Design of RFID Tags and SystemsDesign of RFID Tags and Systems

Prof. Raj MittraPennsylvania State University

University Park , PA, USA

Dr. KVS RaoIntermec Technologies

Everett, WA, USA

June 10, 2007

APS 2007Honolulu, Hawaii

ContentsContents1. Introduction to RFID2. RFID link budget and backscattering fundamentals3. RF front end considerations of ASIC.4. RF matching from ASIC to antenna5. Complete design, simulation and test of RFID tag,

including antenna, ASIC and application examples6. Near-field antenna considerations for RFID tags7. RFID applications, examples, and latest developments8. Conclusions

1. Introduction to RFID

Operating principlesHistoryRFID systems

FrequenciesTags ReadersProtocols

RFID TechnologyRFID TechnologyRFID (Radio Frequency Identification) uses radio frequencies for sending data from the tagged object to the reader.

RFID system components

TagsReadersMiddleware

Operating Principle: LF/HF RFIDOperating Principle: LF/HF RFID

Analogy: loaded transformer

Reader coil antenna creates RF magnetic field Passive tag gets power from the energy of the field responds by loading its own coil antenna with different impedances

LF/HF – low and high frequency

Operating Principle: UHF RFIDOperating Principle: UHF RFID

Analogy: using flashlight and mirror to communicate with Morse codeA

nten

na

Reader antenna transmits RF signal for the tagPassive tag gets power from the energy of the received signal and reflects it, modulating with its own impedance and thus presenting different RCS values

UHF – ultra high frequency

History of RFID History of RFID World War II

German pilots rocked their planes to signal back to the radar that they are comingBritish aviation used “identify friend or foe” system with simple transponder in the nose of the plane

1945 – Soviet eavesdropping device “Buran” (Leo Theremin)1948 - paper “Communication by means of reflected power

(Harry Stockman)1973 – first RFID patent 1999 – Auto-ID center is formed at MIT2004 – EPC Gen2 standard is ratified

RFID vs. Bar CodeRFID vs. Bar CodeRFID tag Bar code

Rewriteable Yes No

Read distance Up to 30 ft Up to 3 ft

Line of sight Not necessary Necessary

Read speed Up to 1500 tags/sec(Gen2)

2 bar codes/sec(trained operator)

Reliability Difficult to damage and counterfeit

Easy to damage and counterfeit

Cost 0.1-1 $ 0.001-0.01$

Equipment cost Approximately the same

RFID Frequency BandsRFID Frequency BandsLF HF UHF

Frequency 125-134 KHz 13.56 MHz

ISO 15693ISO 14443ISO 18000-3up to 1m

up to 64 kbps

860-960 MHz

Standards ISO 11784/5ISO 14223ISO 18000-2

ISO 18000-6

Read range up to 1 m up to 15 m

Data rate up to 9600 bps up to 640 kbps

Applications Logistics, manufacturing, access control

100 kHz 1 MHz 10 MHz 1 GHz 10 GHz100 MHz

13.56 MHz

915 MHz

125-134 kHz

2.45 GHz

LF HFMF UHFVHF

UHF Tag ClassificationUHF Tag ClassificationBy power

Passive Semi-passive Active

By memoryChipless (read only)With chip

• Class 0• Class 1• ISO (18000-6B), • Gen2 (ISO 18000-6C)

1976

1987

1999

Tag evolution

Flexible Passive UHF TagsFlexible Passive UHF TagsUsed in smart labels

Rigid Passive UHF TagsRigid Passive UHF TagsUsed for tagging pallets, machine parts, metal objects

RFID Readers and PrintersRFID Readers and PrintersReaders

Direct conversion transceivers, simultaneous Tx/RxAntennas: separate (switching) or singlePower: up to 1 W (30 dBm)Sensitivity up to -80 dBmDynamic range up to 100 dBWireless or wired database interface

PrintersReader integrated into the printerPrint bar code/text/images on the label and encode RFID inlay

UHF RFID ProtocolsUHF RFID Protocols

Class 0 Class 1 ISO-6B Gen2 (ISO-6C)

Anticollisionmethod

Binary Binary Binary Aloha

Read rate Low Low Low High

Password No No No Write/read/kill

Group select No No No Yes

Read/write No Limited Yes YesEncryption No No No Reader

Gen2 Electronic Product Code (EPC)Gen2 Electronic Product Code (EPC)

Header (8 bit): version number256 combinations

EPC manager number (28 bit): manufacturer268 million combinations

Object class number (24 bit): type of product16 million combinations

Serial number (36 bit): unique68 billion combinations

User data (32 bit).

