December 2010 87 1527-3342/10/$26.00©2010 IEEE

Digital Object Identifier 10.1109/MMM.2010.938571

Stevan Preradovic and Nemai Chandra Karmakar

Stevan Preradovic (stevan.preradovic@ieee.org), and Nemai Chandra Karmakar (nemai.karmakar@eng.monash.edu.au) are with the Department of Electrical and Computer Systems Engineering, Bldg 72 Clayton Campus, Monash University, 3800 VIC Australia.

Chipless RFID: Bar Code of the Future

Radio-frequency identifi cation (RFID) is a wireless

data capturing technique that utilizes radio fre-

quency (RF) waves for automatic identifi cation

of objects. RFID relies on RF waves for data

transmission between the data carrying de-

vice, called the RFID tag, and the interrogator [1], [2].

A typical RFID system is shown in Figure 1. An

RFID system consists of three major components:

a reader or interrogator, which sends the interro-

gation signals to an RFID tag that is to be identi-

fied; an RFID tag or transponder, which contains

the identification code; and middleware software,

which maintains the interface and the software

protocol to encode and decode the identification

data from the reader into a mainframe or personal

computer. The RFID reader can read tags only

within the reader’s interrogation zone. The reader

is most commonly connected to a host computer,

which performs additional signal processing and

has a display of the tag’s identity [3]. The host com-

puter can also be connected via the Internet for

global connectivity/networking.

The vast majority of RFID transponders (or tags)

are usually comprised of an antenna and integrated

circuit (IC) [4]. The IC performs all of the data process-

ing and is powered by extracting power from the inter-

rogation signal transmitted by the RFID reader. These

transponders are called passive due to the fact that they

do not have any on-board power supply. RFID transpon-

ders, which use on-board power supply (such as batteries) are

called active RFID tags. Passive RFID tags offer lower prices at

© DIGITAL VISION

88 December 2010

the cost of shorter reading ranges (up to 3 m) when

compared to the more expensive long-range active

RFID tags (read up to 100 m). Various other transpon-

ders are found on today’s market and are comprehen-

sively presented in [5].

The cost of the entire RFID system is dependent on

the cost of the tag, which is dependant on the cost of its

IC [6]. Therefore, efforts have been put in developing

chipless RFID tags with no ICs, which mean that the

main cost of the tag is being removed. So far, the only

commercially available chipless RFID tag is the surface

acoustic wave (SAW) tag (developed by RF SAW) [7].

This article presents a comprehensive review of chi-

pless RFID tags available on the market and reported

in peer-reviewed journals and conferences. However,

in the quest to be as comprehensive as possible the

authors have also referenced online internet articles

that report novel chipless RFID technologies.

Limitations of Bar Codes and Emergence of Chipless RFID ConceptsBar code labels have been used to track items and

stocks for sometime after their inception in the early

1970s. Though bar codes are printed in marks and

spaces and are very cheap to implement, they pres-

ent undeniable obstacles in terms of their short-range

readability and nonautomated tracking. These limi-

tations currently cost large corporations millions of

dollars per annum [8].

The growing tendency today is to replace bar

codes with RFID tags, which have unique ID codes

for individual items that can be read at a longer dis-

tance. The obstacles of reading range and automa-

tion would be solved using RFID. The only reason

why RFID tags have not replaced the bar code is the

price of the tag. The cost of an existing RFID tag is

still much higher when compared to the price of

the bar code.

The main cost of an RFID tag comes from the chip

embedded as the information-carrying and processing

device in the tag. Significant investments and research

have been focused on lowering the price of the RFID

chip. As a result, the price of the RFID tag has become

lower [9]. However, the price of the RFID tag is still

not competitive when compared to the cost of the bar

code. The recent development of chipless tags without

silicon ICs has lowered the cost of the tags to a level

comparable to that of the bar code. Even though the

technology is still in its infancy, a number of develop-

ments have already been made in the industry, which

we overview here.

