the Reader’sRF power level at which the transponder becomes ener-gized and starts responding to the Reader’s commands.This power threshold level is a complex function thatdepends on the antenna parameters, transponder ICimpedance matching and its activation voltage threshold.Most importantly, the activation power threshold maydepend on the transponder’s location inside a printer. Theexcessive activation power causes a relatively extensivecommunication interval and substantially increases thetransponder placement range (Fig. 1 (a)).

At disproportionate energy levels the reactive field willcover not only the targeted interrogation area but also thesurrounding areas, which is not a problem when dealingwith a single transponder, or when the transponders arespaced far apart. Although this spatial separation local-izes the encoding interval, it also limits the minimumachievable label length. With closely spaced transponders,the interrogation range must be controlled so as to preventaccidental communications with the neighboringtransponders. One way to prevent this collision is toreduce the Reader’s power and consequently the length ofthe transponder placement range. In this case the deliv-ered power is higher than the transponder’s activationpower threshold only for the encoding range and is lowerthan the activation power threshold outside of this range.

Equation (1) demonstrates that the coefficient K,power PA or both can be decreased for power reduction.For some types of antennas the coefficient K is indepen-dent of the antenna-transponder alignment and can bedecreased to low power PT. The drawback of lowering thecoefficient K or power PA is the loss of the RF power mar-gin over the transponder activation power threshold forits placement range, as shown for Antenna #1 in Figure 3.The RF power margin is defined as the maximum sup-pression (in dB) of the Reader’s operational RF powerachieved in the middle of the transponder placementrange, when the power falls to the transponder activationthreshold level and the transponder stops communicat-ing. With a low power margin the interrogation processbecomes unreliable because of the system’s susceptibilityto the deviations of the transponder’s and the antenna’selectrical parameters and their precise tuning. Althoughsome transponders with less than ideal parameters mayhave acceptable performance for long range applications,the power delivered in the near-field may become insuffi-cient and the transponder will be missed. To stabilize theencoding process and make it robust, the RFID systemshould have the highest possible RF power margin.

In the RFID systems with spatially dependent coeffi-cient K, its value changes depending on the antenna-transponder coupling grade, which correlates to the ori-entation and proximity to each other. For such a system,the coefficient K and the activation power achieve theirmaximum values only for the transponder which is clos-est to the antenna; they are noticeably lower for the adja-cent transponders.

A tightly spaced antenna and transponder behave asan air-dielectric variable capacitor, which plates areformed by the transponder and the antenna. Its capaci-tance is proportional to the area of the overlapping sur-

P K PT A= *

Figure 3 · Power delivered to a transponder and RFpower margin.



RFID ANTENNAS

faces of the antenna and the transponder. When thetransponder moves closer to the antenna their mutualsurface grows and the static capacitance increases. Thisincrease in static capacitance improves the antenna-transponder coupling and raises the power delivered tothe transponder as well. Similarly the magnetic couplingincreases when the two current carrying “wires” movecloser to each other. The power transfer coefficient variesalong the communication range and the power deliveredto the transponder throughout the encoding interval sig-nificantly exceeds the transponder’s activation powerthreshold. In this case the RFID system has a high RFpower margin, which is illustrated by the power curve forAntenna #2 in Figure 3.

For a selected antenna-transponder separation dis-tance, an RFID system achieves the maximum RF powermargin when their mutual overlapping area is compara-ble with the transponders’ width. The limiting factor forthe maximum grade of coupling is the impedance inducedby the transponder in the antenna circuit. In very closeproximity this induced complex impedance could cause asevere impedance mismatch between the Reader’s andthe antenna’s ports leading to a drastic reduction of thetransponder’s activation power. To characterize the RFsystem power margin and the antenna-transponder cou-pling grade the encoding field intensity is introduced.

The coefficient K, in addition to being dependent onthe antenna-transponder geometries and their alignmentin the general case, is also a function of the antenna tun-ing frequency and the antenna impedance bandwidth(BW). To justify the antenna bandwidth, at least twoaspects of the RFID system should be taken into account.The first one is the spectrum of modulated signals thatare used by the Readers for transponders interrogation.

For 915 MHz U.S. RFID band, the allocated spectrumis 26 MHz ranging from 902 to 928 MHz. RFID uses fre-quency hopping modulation around the central frequencyof 915 MHz. Although the Readers from different vendorsoperate at the same frequency band, they differ in theirability to handle hopping frequency phase shifts associat-ed with phase difference of signals reflected from theantenna port for different channels. The Readers, whichare based on I-Q synchronous detection of the transpon-der’s re-radiated signals, typically require at their RFport a Standing Wave Ratio (SWR) of 1.4 or less in theoperational band in order to perform reliable interroga-tion. The BW definition for conventional antennas is a fre-quency band over which an antenna has a SWR = 2 or hasits reflection loss or S11 parameter that is less than –9.5dB. The tuning frequency is the center of the antenna’sbandwidth. To obtain a standard BW value of an antenna,the BW at SWR = 2 is calculated from the antenna BW atSWR = 1.4. For example, for a microstrip antenna, the fol-lowing equation from [20] can be used:

(2)

Substituting BW2 = 26 MHz at SWR2 = 1.4 and SWR1= 2 in Equation (2), we can find BW1 = 54.4 MHz. This BWis derived for a precisely tuned 915 MHz antenna.