96 bit (24 hexadecimal symbols)

ReferencesReferencesH. Stockman, “Communication by means of reflected power,”Proc. IRE, pp. 1196–1204, Oct. 1948.A. R. Koelle, S. W. Depp, and R. W. Freyman, “Short-range radio-telemetry for electronic identification, using modulated RF backscatter”, Proceedings of the IEEE, vol. 63, no. 8, Aug. 1975, pp. 1260-1261R. Glidden, et al., “Design of ultra-low-cost UHF RFID tags for supply chain applications”, IEEE Communications Magazine, vol. 42, no. 8, Aug. 2004, pp. 140-151J. Landt, “The History of RFID”, IEEE Potentials, vol. 24, no. 4, Oct.-Nov. 2005, pp. 8-11R. Want, “An Introduction to RFID Technology”, IEEE Pervasive Computing, vol. 5, no. 1, Jan.-Mar. 2006, pp. 25-33

2. RFID Link Budget Calculations and Backscattering Fundamentals

Link budgetTag read range

DefinitionLimitationsMeasurement

RCS (Radar Cross Section)Scalar RCSDifferential RCS

• Scalar • Vector

Link BudgetLink Budget

( ) ( ) pd

GGPP rtta

2

4,, ⎟

⎠⎞

⎜⎝⎛⋅⋅⋅=πλϕθϕθ

Power received by tag antenna in free space (from Friis equation)

tag the todistancewavelength

lossmismatch on polarizati antenna tag theofgain

antennareader theofgain poweroutput eader

antenna by tag receivedpower

−−−−−−−

d

pGG

rPP

r

r

t

a

λ

Tag Read Range

4 th

rtt

PpGGPr τ

πλ

=

IC of sholdpower thre t)coefficien matching impedance (a.k.a.

t coefficiensmission power tran

−

−

thP

τ

Maximum read range

Chip must receive enough power to turn on:

thc PP ≥

Power absorbed by the chip is determined by impedance matching

τ⋅= ac PP

Tag Read Range and Read RateTag Read Range and Read Rate

Read rate in a typical RFID system depends on distance and frequency (figure above is for the case where a tag is tuned to 900 MHz in a specific environment)

90 %

Tag Read RangeTag Read Range

TagChip sensitivityAntenna gainImpedance matchingPolarization matching

ChannelPropagation lossReflectionsInterference

ReaderTransmitter powerReceiver sensitivity

Depends on the following factors:

Tag antenna impedance and gain depend on the properties of tagged object

Ideally, RFID reader detects the tag as soon as it responds.

Interference from other wireless sources can significantly degrade tag range.

Tag LimitationsTag Limitations

Tag 1Antenna gain = 0 dBiChip sensitivity = -10 dBmMaximum range = 16 ft

(900 MHz, 4 W EIRP)

Tag must provide strong modulated backscattered signal for the reader

Tag 2Antenna gain = 10 dBiChip sensitivity = 0 dBmMaximum range = 16 ft

(900 MHz, 4 W EIRP)

!2012 dBPPredbackscatteredbackscatte=−

Two tags may have the same maximum range but signals seen by thereader from these tags may be different

lossmodulation antenna tag theofgain

−−

KGr2

rredbackscatte GP ∝

Propagation Environment LimitationsPropagation Environment Limitations

Path lossFree spaceMulti-path environmentWaveguide environment

Tag detuning on materialsAntenna impedanceAntenna gain

Reader LimitationsReader LimitationsEIRP (US: 4 W, Europe: 3.3 W)Sensitivity (equivalent)

Reader must be able to detect and decode noisy tag signal whose SNR proportional to tag differential RCS

0.059

0.0595

0.06

0.0605

0 10 20 30 40 50 60 70 80 90 100

Time (us)

Vol

tage

(V)

Gen2

ExampleExample

4 W EIRP915 MHz-80 dBm equivalent sensitivity

Free space

-10 dBm chip sensitivity2 dBi antennaperfect impedance match3 dB modulation loss

ExampleExample

Received power vs. distance for tag and reader in example RFID system

Transmitted EIRP (36 dBm)

Typical tag chip sensitivity (-10 dBm)

Typical reader sensitivity (-80 dBm)120 ft

20 ft

Tag Range Measurement ProcedureDistance d to the tag is fixed, transmitted power is variableTest equipment sends queries and analyzes backscattered signalMinimum power Pmin at which tag response is detected allows one to determine tag range at different frequencies for any given EIRP value

minP

EIRPdr =

Range Measurement Facility at IntermecAnechoic chamber

4 ft x 4 ft x 6 ft6 dBi linearly polarized antenna

Measurement EquipmentMeasurement EquipmentNational Instruments PXI RF hardware platform controlled by LabVIEWMeasures the minimum power needed for tag to respondMulti-protocol (ISO, Gen2, etc.)Broadband (800 -1000 MHz)