Difficulties of Achieving Low-Cost RFIDThe use of RFID instead of optical bar codes has not

yet been achieved due to the greater price of the RFID

tag (US$0.10) compared to the price of the optical bar

code (US$0.5) [10]. The reasons why it is difficult to

produce cheap RFID tags are comprehensively pre-

sented in [11]. Fletcher advocates that application spe-

cific IC (ASIC) design and testing along with the tag

antenna and ASIC assembly result in a costly man-

ufacturing process. This is why it is not possible to

further lower the price of the chipped RFID tag. The

basic steps for manufacturing a chipped RFID tag are

shown in Figure 2.

The design of silicon chips has been standard-

ized for more than 30 years, and the cost of building

a silicon fabrication plant is in the billions of U.S.

dollars [12], [13]. Since silicon chips are fabricated

on a wafer-by-wafer basis, there is a fixed cost per

wafer (around US$1,000). As the cost of the wafer is

independent of the IC design, the cost of the RFID

chip can be estimated based on the required silicon

area for the RFID chip. Significant achievements

have been made in reducing the size of the transis-

tors, allowing more transistors per wafer area [14].

Decreasing the amount of transistors needed results

in an even smaller silicon area, hence a lower RFID

chip price. As a result, great efforts have been made

by the Massachusetts Institute of Technology (MIT)

Global

Network

Host

Computer

RFID

ReaderRFID Tag

Clock

Data

Figure 1. Block diagram of a typical RFID system.

ASIC Design

ASIC

Manufacturing

ASIC Testing

Antenna

Manufacture

Tag Assembly

Conversion to

Label/Package

Figure 2. RFID label/tag manufacturing process.

December 2010 89

to design an RFID ASIC with less than 8,000 transis-

tors. Although this will reduce the price of the sili-

con chip, its miniature size imposes limitations and

further handling costs.

The cost of dividing the wafer, handling the die,

and placing them onto a label remains significant,

even if the cost of the RFID chip were next-to-nothing.

The cost of handling the die increases with the use of

smaller-than-standard chips, simply because the elec-

tronics industry is not standardized for them.

Hence, with highly optimized low transistor

count ASICs, implemented assembly processes and

extremely large quantities (over 1 billion) of RFID

chips sold per annum, a minimum cost of US$0.05 is

the reality for chipped RFID tags.

Given the inevitable high cost of silicon chip RFID

tags (when compared to optical bar codes), efforts to

design low-cost RFID tags without the use of tradi-

tional silicon ASICs have emerged. These tags, and

therefore systems, are known as chipless RFID sys-

tems. The expected cost of chipless RFID tags is below

US$0.01. Most chipless RFID systems use the electro-

magnetic (EM) properties of materials and/or design

various conductor layouts/shapes to achieve particu-

lar EM properties/behavior.

Review of Chipless RFID TagsThere have been some reported chipless RFID tag

developments in recent years. However, most are still

reported as prototypes, and only a handful are consid-

ered to be commercially viable or available. The chal-

lenge for researchers when designing chipless RFID

tags is how to perform data encoding without the

presence of a chip. In response to this problem, three

general types of RFID tags can be identified as shown

in Figure 3.

Based on the open literature, it is possible to catego-

rize chipless RFID tags in three main categories:

• time domain reflectometry (TDR)-based chipless

tags

• spectral signature-based chipless tags

• amplitude/Phase backscatter modulation-based

chipless tags.

Time-Domain Refl ectometry-Based Chipless TagsTDR-based chipless RFID tags are interrogated by

sending a signal from the reader in the form of a pulse

and listening to the echoes of the pulse sent by the tag.

A train of pulses is thereby created, which can be used

to encode data.

The advantages of these tags when compared to

chipped tags are low cost, greater reading ranges, and

their applicability in localization/positioning applica-

tions. The disadvantages of these tags are the num-

ber of bits that can be encoded and high-speed RFID

reader RF front-ends required for generating and

detecting short ultrawideband (UWB) pulses.

Various RFID tags have been reported using

TDR-based technology for data encoding. We can

distinguish between nonprintable and printable TDR-

based tags.