The second aspect of the RFID antenna’s BW selectionis associated with the deviations of antenna’s electricaland mechanical parameters. The antenna fabrication pro-cess typically utilizes non-ideal materials and non-idealoperational procedures, which impact the antenna centerfrequency, port impedance, and consequently the inputreflection coefficient (Γ). The reflection coefficient is relat-ed to SWR by the well-known formula:

(3)Γ = −+

SWRSWR

11

BWBW

SWR

SWR

SWRSWR

1

2

1

1

2

2

11

=−( ) ×

−( )

Figure 4 · Antenna reflection coefficient andimpedance bandwidth (BW). (a) BW = 90 MHz; (b) BW =150 MHz.

Using Equation (3) and SWR =1.4, we obtain coefficient Γ = 0.166.This value can be used as the maxi-mum acceptable level of reflection forthe bandwidth. If the resonant fre-quency of a narrowband antennachanges noticeably, the antennainput reflection loss at the opera-tional frequency increases, and thepower transfer coefficient K drops.For example, a microstrip antennabased on the substrate materialIS410 (ISOLA) with the dielectricconstant ε = 4.25 ±0.15 and BW = 90MHz can have a resonant frequencyfrom 900 to 930 MHz (Fig. 4 (a)) andmaintain an unacceptably highreflection coefficient |Γ| below 0.24(SWR = 1.63) for the 902-928 MHzfrequency span instead of the Γ =0.166 required for SWR = 1.4. If anantenna based on the same dielectricmaterial has BW = 150 MHz, itsreflection coefficient magnitude is|Γ| <0.14 for the 902-928 MHz band(Fig. 4 (b)) and it complies with theReader SWR requirements.

Deviations of other antennaparameters including the thicknessof the substrate and the coppercladding have less influence on theantenna resonant frequency than thedielectric constant and the BW of 150MHz can be considered as a conser-vative estimate for the desirableantenna bandwidth in order to toler-ate technological deviations of theantenna’s electrical and mechanicalparameters. On the other hand, anexcessive antenna bandwidth is notadvantageous. Antennas with sub-stantially wider than necessarybandwidth could potentially be sus-ceptible to the electro-magnetic inter-ferences caused by the printer’s near-by electrical and electronic devices.

The chosen impedance bandwidthcriterion thus represents a character-istic of the antenna in terms of thetechnological stability and theEMI/RFI immunity.

[This article will continue in thenext two issues of High FrequencyElectronics, beginning with with sec-tion 3. Printer-Encoder Environment.All references will be listed at the endof the final installment.]

Author Information Boris Y. Tsirline is the Principal

Engineer at Zebra TechnologiesCorporation in Vernon Hills, IL. Hereceived a BS and MS degrees in RF& Microwave Engineering fromMoscow Aviation University, Russiain 1973 and a PhD in EE fromMoscow State University in 1986.Before moving to the US in 1992, heserved as a Director of R&D atAutomotive Electronics and Equip-ment Corp., Russia, developing mili-tary and aerospace electronic sys-tems. He has been in the AutomaticIdentification and Data Captureindustry since 1995; first as an RFEngineer involved in LF RFID designat TRW, and then at ZebraTechnologies Corporation since 1998.He managed the development ofZebra’s first HF RFID printer-encoder and established the designmethodology for HF and UHF spa-tially selective transponder encodingmodules used throughout the corpo-ration divisions for RFID labels andcards printers. Dr. Tsirline holdsthree non-classified Russian and twoUS patents and has numerous pend-ing patents for RFID enhancements.He can be reached by e-mail [email protected].

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RFID ANTENNAS

UHF RFID Antennas for Printer-Encoders—Part 2: Antenna Types


Part 2 of this articlecontinues the dis-cussion of RFID

antennas with a look atthe operating environ-ment. of antenna and itsassociated RFID tagprinter-encoder. Readers

may wish to have Part 1 available to follow theauthor’s references to earlier comments andfigures.

3. UHF Printer-Encoder EnvironmentSmart Label design restricted by the

antenna-transponder interaction in very closeproximity is complicated further by thePrinter-Encoder environment. A commonarchitecture of a thermal transfer UHF RFIDPrinter-Encoder (Fig. 5) yields an arrange-ment of four key internal areas: the mediasupply zone, the following adjacent transpon-ders zone, the targeted transponder zone andthe encoded transponder zone. This represen-tation assumes that the antenna is positionedunderneath the printhead and behind theplaten roller. During printer operation, thetransponders pass through the zones sequen-tially from the media roll to the printhead.Each zone can be active or inactive in terms ofits ability to activate the transponders locatedin it. The zone lengths are correlated to theprinter structure, the antenna constructionand its dimensions. Consequently the zonesimpact the Smart Label design and imposeconstraints on the minimum Smart Labelpitch and on the two transponder placementparameters: the placement starting distanceand the transponder placement range (Fig. 1).

Media supply zone usually has a fixed

length. This zone is relatively far away fromthe antenna and is inactive. In case of para-sitic activation of the transponders in thiszone, the simplest solution regardless of theantenna and label dimensions is shielding.

Following adjacent transponders zone hasa variable length. Typically the design goal isto make this inactive zone as long as possiblein order to be able to process densely spacedshort labels and achieve a short pitch. If anantenna positioned right after the platenroller has intensive radiation or an extensivelongitudinal length, the inactive followingadjacent transponders zone shrinks. In thiscase the printer can still process narrow labelsbut with an extended transponder placementrange. The printer requires an increased pitchon the liner (Fig. 1(b)). This approach leads toa noticeable waste of liner material and limitsthe minimum label length. In order to protectthe transponders against activation in this

Understanding antennaperformance is one key to

obtaining accurate andreliable writing of RFID tags,especially in a high-volume

automated system

Figure 5 · Common architecture of thermaltransfer UHF RFID Printer-Encoder.