RFID Tag Scalar RCSRFID Tag Scalar RCSScalar RCS of an RFID tag determines the backscattered power when the tag antenna loaded with constant impedance loadBackscattered power = Power scattered from the open-circuited antenna + Power re-radiated by the loaded antennaOpen-circuited thin wire antennas scatter little power compared to when they are loaded (minimum scattering)For minimum scattering loaded antennas, re-radiated power can be calculated from the simple equivalent circuit

radiatedreradiatedrecircuitedopenredbackscatte PPPP −−− ≈+=

ginco

redbackscatte

SP

min

=σ density power power

−−

SP

Scalar RCS DerivationScalar RCS Derivation

Power collected by the loaded antenna

impedance antennaimpedance chip

wavelengthEIRP ed transmitt

area effective antennaantenna tag theofgain

−+=−+=

−−

−−

aaa

ccc

tt

e

jXRZjXRZ

GPAG

λ

Gr

GPASP ttea π

λπ 44

2

2 ⋅==

Power re-radiated by the loaded antenna(power dissipated in Za) G

ZZRPP

ca

aaradiatedre 2

24+

=−

2

222

ca

aradiatedre

ZZRG

SP

+== −

πλσScalar radar cross-section

K-factorK-factorK-factor shows how much power is re-radiated

2

24

ca

a

ZZR

K+

=22

24

aa

a

XRR+

0cZ *aZ ∞

K 1 0

Power re-radiated by complex conjugate loaded antenna is normalized by the power re-radiated by the short circuit loaded antenna

Measurement Methodology

Antenna gain is 6 dBi, distance to the tag is 0.5 m (to power up tag IC)

Anechoic chamber, antenna, network analyzer, tag tester Return loss is measured without a tag (for calibration) and then with the tag present inside the chamber

RFID tag

Antenna

Tags used in Measurements Tags used in Measurements

16 m

m

Return Loss Calibration (without Tag)Return Loss Calibration (without Tag)

Measured Return Loss (with Tag)Measured Return Loss (with Tag)

( )22

432

114

tGrS

λπσ =RFID tag scalar RCS

Comparison of Theory and DataComparison of Theory and Data

RFID Tag Differential RCSRFID Tag Differential RCS

Differential RCS of an RFID tag is an important parameter which determines the power of the modulated tag signal received by the reader

Non-coherent receiver can only register a magnitude difference between two scalar RCS values (scalar differential RCS)

Coherent receiver can detect both amplitude and phase of the signal and hence can register a vector difference between two RCS values (vector differential RCS

Vector Differential RCS DerivationVector Differential RCS Derivation

ac

ac

ZZZZ

+

−=

2,1

*2,1ρ

GRIIP absdif2

21.. 21

−=

( ) ( )2,12,1

.2,1 12

ρ−=+

=a

o

ca

o

RV

ZZVI

221

22..

4ρρ

πλσ −==Δ

GS

P bsdifRFID tag differential RCS

Currents in the antenna due to two different loads

Differential backscattered power

Load reflection coefficient

Vector Differential RCS on Smith ChartVector Differential RCS on Smith Chart

ac

ac

ZZZZ

+

−=

2,1

*2,1ρ

Measurement SetupMeasurement Setup

( )2

43

24λπσ d

GPP

tt

received=Δ

Received power is measured directly in the signal analyzer

Comparison of Theory and DataComparison of Theory and Data

-40

-35

-30

-25

-20

870 880 890 900 910 920 930Frequency (MHz)

diffe

rent

ial R

CS

(dB

sqm

)

Theory

Data

ReferencesReferencesD. D. King, “The measurement and interpretation of antenna scattering”, Proc. IRE, 1949, 37, (7), pp. 770 – 777R. Harrington, “Electromagnetic scattering by antennas”, IEEE Transactions on Antennas and Propagation, 1963, 11, (5), pp. 595-596R. C. Hansen, “Relationships between antennas as scatterers and as radiators”, Proceedings of the IEEE, 1989, 77 (5), pp. 659-662K. Schneider, “A re-look at antenna in-band RCSR via load mismatching”, Proc. of IEEE Ant. and Prop. Soc. Int. Symposium, June 1996, pp. 1398-1401L. Penttila, M. Keskilammi, L. Sydanheimo, and M. Kivikoski, “Radar cross-section analysis for passive RFID systems”, IEE Proceedings on Microwaves, Antennas and Propagation, 2006, 153, (1), pp. 103-109P. V. Nikitin and K. V. S. Rao, “Theory and measurement of backscattering from RFID tags”, IEEE Antennas and Propagation Magazine, vol. 48, no. 6, pp. 212-218, December 2006P. V. Nikitin, K. V. S. Rao, and R. Martinez, “Differential RCS of RFID tag”, Electronics Letters, vol. 43, no. 8, pp. 431-432, April 2007