An example of a nonprintable TDR-based chipless

RFID tag is the SAW tag, for example, developed by

RFSAW Inc. [15]. SAW tags are excited by a chirped

Gaussian pulse sent by the reader centered around

2.45 GHz [16]–[20]. A SAW tag is shown in Figure 4.

The interrogation pulse is converted to a SAW using

Chipless RFID Tags

TDR based Spectral

Signature Based

Nonprintable Printable Chemical Planar Circuits

TFTC

Delay-Line-

Based Tags

Nanometric

MaterialsCapacitively

Tuned Dipoles

Space Filling

Curves

LC Resonant

Ink-Tattoo

Chipless RFID

Amplitude/Phase Backscatter

Modulation Based

Left-Hand (LH)Delay Lines

Multiresonator

Based

Stub-LoadedPatch Antenna

Remote ComplexImpedance

CarbonNanotube Loading

Multiresonant

Dipoles

TDR Based

SAW Tags

Figure 3. Classification of chipless RFID tags. TDR: Time-domain reflectometry; SAW: surface acoustic wave; TFTC: thin-film-transistor circuit.

90 December 2010

an interdigital transducer (IDT). The SAW propagates

across the piezoelectric crystal and is reflected by a

number of reflectors, which create a train of pulses with

phase shifts [21]–[28]. The train of pulses is converted

back to an EM wave using the IDT and detected at the

reader end where the tag’s ID is decoded [29]–[38].

Printable TDR-based chipless tags can be found

either as thin-film-transistor circuit (TFTC) or

microstrip-based tags with discontinuities. TFTC

tags are printed at high speed on low-cost plastic

film [39]. TFTC tags offer advantages over active and

passive chip-based tags due to their small size and

low power consumption. They require more power

than other chipless tags but offer more functional-

ity. However, low-cost manufacturing processes for

TFTC tags have not yet been developed. Organic

TFTC could provide a cost-effective solution [40].

One of the institutes working on organic TFTC

development is the National Institute of Advanced

Industrial Science and Technology (AIST) in Japan.

An organic TFTC printed on flexible plastic film is

shown in Figure 5. Another issue is the low electron

mobility, which limits the frequency of operation up

to several megahertz.

Delay-line-based chipless tags operate by using

a microstrip discontinuity after a section of delay-

line, as reported in [41]–[43]. A delay-line-based

chipless tag is shown in Figure 6. The tag is excited

by a short pulse (usually 1 ns) EM signal. The inter-

rogation pulse is received by the tag and reflected

at various points along the microstrip line creating

multiple echoes of the interrogation pulse, as shown

in Figure 7. The time delay between the echoes is

determined by the length of the delay-line between

the discontinuities. This type of tag is a replica of the

SAW tag using microstrip technology, which makes

it printable. Although initial trials on this chipless

technology have been reported, only 4 bits of data

have been successfully encoded, which shows the

limited potential of this technology.

Spectral-Signature-Based Chipless TagsSpectral signature-based chipless tags encode data

into the spectrum using resonant structures. Each data

bit is usually associated with the presence or absence

of a resonant peak at a predetermined frequency in the

Antenna

Interdigital

Transducer

Reflectors

Figure 4. Circuit architecture of a surface acoustic wave tag [5].

Figure 5. Organic-thin-film-transistor circuit printed on flexible plastic film. [Courtesy of National Institute of Advanced Industrial Science and Technology (www.aist.go.jp) (www.aist.go.jp/aist_e/latest_research/2008/20080728/20080728.html), reprinted with permission.]

Antenna TransmissionDelay Line

Place forSensorIntegration

Delay Line

Figure 6. Delay-line-based chipless tag with patch antenna and delay line [43].

Amplitude

Input Signal

Reflected Signal

“Generated ID: 011”

Reflected Signal

“Generated ID: 110”

Pulse Position

Modulation Code

Representation00

000

101

001

1

011

100

101

110

110

111Time

Figure 7. Interrogation and coding of delay-line-based chipless tag [43].