From October 2007 High Frequency ElectronicsCopyright © 2007 Summit Technical Media, LLC


RFID ANTENNAS

zone shielding component may be used. The disadvantageof a shielding solution is that the geometries of the shield-ing components depend on the transponder dimensionsand involve adjustment for every new transponder form-factor.

Targeted transponder zone generally depends on theantenna and the transponder dimensions as well as theReader’s RF power. This zone is active, of course. Whenthe 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 pitchin order to avoid collisions or transponder re-encodingwith the wrong data. A short antenna, closely positionedto the platen roller, affords a short placement starting dis-tance for the transponders and their short transponderplacement range.

Relatively short labels often have a partition distancebetween them that is only a fraction of their width (Fig.1(d)). Consequently, transponders embedded into suchlabels are grouped close to each other. In this densearrangement all transponders can be activated simultane-ously by a “low” resolution antenna. The long range RFIDsystems commonly employ an anti-collision technique forprocessing a group of transponders. This technique isimpractical for Printer-Encoders because it is unable toidentify single targeted transponder. Only an antennawith spatial selectivity can work with a single closesttransponder without activating the adjacent ones. Thehigher the spatial selectivity of an antenna, the shorterthe transponder placement range. In the best case thetransponder placement range can equal the transponder’swidth. The shielding components can also be used to formthe targeted transponder zone or to limit its longitudinallength with the same disadvantages as for the followingadjacent transponders zone application described above.

Encoded transponder zone length mainly depends onthe antenna field strength and the printer componentssurrounding this area. The encoded transponder zoneshould be inactive. The antennas with highly intensiveelectro-magnetic field and some printer components withthe wave re-radiating ability can inadvertently make thiszone active. In this case every encoded and printed labelmust be either peeled or torn off in order to prevent thetransponders collision or an incorrect re-encoding. Theneed to take off the encoded transponders prohibits print-er operation in the batch processing mode. Alternativelythe label pitch may be increased such that the encodedtransponder leaves this zone when the next transponderarrives for an encoding. The application of the shieldingcomponents suppressing electro-magnetic field in theencoded transponder zone imposes functional limitationson the printer. For example, the shielding elements mount-ed in this zone conflict with the use of an external cutter.

Two criteria are proposed for the integral characteri-zation of relations between the antennas construction,the printer zone dimensions and the Smart Label designparameters. The first criterion is the antenna structuralfeasibility, which reflects the space required for theantenna installation and designates the interval occupiedby the antenna along a transponder’s path. The secondcriterion is the transponder placement boundaries, whichcharacterizes the antenna spatial selectivity and theassociated transponder placement parameters of theSmart Label.

These four criteria established above are intended tobe utilized for the comprehensive study and comparisonof the existing UHF antennas for stationary and mobileRFID Printer-Encoders and also to determine the corre-lation between the printer encoding function and theSmart Label parameters limitations.

4. UHF Antennas for Stationary Printer-EncodersMicrostrip, stripline and others PCB transmission

lines developed primarily for RF energy transfer havebecome accepted as antennas by UHF Printer-Encodersand by other RFID close proximity applications. Theirplanar structure, ability to handle relatively high RFpower and inexpensive, precise fabrication process enableeasy integration. Any transmission line antenna signifi-cantly changes its behavior and electrical propertiesdepending on the line length and its terminating status.There are two TL antenna types: Open TL type based onthe open TL and Terminated TL type—antennas based onthe loaded TL.

Antenna Based on Open TL An Open TL antenna type is represented by a quarter-

wavelength microstrip patch antenna (Fig. 6) [21], [22].The patch antenna for close proximity applications differs


Figure 6 · Open Transmission Line antenna—λλ/4 wave-length microstrip patch.



RFID ANTENNAS

from a conventional patch antenna in having shieldingcomponents along the non-radiating patch sides and amuch narrower radiating edge in order to decrease fieldstrength in radiating near- and far-field zones.

Antenna Structural FeasibilityThis antenna is based on PCB and enclosed in shield-

ing case with one open side. The antenna is arranged inparallel with a transponder in an encoding area andresides in interval of 20-25 mm (including its mountingcomponents) behind the platen roller.

Transponder Placement BoundariesThe antenna is positioned close to the platen roller

and provides a short transponder placement range andplacement starting distance, which allows the processingof short Smart Labels with a short pitch.

The antenna shielding elements are engaged to limitthe transponder interrogation interval. Electro-magneticshielding is probably the oldest method of insulating thetransponder designated encoding area. In RFID technolo-gy shielding was initially employed for selective singletransponder testing in the presence of others [23]. Theshielding disadvantage appears when transponder form-factors change frequently, for example, for different labelsizes, and so do the geometries of the shielding elements.

Encoding Field Intensity Parallel alignment of the antenna with a transponder

in the encoding area ensures improved coupling.However, because electrical charges are highly accelerat-ed at the open edge of the antenna, it has very strongreactive and radiating near-field intensity. The antennaenergy efficiency is very high and a transponder encodingat 5-10 mm from the antenna requires a few milliwatts ofthe Reader RF power. Shielding elements create losses inthe antenna near-field and change its distribution aroundthe antenna. Shielding reduces energy in the area of adja-cent transponders but works inefficiently for radiatingnear- field. A strong antenna electric field can potentiallyactivate the transponders in encoded transponder zone orin following adjacent transponders zone (Fig. 5) and thusthis antenna requires RF power control to reduce thisfield. The collision risk drives the Reader operational RFpower down to the level that is insufficient to activatetransponders in the adjacent zones and significantlydecreases system power margin as illustrated by Antenna#1 in Figure 3. Magnetic field mostly concentrated nearthe grounded edge partly contributes to the transponderactivating power.