3. RF Front End Considerations of ASIC

Tag Block DiagramRectifier and Voltage MultiplierModulator and DemodulatorTag Equivalent Circuit ExampleDesign Criteria

Tag Block DiagramTag Block Diagram

Supplies DC voltage

Changes tag input impedance

Detects signal envelope for data decoding

Rectifier and Voltage MultiplierRectifier and Voltage Multiplier

RFV

( )DRFOUT VVNV −≈

OUTV

- number of diodes - amplitude of RF input signal from tag antenna- forward voltage of Schottky diodes (~0.2V)

RFVDV

N

Modulator and DemodulatorModulator and Demodulator

Modulator

DemodulatorAntenna terminals

Antenna terminals

Tag Equivalent Circuit Example

Front End Impedance Calculation

Impedance calculation of the front-end (ohms) :

Rpad 102:= Cpad .0991:= Zpad Rpad103− i⋅

2 π⋅ fr⋅ Cpad⋅+:= Rs 123:= Rj 300000:= Cj .02:=

Zdiode Rs1

1Rj

1

10−( )3 i⋅2 π⋅ fr⋅ Cj⋅

+⎡⎢⎢⎣

⎤⎥⎥⎦

+:=Rnwell 100:= Cnwell .074:= Znwell Rnwell103− i⋅

2 π⋅ fr⋅ Cnwell⋅+:=

Zpwrpt Zdiode1

1300000

1

10−( )3 i⋅2 π⋅ fr⋅ 15⋅

+⎡⎢⎢⎣

⎤⎥⎥⎦

+:= Zsigpt Zdiode1

1300000

1

10−( )3 i⋅2 π⋅ fr⋅ 1.5⋅

+⎡⎢⎢⎣

⎤⎥⎥⎦

+:=

Zsp 5000103− i⋅

2 π⋅ fr⋅ 1.⋅+:=

Zchip1

1Zpad

1Znwell

+1

Zdiode+

1Zpwrpt

+1

Zsigpt+⎛

⎜⎝

⎞⎟⎠

:= Zchip 36.802 747.014i−=

Package Impedance Calculation and Matching

ML 0.954−=ML 10 log τ( )⋅:=Mismatch loss in dB is :

τ 0.803=τ1 τtag( )2−⎡⎣ ⎤⎦ 1 τpkg( )2

−⎡⎣ ⎤⎦⋅

1 τtag τpkg⋅−( )2:=τ

4 Re Zpkg( )⋅ Re Ztag( )⋅

Zpkg Ztag+( )2:=

This mismatch can be calculated from the transmission coefficient which can be expressed as

Zpkg 19.869 534.241i−=τtag 0.994=τpkg 0.993=τtagZtag 50−

Ztag 50+:=τpkg

Zpkg 50−

Zpkg 50+:=

Ratio 1.028=RatioZchip( )2

Re Zchip( )Re Zpkg( )

Zpkg( )2⋅:=Zpkg 19.869 534.241i−=Zpkg Z11

Z12 Z21⋅

Z22 Zchip+( )−:=

Z22 Z11:=Z21 Z12:=Z12 Zt4:=Z11 Zt4 XL+:=

Zcfl Zchip XL+:=Zt4 0103−

2 π⋅ fr⋅ Cpkg⋅i⋅+:=XL 2 π⋅ fr⋅ Lpkg⋅ i⋅:=Lpkg 1.5:=Cpkg .085:=

Parameters of the package :

Z tag 20 554 1i⋅+:=

Front End Design ConsiderationsFront End Design Considerations

Rectifer/voltage multiplerHaving a high RF to DC power conversion efficiency Providing sufficiently high output voltage (~2 V) to power up digital logic

ModulatorChoosing the modulation typeSelecting the impedance states to achieve high modulated backscattered power

Forward link limitation on tag range

Return link limitation on tag range

ReferencesReferencesU. Karthaus and M. Fischer, “Fully integrated passive UHF RFID transponder IC with 16.7 u/W minimum RF input power”, IEEE Journal of Solid-State Circuits, Volume 38, Issue 10, Oct. 2003, pp.1602 - 1608

G. De Vita and G. Iannaccone, “Design Criteria for the RF Section of UHF and Microwave Passive RFID Transponders”, IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 9, Sept. 2005, pp. 2978-2990