December 2010 91

spectrum. The advantages of these tags are that they

are fully printable, robust, have greater data storage

capabilities than other chipless tags, and are low cost.

The disadvantages of these tags are large spectrum

requirements for data encoding, chipless tag orienta-

tion requirements, size, and wideband dedicated RFID

reader RF components. So far, seven types of spectral

signature-based tags have been reported, and all seven

are considered to be fully printable. We can distin-

guish two types of spectral signature tags based on the

nature of the tag: chemical and planar circuit.

Chemical tags are designed from a deposition

of resonating fibers or special electronic ink. Two

companies from Israel use nanometric materials to

design chipless tags. These tags consist of tiny par-

ticles of chemicals, which exhibit varying degrees of

magnetism, and, when EM waves impinge on them,

they resonate with distinct frequencies, which are

picked up by the reader [44]. They are very cheap and

can easily be used inside banknotes and important

documents for anticounterfeiting and authentication.

CrossID, an Israeli paper company, claims to have 70

distinct chemicals, which would provide unique iden-

tification in the order of 270 (more than 1,021) when

resonated and detected suitably [45]. Tapemark also

claims to have nanometric resonant fibers, which are

5 µm in diameter and 1 mm in length [46]. These tags

are potentially low cost and can work on low-grade

paper and plastic packaging material. Unfortunately,

they only operate at frequencies up to a few kilohertz,

although this gives them very good tolerances to metal

and water.

Ink-tattoo chipless tags use electronic ink patterns

embedded into or printed onto the surface of the object

being tagged. Developed by Somark Innovations [47],

the electronic ink is deposited in a unique bar code

pattern, which is different for every item. The system

operates by interrogating the ink-tattoo tag by a high

frequency microwave signal (>10 GHz) and is reflected

by areas of the tattoo, which have ink creating a

unique pattern which can be detected by the reader.

The reader detection is based on spatial diversity cre-

ated by the presence or absence of ink particles on the

tagged surface. The reading range is claimed to be up

to 1.2 m (4 ft) [48], [49]. In the case of animal ID, the

ink is placed in a one-time-use disposable cartridge.

For nonanimal applications, the ink can be printed on

plastic/paper or within the material. Based on the lim-

ited information available for this technology (which

is still in the experimental phase) we assume that it is

spectral signature based.

Planar circuit chipless RFID tags are designed using

standard planar microstrip/coplanar waveguide/

stripline resonant structures, such as antennas, filters,

and fractals. They are printed on thick, thin, and flex-

ible laminates and polymer substrates. Capacitively

tuned dipoles were first reported by Jalaly [50]. The

chipless tag consists of a number of dipole antennas,

which resonate at different frequencies. The capaci-

tively tuned dipole tag is shown in Figure 8. When the

tag is interrogated by a frequency sweep signal, the

reader looks for magnitude dips in the spectrum as a

result of the dipoles. Each dipole has a 1:1 correspon-

dence to a data bit. Issues regarding this technology

include tag size (lower frequency longer dipole—half

wavelength) and mutual coupling effects between

dipole elements.

Space-filling curves used as spectral signature

encoding RFID tags were first reported by McVay [51].

The tags are designed as Piano and Hilbert curves

with resonances centered around 900 MHz. The tags

represent a frequency selective surface, which is

manipulated with the use of space-filling curves (such

as the Hilbert and Piano curves). The space-filling

curve exhibits an interesting property of resonating

at a frequency, which has a wavelength much greater

than its footprint. This is an advantage since it allows

the development of small footprint tags at UHF ranges.

Figure 9 shows the 5-bit space-filling curve chipless

tag, which comprises an array of five second-order

… First Bit 11th Bit

Laminate (Dielectric)

Dipole (Conductor)

Figure 8. Capacitively tuned dipoles arranged as a 11-bit chipless RFID tag.

–15

–20

–25

–30

–35

–40

–45

–50

RC

S (

dB

)

0.5 0.6 0.7 0.8 0.9Frequency (GHz)

1

y

x

Ey

Figure 9. Five-bit piano-curve-based tag and tag radar-cross-section spectral signature [51].