Impedance BandwidthAntenna feeding port impedance match is achieved by

finding the appropriate point close to the grounded edge

of the patch (Fig. 6). Bandwidth of patch antennas with-out a shield is narrow, approximately 30 to 50 MHz. Inorder to tolerate parameter deviations the geometry ofevery antenna must be adjusted for frequency tuning andimpedance matching.

Antennas Based on Terminated TLIn contrast to Open TL, antennas based on

Terminated TL could be resonant or not-resonant. Theymay have wide or narrow bandwidths depending on theTL length and the terminating load value. In the mostcommon case a terminated TL exhibits three specific fea-tures that have defined three trends in UHF antennadevelopment for very close proximity RFID applications.These features are related to the TL input impedance.

The input impedance ZIN of any loss-free TL havingcharacteristic impedance ZC, a length l and terminated bya load ZL in general is described [24] as

(4)

where ß is the phase constant, which for a uniform,loss-free TL is inversely proportional to a wavelength λλand is given by

(5)

Substituting (5) in (4) we obtain:

(6)

There are three conclusions of interest from equation (6).

1. If TL characteristic impedance ZC meets the con-dition:

(7)

Then substitution of (7) in equation (6) gives:

(8)

In reference to (8) the impedance ZIN is theoreti-cally independent of TL length and equal to theterminating load for any frequency. Although inreality the bandwidth is limited by parasiticeffects associated with non-ideal TL components,it can easily reach 5 to 6 GHz. In this case voltagestanding wave ratio (VSWR) of the TL is about 1;voltage along the whole TL length is equal to the

Z ZIN L=

Z ZC L=

Z ZZ jZ

Z jZIN C

L C

C L

=+

+

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

tan

tan

2

2

πλπλ

β πλ

= 2

Z ZZ jZZ jZIN C

L C

C L

= ++

⎛

⎝⎜

⎞

⎠⎟

tantan

ββ

input voltage. The electric field strength distribu-tion around the TL is also homogeneous.

2. If the TL length is a quarter-wavelength:

(9)

Substituting ßl = π/2, from (5) and (9) in (6)obtain:

(10)

The ability of TL to transform load impedance(10) is widely used for impedance matching in thevicinity of one particular operational frequency(ƒ0).

3. If the TL length satisfies the condition:

(11)

Substituting ßl = π, from (5) and (11) in (6) obtain:

(12)

Equation (12) is valid for any impedance value ZC forone particular frequency ƒ0. TL experiences a standingwave with SWR ≥ 1 depending on how much impedanceZC differs from impedance ZL. In an extreme case for ahuge mismatch SWR >> 1, the voltage amplitudes nearthe edges of a λ/2 wavelength TL are in anti-phase andcan attain almost a double the input voltage value. Thisvoltage amplification increases the electric field strengthimmediately adjacent to the TL and for SWR >> 1 isalmost equivalent to the input power increase of up to 4times for the matched TL.

Antennas Based on Terminated Non-Resonant TL The so-called Terminated Non-Resonant TL antennas

are presented by “Two-Wire” TL [25] (Fig. 7(a)) formed bytwo PCB traces and by a combined arrangement of twomicrostrip transmission lines [26] (Fig. 7(b)). This groupof antennas utilizes the TL phenomena (8). For all anten-nas based on terminated TL, their electrical charges slow-ly accelerate at the edges. Therefore, the antennas have aweak radiating near- and far-field intensity, while highcurrent provides relatively strong reactive near-field.

Antenna Structural FeasibilityBoth antennas, based on a highly technological PCB

fabrication process, have an orthogonal alignment of theirtraces with the targeted transponder. Their structurestake up to 45-60 mm in longitudinal length (including the

mounting elements) from the platen roller back to themedia roll. The antennas are very convenient for imple-menting a transponder interrogation method known as“encoding on the run” along the media feed direction. Thedistance between the “wires” on the dielectric substrate is20-40 mm (Fig. 7(a)). The combined structure, “DualMicrostrip” transmission line, is 45-60 mm in length withtwo microstrips 20-40 mm apart (Fig. 7(b)).

Transponder Placement BoundariesBoth antennas have an excellent selectivity outside of

the targeted transponder zone (Fig. 5) to prevent commu-nications with adjacent transponders; however, the zoneitself is much wider than a transponder width. Antennasmust be field upgradeable for different transponder form-factors and redesigned to adjust the transponder place-ment range (Fig. 1(b)). With these antennas a long pitch isrequired to encode short Smart Labels.

Encoding Field IntensityDepending on permittivity of the dielectric substrate,

antennas can have a width of traces approximately 1.5-3mm either for the “Two-Wire” TL or for the “Dual-

Z f ZIN L0( ) =

l = λ / 2

Z fZZIN

C

L0

2

( ) =

l = λ / 4

Figure 7 · Antennas based on Terminated Non-Resonant TL. (a) “Two-Wire” transmission line; (b) “DualMicrostrip” transmission line.

October 2007 41



RFID ANTENNAS

Microstrip” TL to attain characteristic impedance of 100ohms. Because of the antenna-transponder orthogonalorientation, the antennas form a small mutual staticcapacitance and have a loose coupling with transponders.The areas of the electric field strength for the “Two-Wire”TL are not quite close to transponder’s most sensitiveedges. Both antennas have comparatively low power effi-ciency but could have a high RF power margin. The areasof intensive electric field of the “Dual-Microstrip” struc-ture are positioned closer to the sensitive transponderedges but the mutual overlapping area is small and thecoupling grade is still low. The electric field strength ishomogeneous along both transmission lines and ampli-fied by transformer usage. Magnetic field surroundingevery TL is practically not contributing to transpondersactivation power.