J.-P. Curty, N. Joehl, C. Dehollain, and M. J. Declercq, “Remotely powered addressable UHF RFID integrated system”, IEEE Journal of Solid-State Circuits, Volume 40, Issue 11, Nov. 2005, pp. 2193 - 2202

4. RF matching from ASIC to antenna

Equivalent CircuitPower transmission coefficient

Impedance contoursNormalized contours

Reflection coefficientSmith chart

Tag Equivalent CircuitTag Equivalent CircuitPower absorbed by the chip is determined by impedance matching

impedance antennaimpedance chip

t coefficiensmission power tran antenna thefrom availabepower chip by the absorbedpower

−+=−+=

−−−

aaa

ccc

a

c

jXRZjXRZ

PP

τ

24

ac

ac

ZZRR

+=τ

Antenna

Chip

τ⋅= ac PP

Impedance Matching Contour ChartImpedance Matching Contour Chart

( ) ( )2

22

21412τ

ττ

−=++⎟⎟

⎠

⎞⎜⎜⎝

⎛⎥⎦⎤

⎢⎣⎡ −− c

cacaRXXRR

24

ac

ac

ZZRR

+=τ

( ) ( )τ

acacac

RRXXRR 422 =+++

Contours of constant transmission coefficient are circles on (Ra,Xa) planeCenter of circles corresponds to perfect matchCircle radius depends on chip impedance

⎟⎠⎞

⎜⎝⎛ −12τcR

aX−

ττ

−12 cR

1=τ

Normalized Impedance Matching Contour ChartNormalized Impedance Matching Contour Chart

( ) ( )2

22

1412ττ

τ−

=−+⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦⎤

⎢⎣⎡ −− Qxr aa

⎟⎠⎞

⎜⎝⎛ −12τ

( ) ( )τ

aaa

rQxr 41 22 =−++

Contours of constant transmission coefficient are circles on (ra,xa) planeCenter of circles corresponds to perfect matchCircle radius depends only on transmission coefficient

Q

ττ

−12 ( ) ( )14

22aa

a

xQrr

+++=τ

c

c

c

a

c

a

RX

RX

RR

−=

=

=

Q

x

r

a

a

1=τ

Complex Impedance Matching on Smith ChartComplex Impedance Matching on Smith Chart

ca

ca

ZZZZ

+−

=*

ρ

( )( ) ccaa

ccaa

RXXjRRXXjR

+++−++

=ρ

Smith chart is normalized to real impedance RcQuantity plotted is Ra+j(Xa+Xc)Origin corresponds to perfect match

21 ρτ −=

Range vs. Transmission CoefficientRange vs. Transmission Coefficient

Shows the effect of better matching on relative range improvementMaximum possible range is when the tag is perfectly matched

τ∝r

ReferencesReferencesK. Kurokawa, “Power waves and the scattering matrix”, IEEE Transactions On Microwave Theory and Techniques, 1965, MTT-13, (3), pp. 194–202

K. V. S. Rao, H. Heinrich, R. Martinez, “On the analysis and design of high-performance RFID tags," IEEE Workshop on Automatic Identification Technologies, pp. 68-69, March 2002

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

5. Complete Design, Simulation and Test of RFID Tag, including Antenna, ASIC and

Application Examples

Tag design processTag characteristicsPerformance chartDesign examples

Most flexible tags are dipoles (folded, loaded, stubbed, meandered, fed with slotline, crossed, combined with loops, etc.)

RFID Tag Design: Art or Science?RFID Tag Design: Art or Science?

RFID Tag MaterialsRFID Tag Materials

Dielectric SubstrateFR4 (rigid)PET (flexible)High permittivity dielectrics Paper

Antenna TraceCopper (most expensive)Aluminum (less expensive)Silver ink (cheapest)

Chip packagingFlip-chipWire-bondedSMD packages (TSSOP, MSOP)

RFID Tag Design ProcessRFID Tag Design Process

Select the application and define tag requirements

Determine the materials for antenna construction

Determine RF impedance of packaged ASIC

Identify the type of antenna and its parameters

Perform parametric study and optimization

Build and measure prototypes

Design requirements

met?