Impedance BandwidthBoth transmission lines are terminated by loads

matching their characteristic impedances. They haveSWR ~1 over a frequency band that is much wider than 1GHz. The “Two-Wire” TL width W1 defines its character-istic impedance that is about 300 ohms. To satisfy the con-dition (7) TL is loaded by a 300 ohm resistor. An RF trans-former with impedance ratio equal to 6 is used to providethe 50 ohm antenna port impedance match and anti-phase voltages. The combined structure—“DualMicrostrip” transmission line (Fig. 7(b)), loaded by two100-ohm resistors R1, makes the characteristicimpedance of the antenna independent of the distance D.It also uses an RF transformer with the impedance ratioof 2 for impedance matching and phase shifting.

Antennas Based on Terminated Uniform Resonant TLThe second type of antennas is based on terminated

but mismatched TL. The so-called Terminated UniformResonant TL antennas are demonstrated by the λ/4 (Fig.8(a)) and the λ/2 (Fig. 8(b)) length of the uniformmicrostrip TL. This group of antennas realizes TL phe-nomena (10) and (12) respectively. Antenna portimpedance is matched to the system impedance withoutadditional matching network.

Antenna Structural FeasibilityBoth antennas are in parallel alignment with the tar-

geted transponder and occupy a 20-30 mm intervalbehind the printer’s platen roller.

Transponder Placement BoundariesThese antennas allow a printer to achieve a short

transponder placement starting distance 10-15 mm andplacement range 20-25 mm for transponders with dimen-sions 8 × 95 mm or 10 × 95 mm [11, 13]. The pitch for thelabels is in the range of 40-50 mm.

Encoding Field Intensity The microstrip TL base element for these antennas

has a lower characteristic impedance ZC than the loadimpedance ZL and therefore a wider than non-resonantTL conductive strip, which increases static capacitanceand a coupling with a transponder. The impedance mis-match causes a wave reflection with standing wave ratioSWR >1 along the line and increases the electric fieldstrength above the line. The reflection coefficient Γ is acomplex voltage (current) ratio, which may be expressedin terms of the antenna characteristic impedance andload impedance (ZC and ZL) correspondingly:

(13)

Substituting (13) in (3) we obtain

(14)

The equation (14) shows that an increase in ratiobetween the load impedance ZL and the microstripimpedance ZC causes an amplification of SWR and makesstronger electric field above the TL. The impedance ZC isinversely proportional to the conductive strip width W2 orW3 (Fig. 8(a) and (b)). For both antennas the conductivestrip widths can be made comparable to the transponderwidth and RF power margin can attain 3-6 dB level with-out a significant expansion of the encoding range.

The quarter-wave TL antenna contributes to transpon-der power delivery by electric field at one side of the TL andby magnetic field at the transponder’s center. The half-wave TL antenna is twice as long, has double the mutualstatic capacitance with a transponder, and therefore main-tains an enriched coupling and encoding field intensity.

Impedance BandwidthThe geometries of λ/4 and λ/2 TL antennas, terminat-

ed by mismatched loads, define their resonant frequencyand consequently their bandwidth. The bandwidth ∆ƒ ofthe quarter-wave TL antenna can be obtained from [27],

(15)

Applying equations for microstrip characteristicimpedance and strip width from [28], the bandwidth ofthe quarter-wave TL antenna is calculated using equation(15) for the frequency 915 MHz as a function of the stripwidth for impedance ZL in the range of 2 to 8 ohms (Fig.8(c)). The plot shows that the strip width W2 can beincreased up to 35 mm without violating the justifiedantenna bandwidth of 150 MHz.

∆ ΓΓ

f fZ Z

Z Zm

m

L

L

= −−

∗−

⎡

⎣⎢⎢

⎤

⎦⎥⎥

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪0 2

0

0

24

1

2π

arccos

SWRZZ

L

C

=

Γ = −+

Z ZZ Z

L C

L C

Substituting the antenna length l = λ/2 in (6) for port impedance ZIN, forthe half-wave TL antenna the reflection coefficient Γ is:

(16)

where

For the maximum reflection coefficient Γm = 0.333 in (16) that correspondsto SWR = 2, θm and the bandwidth ∆ƒ can be obtained,

(17)

where

and ƒm corresponds to Γm.

Using equations for microstrip characteristic impedance and strip widthfrom [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 915MHz. In order to comply with the requirement of ∆ƒ = 150 MHz, the stripwidth W3 of the half-wavelength TL antenna should not exceed 14 mm. Thisbandwidth restriction limits the transponder placement range in the casewhen printer design requires a wide transponder encoding area.

θ πmmff

=0

∆f f m= −⎛⎝⎜

⎞⎠⎟

2 10

θπ

θ β θ π= =lff

;0

Γ =−( ) + ( ) −( )

( ) + +(Z Z Z Z Z Z

Z Z Z Z

L L L

L L

04 4 2 4

02

02 2 2 2

02

02 2

4

4

tan tanθ θ

))⎢⎣⎢

⎥⎦⎥

2 2tan θ

Figure 8 · Antennas based on Terminated Uniform Resonant microstrip TLfor 915 MHz band. (a) λλ/4 TL antenna; (b) λλ/2 TL antenna; (c) λλ/4 TL band-width vs. width; (d) λλ/2 TL antenna bandwidth vs. width.



RFID ANTENNAS

Antennas Based on Terminated Tapered Resonant TLAnother 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 antennasa relatively wide bandwidth and an increased grade ofcoupling with transponders. This goal is accomplished byimplementing a method previously developed for band-width enhancement of impedance matching TL trans-formers. This method is based on the theory of smallreflections [24] applied to a tapered (non-linear) profile ofcharacteristic impedance for any TL. Antennas are pre-sented by the λ/4 wave and the λ/2 wave non-uniformmicrostrip TL.

Antenna Structural FeasibilityThe width of the quarter-wave non-uniform

microstrip TL is tapered from W4 to W5 (Fig. 9(a)). Theedge widths of the half-wave non-uniform microstrip TLantenna are W6 (Fig. 9(b)). Both antennas can be madewider than the widths of uniform microstrip TL anten-nas. The corresponding lengths of non-uniformmicrostrip TL antennas are shorter than lengths of uni-form ones because of the extension of the sides of thetapered microstrip TL. The considered example is thehalf-wave microstrip linear width (non-linear character-istic impedance) taper TL (Fig. 9(b)). The width of the TLvaries linearly from 18 to 4.5 and back to 18 mm, thedielectric constant of the substrate is 4.25, and theheight of the substrate and the length of the strip are 1.6mm and 65 mm respectively.

Transponder Placement BoundariesTerminated Tapered Resonant TL antennas can pro-

vide the same placement starting distance and placementrange compared to the Terminated Uniform Resonant TLantennas with equally wide conductive strip. For anextended transponder placement range the tapered con-ductive strip can be made wider without sacrificing theantenna bandwidth.

Encoding Field Intensity Field distribution above the quarter-wave terminated

tapered TL antenna (Fig. 9(a)) covers only a part of thetargeted transponder thus delivering half the power ofthe half-wave TL antenna. Electric and magnetic fielddistribution of the half-wave terminated tapered TLantenna (Fig. 9(b)) is concentrated at the most field sen-sitive transponder areas. The antenna with linearly vari-able width at the input end W6 = 18 mm maintains agreater mutual static capacitance with the transponderand provides a higher spatial selectivity than the uniformTL antenna with the narrower conductive strip. The RFpower margin can achieve 6 dB without a significantincrease in the transponder encoding range.

Impedance BandwidthLike other terminated resonant TL antennas, the

tapered TL antennas have their port impedance of 50ohm without an additional matching network. In contrastto the uniform TL, the λ/2 wavelength linearly tapered

Figure 9 · Antennas based on Terminated TaperedResonant microstrip TL: (a) λλ/4 TL antenna; (b) λλ/2 TLantenna; (c) S11 parameter for λλ/2 TL antenna dimen-sions 4.5 ×× 18 ×× 65 mm.

width microstrip TL antenna has a widened bandwidth. Measured reflectionloss (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 taperimplementation for the λ/4 wave microstrip TL is not necessary for the band-width enhancement unless dictated by other design reasons. The uniform λ/4wave microstrip TL with a strip width W4 of up to 30 mm already has BW inthe 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 replacedwith an opposite impedance step provides the equal input and outputimpedances. It implies that the two parts of λ/4 wave tapered TL can be usedas building blocks for tapered λ/2 wave TL antennas.

It was shown by Collin [24] that reflection coefficient of tapered TL is:

(18)

where z is the position along the taper, L is the taper length, Z is the tapervariation, Z0 represents the reference impedance at the input end of the taper.

There are numerous solutions for (18) available for several characteristicimpedance 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 coefficientcan be obtained [27]:

(19)

This simplified solution (19) assumes TEM propagation mode for TL andboth 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 lengththen is shorter than λ/2 wavelength. For a maximum allowed reflection coef-ficient in the pass band the taper profile introduced by Klopfenstein has theshortest total length.

The reflection coefficient along a non-uniform TL can be described by anon-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 fromAnsoft Corporation [35].

[This final part of this article series will appear in the nextissue of HighFrequency 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 anda PhD in EE from Moscow State University in 1986. He has been in theAutomatic Identification and Data Capture industry since 1995; first as anRF Engineer involved in LF RFID design at TRW, then at Zebra TechnologiesCorporation, where he has been since 1998. He can be reached by e-mail [email protected].

ΓINL j LLn

ZZ

eL

L= −1

2 0

β ββ

sin

ΓINj z

L

f eddz

Z dz( ) = ( )−∫12

2

0

β ln



RFID ANTENNAS

UHF RFID Antennas for Printer-Encoders—Part 3: Mobile Equipment


Antennas for RFIDapplications haveunique require-

ments, particularly forthe small spaces insideportable or mobile equip-

ment. This final installment of this series ofarticles looks at antennas for these types ofRFID printer-encoders, followed by summarycomments for the entire series and an exten-sive list of references.

UHF Antennas for Mobile Printer-EncodersSpace saving for mobile RFID printer-

encoders is the biggest concern. Printersrequire UHF antennas to be slim, because thespace available for their installation is verylimited. In addition to the geometric con-strains, the antennas must enable the encod-ing of short labels on a short pitch. Terminatedtapered resonant stripline TL antennas aremost qualified to meet these stringent require-ments of the portable printers. The striplineTL antennas are ultra-compact and conformal.They fit in the space near the printhead andcan provide a short transponder placementrange. These antennas have received the high-est acceptance for transportable and station-ary RFID printer-encoders. Antennas are pre-sented by the half-wave stripline (Fig. 10(a))and a double-conductor stripline (Fig. 10(b))linear taper width TL.