Design is ready

EM Software used at IntermecEM Software used at Intermec

Ansoft HFSSThick (rigid) tags, chip packaging modeling

DesignerThin (flexible) tags

Pallet tag

Rigid tag

Meander tag

Main Tag CharacteristicsMain Tag Characteristics

RangeBandwidthOmni-directionality

Tag Antenna Performance ChartTag Antenna Performance Chart

or - range of the tag with 0 dBi antenna and perfect impedance match

RFID Tag Design ExampleRFID Tag Design Example

RFID tag for smart label applications on variety of materialsRange: at least 15 ft in 860-960 MHz band (free space, 4W EIRP)Size limitation: 4 x 4 inMaterials: 1 mil copper on 2 mil polyester substrate Chip: ISO 18000-6B Attachment: flip-chip, packaging variations possible

Requirements

Tag antennaS-structure

WidebandTunable to accommodate for:

• Chip packaging variations• Different tagged materials

Packaged Chip ImpedancePackaged Chip ImpedanceTheoretical

Modeling/simulation with HFSS

PracticalMeasure packaged chip impedance at different power levels with a network analyzer

TSSOP Flip-chip

Chip Impedance vs. Frequency and PowerChip Impedance vs. Frequency and Power

Impedance of RFID Gen2 Chip (Freq 900)

-10

-5

0

5

10

15

20

25

30

-20 -15 -10 -5 0 5

Power Delivered to Chip (dBm)

R

-150

-130

-110

-90

-70

-50

-30

-10

Xj

R(Ohms)

Impedance of RFID Gen2 Chip (-10 dBm)

-10

-5

0

5

10

15

20

25

30

800 820 840 860 880 900 920 940 960 980 1000

Frequency

R

-150

-130

-110

-90

-70

-50

-30

-10

Xj

R(Ohms)

Frequency dependence Power dependence

Tag Design Process with DesignerTag Design Process with DesignerCreate parametric geometry

Specify excitation

Write post-processing variables

Parametric StudyParametric StudyIdentify key parameters and fix othersRun parametric analysis, plot and examine the rangeIdentify optimal parameter combinations

Parametric Study (S-antenna)Parametric Study (S-antenna)Fixed parameters

L=H=80 mm W1=0.5 mm

Variable parametersW2=W3=W L3

Optimal parameter combinationW=10 mmL3=80 mm (untrimmed), 40 mm (trimmed for 915 MHz band)

L3 varied W varied

Experimental VerificationExperimental Verification

Make several prototypes (use X-acto knife)Compare with experimental dataIdentify sources of discrepancy

0

5

10

15

20

25

30

800 820 840 860 880 900 920 940 960 980 1000Frequency (MHz)

Ran

ge (f

t)

DataSimulation

L3 = 40 mm

Free space, 4 W EIRP

Designer simulation has frequency offset compared to data (HFSS agrees with data well)

Final Design of S-antenna*Final Design of S-antenna*Tuning by trimming (punching)

Prototype

Gain pattern

* patent pending

80 mm

80 mm

0.5 mm

10 mm10 mm

Performance on Different MaterialsPerformance on Different Materials

0

5

10

15

20

25

30

820 840 860 880 900 920 940 960 980

Frequency (MHz)

Ran

ge (f

t)

Free spaceCardboardPlastic

L3 = 40 mm Free space, 4 W EIRP

S-antenna tag can be easily tuned for any material

Minimum required range

Another Design ExampleAnother Design Example

Intellitag ID card915 MHz bandISO 18000-6B

Antenna was modeled and simulated with Ansoft Designer

70 mm x 22 mm4 mil FR4 substrate with copper traceFairchild RFID ASIC chip

Experimental and Simulation ResultsExperimental and Simulation Results

RFID tag characteristics depend on surrounding material

EIRP=4 W

Free space resonant frequency

Resonant frequency inside plastic card

ReferencesReferencesP. R. Foster and R. A. Burberry, “Antenna problems in RFID systems”, IEE Colloquium on RFID Technology, October 1999, pp. 3/1-3/5

K. V. S. Rao, P. V. Nikitin and S. Lam, “Antenna Design for UHF RFID Tags: a Review and a Practical Application”, IEEE Transactions on Antennas and Propagation, vol. 53, no. 12, pp. 3870-3876, December 2005

D. M. Dobkin and S. M. Weigand, “UHF RFID and tag antenna scattering (Part I: Theory, Part II: Experimental Results)”, Microwave Journal, vol. 49, no. 5-6, May-June 2006

T. C. Chau, B. A. Welt, and W. R. Eisentadt, “Analysis and Characterization of Transponder Antennae for Radio Frequency Identification (RFID) Systems”, Packaging Technology and Science, no. 19, pp. 33-44, 2006

6. Near-field Antenna Considerations for RFID Tags

Antenna field regions Reader-tag antenna coupling Material penetration and antenna sizeNear Field UHF RFID OptionsMeasurements

Antenna Field RegionsAntenna Field Regions

DRFID reader

Antenna

D

r

RFID reader

Reactive

πλ 2/=r

λ/2 2Dr <

λ/62.0 3Dr =

Electrically small antenna

Electrically large antenna

πλ 2/≈rRadiating near field

Reactive near field

Near-field theory is the same for LF, HF, and UHF RFID(comes from Maxwell’s equations)

Reader-Tag Antenna CouplingReader-Tag Antenna Coupling

τρ CPP readerchip =2

4

tr

tr

ZZRR

+=ρ 2

4

ac

ac

ZZRR

+=τ

Reader impedance matching coefficient

Tag impedance matching coefficient

Coupling coefficient C depends on:Reader and tag antenna geometries;Relative position of antennas (distance and orientation);Environment, including any objects near antennas.