Antenna Structural FeasibilityStripline TL antennas, which are arranged

in parallel with the transponder in the encod-ing area, occupy a very small space behind theplaten roller (Fig. 11). These antennas allowselective encoding of densely spaced transpon-

ders on the liner without activation of adja-cent transponders. Examples of a striplineand double-conductor stripline TL antennasare built on PCB substrate and have dimen-sions of 3.5 × 18 × 100 mm and 6 × 14 × 100mm, respectively. The internal conductor strip(strips for a double-conductor stripline) is

The final installment of this series looks at antennas

for mobile or portable RFIDprinter-encoder equipment

Figure 10 · Structure of terminated taperedstripline TL antennas: (a) single conductor TLantenna; (b) dual-conductor stripline TLantenna.

From November 2007 High Frequency ElectronicsCopyright © 2007 Summit Technical Media, LLC


RFID ANTENNAS

enclosed by two ground planes, stitched by vias along theother three sides of the antennas to organize electricwalls and reduce parasitic radiation. The inner layer pro-file (Fig. 10 (a)) is a modified bow-tie shape with the widthlinearly varied from 9 to 4.5 and back to 9 mm for thestripline and from 10 to 3 to 10 mm for two strips of thedouble-conductor TL antenna. The dielectric constant ofboth substrates is 4.25 and their height is 3.5 and 6 mmaccordingly. The length of the single stripline TL is 64 mmand for double-conductor line is 57 mm. The narrow cen-ter part of the inner layer is positioned close to the activeedge of the TL in order to concentrate magnetic field atthe center of this edge. This position of the maximummagnetic field usually corresponds to the center of a tar-geted for encoding transponder and supports an optimalenergy transfer for the symmetrical antenna-transponderalignment.

Transponder Placement BoundariesThe single stripline TL antenna with a thickness of

only 3.5 mm improves printer’s performance by providinga short transponder placement starting distance from thelabel’s leading edge. It enables individual encoding ofshort Smart Labels with a short pitch comparable to thetransponders width (Fig. 1 (d)). The double-conductorstripline TL antenna with a thickness of only 6 mm wasdeveloped for specific Smart Labels requiring a longertransponder placement range and higher antenna energyefficiency than the single stripline TL antenna.

Encoding Field Intensity Both antennas are in parallel alignment with target-

ed transponders and are coupled with them by one openlong side edge. The electric field strength distributionsimulated using Ansoft HFSS for the single stripline TLantenna (Fig. 12 (a)) and for the double-conductorstripline TL antenna (Fig. 12 (b)) shows optimal shape for

coupling with a dipole type transponder antenna (Fig. 2).The capacitive coupling maintained by the stripline TLantenna is relatively weak and permits very close posi-tioning to transponders. The stripline TL antenna is lessspatially selective than the microstrip TL antenna but itsRF power margin is still about 3 dB without a significant


Figure 11 · Printer zones with stripline TL antenna.

Figure 12 · HFSS simulation of tapered stripline TL. (a)single conductor TL antenna—E field; (b) dual-conduc-tor stripline TL antenna—E field; (c) S11 for dual-con-ductor stripline TL antenna.



RFID ANTENNAS

change in the encoding range. The double-conductorstripline TL antenna in comparison with a single strip TLhas improved field intensity due to a higher SWR gener-ated by an increased load. Its power efficiency, spatialselectivity and coupling grade with a transponder are alsoincreased due to a larger effective edge area. The doublestripline TL antenna has an RF power margin in excessof 6 dB.

Impedance BandwidthThe port impedance of a single conductor stripline TL

antenna is 50 ohms. For the double-conductor TL anten-na the port impedance of 50 ohms is realized without anadditional matching network by connecting in paralleltwo strips, each loaded by a 100 ohm resistor. Both anten-nas utilize the same principles for bandwidth improve-ment as other tapered TL antennas and have a widened

bandwidth. They are shorter than λ/2. A solution forreflection 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 abovesamples the single stripline (Fig. 13 (a)) and double-con-ductor stripline (Fig. 13 (b)) TL antenna S11 parametersdemonstrate bandwidths in excess of 150 MHz. By vary-ing individual strip lengths the multi-conductor striplineTL antenna enables further increase in bandwidth,antenna sensitivity, spatial selectivity, power efficiencyand transponder placement range.

ConclusionsThe 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 interrogatetransponders without RF power suppression. Increasedavailable power delivered by the terminated resonant TLantennas to the encoding interval tolerates usage oftransponders with large variation of their resonance fre-quency and activation power threshold. Moreover,enlarged bandwidth of terminated tapered resonant TLantennas allowed using inexpensive RoHS PCB dielectricmaterials with fairly wide deviations of permittivity,thickness of a substrate and copper cladding.

The proposed miniature stripline TL antennas, withtheir compressed encoding range, permit portable print-er-encoders to work with short, densely spaced SmartLabels. The stripline antennas geometry, their conductivestrip dimensions, and bandwidth obtained from AnsoftHFSS modeling for RFID 915 MHz band, have been veri-fied empirically and found to be in a good agreement.Antenna analysis, mostly concentrated on microstrip andstripline terminated TL, imposed no restrictions on thetype of TL. Other TL structures, for example, the coplanarwaveguide or the slotline, may also be considered asbuilding blocks of antennas for close proximity RFIDapplications. Conclusively the stripline TL antenna isjudged as a vital component for RFID applications involv-ing equipment miniaturization or having spatial con-straints for an antenna installation.

Besides RFID printer-encoders, there are many moreapplications of compact UHF antennas, including accesscontrol (Homeland Security market), item-level RFID forconveyors, testing small transponders during their highvolume manufacturing, quality validation in the SmartLabels conversion process (Industrial market), and scan-ners of RFID Smart credit cards (Financial market). It isbelieved that presented information on UHF antennaswill be helpful in selection of UHF Printer-Encoder and aswell as a tutorial guide for RFID newcomers. Althoughthe terminated TL antennas have low far-field radiation,they are still a source of UHF electromagnetic energy.

Figure 13 · Reflection loss S11 for stripline TL antennasamples. (a) single conductor TL antenna: 4.5 ×× 9 ×× 64mm; (b) dual-conductor TL antenna: 2× (3 ×× 10 ×× 57mm).



RFID ANTENNAS

Antenna mounting elements and nearby metal-plasticcomponents can easily create a parasitic wave-guidingstructure for this energy transmission, causing excessiveunintentional RF radiation that can interfere with thetransponder encoding process. UHF terminated TLantennas have relatively low RF power efficiency inexchange for their spatial selectivity and thus, representan improvement of energy conversion, and can be consid-ered as a subject for further research.

Parts 1 and 2 of this series are available as PDF down-loads from the Archives section of this magazine’s Web site:www.highfrequencyelectronics.com

AcknowledgementsThe author would like to thank Zebra Technologies

Corporation and its associates K. Torchalski, Director ofRFID, and M. Schwan, System Manager for their helpfuland productive discussions regarding UHF RFID Printer-Encoders development, M. Fein, RF Engineer for hisHFSS counseling, and R. Gawelczyk, EngineeringTechnician for his outstanding support and assistance inantenna fabrication, testing and evaluation. The authoralso would like to thank S. Kovanko, EE Engineer forcarefully reading parts of the manuscript.

References 1. “Item-Level Visibility in the Pharmaceutical Supply

Chain: a Comparison of HF and UHF RFID Technologies,”White Paper, Philips Semiconductors, TAGSYS, TexasInstruments Inc., July 2004. http://www.tagsysrfid.com/modules/tagsys/upload/news/TAGSYS-TI-Philips-White-Paper.pdf

2. M.C. O’Connor, “Study Shows Big Growth for RFIDPrinter-Encoders,” RFID Journal, Inc., July 25, 2006.http://www.rfidjournal.com/article/articleprint/2515/-1/1/

3. M.C. O’Connor, “RFID Changing Buying Behavior,”RFID Journal, Inc., July 21, 2006. http://www.rfidjournal.com/article/articleprint/2508/-1/1/

4. L.G. Maloratsky, Passive RF & MicrowaveIntegrated Circuits, Newnes, 2003.

5. K.V.S. Rao, P.V. Nikitin, S.F. Lam, “Antenna Designfor UHF RFID Tags: A Review and a Practical Appli-cation” IEEE Transactions on Antennas and Propagation,Vol. 53, No. 12, pp. 3870-3876, December 2005.

6. “Texas Instruments Gen 2 Inlay,” RI-UHF-00C02 -Product Bulletin, 2006. http://www.ti.com/rfid/docs/manu-als/pdfspecs/ri-uhf-00c02_prodbulletin.pdf

7. D.M. Dobkin, S.M. Weigand, “UHF RFID and TagAntenna Scattering, Part I: Experimental Results,”Microwave Journal, Vol. 49, No. 5, pp. 170-190, May 2006.

8. D.M. Dobkin, T. Wandinger, “A Radio-OrientedIntroduction to RFID—Protocols, Tags and Applications,”High Frequency Electronics, Vol. 4, No. 8, pp. 32-46,August 2005.

9. D.M. Dobkin, S.M. Weigand, “UHF RFID and TagAntenna Scattering, Part II: Theory,” Microwave Journal,Vol. 49, No. 6, pp. 86-96, June 2006.

10. (284) T. Breahna, D. Johns, “Simulation SpicesRFID Read Rates,” Microwaves and RF, pp.66-76, March2006.

11. P.V. Nikitin, K.V.S. Rao, S.F. Lam, V. Pillai, R.Martinez, H. Heinrich, “Power Reflection CoefficientAnalysis for Complex Impedances in RFID Tag Design,”IEEE Transactions on Microwave Theory and Techniques,Vol. 53, No. 9, pp. 2721-2725, September 2005.

12. C.A. Balanis, Antenna Theory: Analysis andDesign, 2nd Edition, John Wiley & Sons, 1996.

13. C. Capps, “Near field or far field?,” EDN Magazine,pp. 95-102, August 16, 2001.

14. I. Straus, “Loops and Whips, Oh My!,” Conformity,pp. 22-28, August 2002.

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Author Information Boris Y. Tsirline is the Principal

Engineer at Zebra TechnologiesCorporation. He received a BS andMS degrees in RF & MicrowaveEngineering from Moscow AviationUniversity, Russia in 1973 and a PhDin EE from Moscow State Universityin 1986. Before moving to the US in1992, he served as a Director of R&D

at Automotive Electronics and Equip-ment Corp., Russia, developing mili-tary and aerospace electronic sys-tems. He has been in the AutomaticIdentification and Data Captureindustry since 1995. He managed thedevelopment of Zebra’s first HF RFIDprinter-encoder and established thedesign methodology for HF and UHF

spatially selective transponderencoding modules used throughoutthe corporation divisions for RFIDlabels and cards printers. Dr. Tsirlineholds three non-classified Russianand two US patents and has numer-ous pending patents for RFIDenhancements. He can be reached bye-mail at [email protected].

U H F R F I D Antennas For Printer Encoders

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