Types of CouplingTypes of CouplingRadiative (far field)

Different regions of reader antenna impedance (shown for a dipole antenna).

Non-radiative (near field)Inductive (magnetic)Capacitive (electric)

Hertzian Dipole FIeldsHertzian Dipole FIelds

θββπ

βη β cos)(

1)(

12 32

2rj

r erjrj

lIE −⎟⎟⎠

⎞⎜⎜⎝

⎛+Δ=

θβββπ

βη βθ sin

)(1

)(11

4 32

2rje

rjrjrjlIE −

⎟⎟⎠

⎞⎜⎜⎝

⎛++Δ=

θββπ

β βφ sin

)(11

4 2

2rje

rjrjlIH −

⎟⎟⎠

⎞⎜⎜⎝

⎛+Δ=

Near field decays as 1/r^2 and 1/r^3Far field decays as 1/r

x

E

z

Energy radiation

Energy storage

r

r EH

Near field Far field

Far Field Coupling (Radiative)Far Field Coupling (Radiative)Mutual effect of antennas is minimalAntenna impedance and gain can be specified independently of each

other and of distance between the antennasLong range, associated with electromagnetic plane wavesReader transmits a modulated signal which decays in free space as 1/rTag responds by modulating the backscattered signal from the tag

pGLGC rpatht=

2

4 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

dLpath π

λ

In free space, path loss is:

RFID reader

RFID tag

Near Field Coupling (Non-radiative)Near Field Coupling (Non-radiative)

Inductive Coupling (magnetic): most energy is stored in magnetic fieldExample: multiple turn coil, axial magnetic field decays as 1/r^3RFID application: HF RFID systems Field is mostly affected by the presence of magnetics and conductors

Capacitive Coupling (electric): most energy is stored in electric fieldExample: parallel plate capacitorRFID application: UHF RFID printer couplerField is mostly affected by the presence of dielectrics and conductors

Mutual effect of antennas cannot be ignoredAntenna impedance and gain become dependent on mutual antenna position and orientation

Magnetic Coupling Example (HF RFID)Magnetic Coupling Example (HF RFID)Basic theory: Faraday’s law (induced voltage is proportional to the magnetic flux rate of change)

If the tag antenna is small, the magnetic field created by the reader antenna is not perturbed by the tag and the coupling coefficient is:

α2222 BSNfC ∝

dtdV Φ

−=

RFID reader antenna RFID transponder

Material Penetration and Antenna SizeMaterial Penetration and Antenna Size

fσμπδ 1=

Frequency 125 KHz (LF) 13.56 MHz (HF) 900 MHz (UHF)

Wavelength 2400 m 22.1 m 0.33 m

Skin depth(aluminum)

230 um 22.3 um 2.7 um

Field penetration into materials is limited by skin depth For near field operation antenna size can be much smaller than wavelengthTo radiate efficiently into the far field, antenna size needs to be comparable with wavelength

frequencytyconductivitypermeabili

−−−

fσμ

Item Level UHF RFID System OptionsItem Level UHF RFID System OptionsReader Antenna

Reader Power

Tag Read zone

1

2

Standard Full

3

4

Standard Low

LargeStandard

Standard

Short range1.Mistuned 2.Special

Small

Small for short range tagsLarge for standard tags

Special Small

Standard Full

AnySpecial

Standard – RF antennas which radiate well into the far field Special – RF antennas which generate primarily near field

Tag Range for Option 2 (Low Reader Power)Tag Range for Option 2 (Low Reader Power)

048

1216202428

800 820 840 860 880 900 920 940 960 980Frequency (MHz)

Tag

rang

e (ft

)

EIRP=4 WEIRP=0.04 W

RFID tag: TI Dallas

Experimental Test SetupExperimental Test Setup

Antenna (Sinclair LPD) Tag (TI Dallas)

Inside anechoic chamber

Minimum Power vs. DistanceMinimum Power vs. Distance

-6

-2

2

6

10

14

800 830 860 890 920 950 980Frequency (MHz)

Min

imum

pow

er (d

Bm

) 242220181614121086

d (in)

RFID reader antenna: Sinclair, RFID tag: TI Dallas

Far Field BoundaryFar Field Boundary

-6-4-202468

1012

0 2 4 6 8 10 12 14 16 18 20 22 24Distance (in)

Min

imum

pow

er (d

Bm

)

Theory (far field)Data

Reader antenna: Sinclair, RFID tag: TI Dallas

Liquids Demo: UHF RFID Tag in GatoradeLiquids Demo: UHF RFID Tag in Gatorade

Standard UHF RFID reader antennaStandard UHF RFID tag Tag is immersed into the bottle of Gatorade

Liquids Demo: Measurement ResultsLiquids Demo: Measurement Results

23

24

25

26

27

28

840 860 880 900 920 940Frequency (MHz)

Pow

er (d

Bm

)

Tag can be read using as little as 250 mW reader power at 890 MHz

Tag response

Near Field UHF RFID TagsNear Field UHF RFID TagsLF and HF RFID Tags can not work in far field

Physical sizes of the antenna ( both reader and tag) need to be large to operate in far field

UHF RFID Tags can work both in near and far fieldRadiation mechanism includes both near and far fields for realizable physical sizes of the antenna (both reader and tag)

Near Field ConsiderationsNear Field ConsiderationsBasic physics and design methodology are the same for RFID at LF, HF, and UHF

Near field UHF RFID can be used for item level tagging

Magnetic or electric coupling can be used for near field UHF RFID depending on application

UHF has the advantage of smaller antenna size for both near and far field operation

ReferencesReferencesC. A. Balanis, “Antenna theory: analysis and design”, John Wiley & Sons, 1997R. Bansal, “Near-field magnetic communication, IEEE Antennas and Propagation Magazine, Vol. 46, No. 2, Apr. 2004, pp. 114 – 115“Item-level visibility in the pharmaceutical supply chain: a comparison of HF and UHF RFID technologies”, white paper by Philips, TAGSYS, and Texas Instruments, available at http://www.tagsysrfid.com/modules/tagsys/upload/news/TAGSYS-TI-Philips-White-Paper.pdfT. Lecklider, “The world of the near field”, Evaluation Engineering, October 2005, available at http://www.evaluationengineering.com/archive/articles/1005/1005the_world.asp P. V. Nikitin and K. V. S. Rao, “An Overview of Near Field UHF RFID”, IEEE RFID Conference, Grapevine, TX, March, 2007, pp. 167-174

7. RFID Applications, Examples, and Latest Developments

ApplicationsExamplesLatest Developments

RFID ApplicationsRFID Applications

LogisticsManufacturingAccess controlAnti theftWireless pay systemsDocuments/cards

Automobile industry

Aviation

Pharmaceuticals

Application: Supply Chain LogisticsApplication: Supply Chain Logistics

RFID portalsDock-systemsDoorsBays

RFID forklifts and loaders

Large retailers (Wal-Mart, Target, Metro) operate on profit of less than 5%Saving only 1% significantly increases their competitiveness

Application: Border ControlApplication: Border Control

US-Canada borderSpecial lane for owners of NEXUS pass (plastic card with photograph and RFID tag)Equipment and tags are supplied by Intermec

Program NEXUSProgram NEXUS

Application: AirportsApplication: AirportsPassenger registration, passport control, passenger trackingBaggage and cargo tagging

Application: AirplaneApplication: Airplane

RFID for part trackingReduces time for:

• Part identifications• Locating spare parts• Service document writing

Prevents usage of counterfeit and used partsAllows to control part assembly procedure

Boeing 787 Dreamliner will have RFID tags on ~2000 critical (expensive or requiring frequent or regular servicing) parts

Application: ID CardsApplication: ID Cards

Badges and ID cardsGen2

• 869 or 915 MHz• Read range up to 10 m

Latest DevelopmentsLatest DevelopmentsUHF Near Field RFID

Tags can work close to metal/liquidsCompanies: Smartcode, RsiID

Multi-protocol tagsSingle tag (multi-protocol IC with special antenna) can work in several bands (LF, HF, UHF)Companies: TwinLinx, Texas Instruments

RFID Reader ChipsetsSeveral chips do most of RFID reader functionsCompanies: Intel, WJ, Anadigm

Organic/polymer electronicsLow cost, printable, biodegradable tagsCompanies: OrganicID, PolyIC, ORFID

8. Conclusions8. Conclusions

RFID tag antenna design is both art and science

Best results are obtained when tag antenna gain is maximized and tag antenna is well matched to ASIC

EM software tools are necessary for RFID tag design modeling and optimization

Accurate wideband tag range measurement capability is crucial for quality tag design implementation and performance verification

Thank you!

kvs.rao@intermec.committra@engr.psu.edu

Questions?