92 December 2010

Piano curves, which create five peaks in the radar

cross-section (RCS) of the tag. The chipless tag was suc-

cessfully interrogated in an anechoic chamber. Only 5

bits of data have been reported to date. The advantage

of the tag is its compact size due to the properties of the

space-filling curves. However the disadvantage of the

tag is that it requires significant layout modifications

in order to encode data.

LC Resonant chipless tags comprise a simple coil,

which is resonant at a particular frequency. These tags

are considered 1-bit RFID tags. The operating prin-

ciple is based on the magnetic coupling between the

reader antenna and the LC resonant tag. The reader

constantly performs a frequency sweep searching

for tags. Whenever the swept frequency corresponds

to the tag’s resonant frequency, the tag will start to

oscillate, producing a voltage dip across the reader’s

antenna ports. The advantage of these tags is their

price and simple structure (single resonant coil), but

they are very restricted in operating range, informa-

tion storage (1 bit), operating bandwidth, and multiple-

tag collision. These tags are mainly used for electronic

article surveillance (EAS) in many supermarkets and

retail stores [52].

The Multiresonator-based chipless RFID tag was

designed and patented by the authors at Monash Uni-

versity [53]. The chipless tag comprises three main

components: the transmitting (Tx) and receiving (Rx)

antennas and multiresonating circuit. A block dia-

gram of the integrated chipless RFID tag with basic

components is shown in Figure 10.

The multiresonator-based chipless RFID tag con-

sists of a vertically polarized UWB disc-loaded mono-

pole Rx tag antenna, a multiresonating circuit, and a

horizontally polarized UWB disc-loaded monopole

Tx tag antenna [54]–[57]. The tag is interrogated by

the reader by sending a frequency swept continuous

wave signal. When the interrogation signal reaches

the tag, it is received using the Rx monopole antenna

and propagates towards the multiresonating circuit.

The multiresonating circuit encodes data bits using

cascaded spiral resonators, which introduce attenu-

ations and phase jumps at particular frequencies of

the spectrum. After passing through the multireso-

nating circuit, the signal contains the unique spectral

signature of the tag and is transmitted back to the

transmitter using the Tx monopole tag antenna. The

Rx and Tx tag antennas are cross-polarized in order

to minimize interference between the interrogation

signal and the retransmitted encoded signal contain-

ing the spectral signature. Figure 11 shows a 35-bit

tag designed on Taconic TLX-0 (er 5 2.45, h 5 0.787

mm, tan d 5 0.0019).

The main differences between the multiresonator-

based tag and those reported in the previous sections

are that the tag encodes data in both amplitude and

phase (Figures 12 and 13), the tag operates in the UWB

region, the tag supports simple spiral shorting data

encoding [58] and the tag responses are not based

on RCS backscattering but on retransmission of the

cross-polarized interrogation signal with the encoded

unique spectral ID. The chipless tag is designed for

printing on the Australian polymer banknote as an

anticounterfeiting security feature.

The Multiresonant dipole-based chipless RFID tag

is based on a similar concept as the multiresonator-

based chipless tag. However, the tag’s designers seek

to build on the concept of the multiresonator tag by

replacing the stop-band spiral resonators and the sec-

ond tag antenna with a novel multiresonant dipole

antenna [59]. The multiresonant dipole antenna com-

prises a set of parallel loop antennas, which resonate at

different frequencies. Each loop antenna corresponds

to a single bit of data. The multiresonant dipole-based

chipless RFID tag is shown in Figure 14. From Figure

14, it is clear that the tag receives the reader’s wideband

interrogation signal by the Rx UWB monopole antenna

and retransmits only certain frequencies, hence encod-

ing a unique spectral signature in the response signal

sent by the Tx multiresonant dipole antenna.

The multiresonant dipole antenna comprises a

series of folded half-wave dipole antennas. The dipole

arms etched out in the bot-

tom (ground) layer are fed by

a prolongation of the ground

plane with the prolongation

impedance being 50 V. The

half wavelength dipole anten-

nas produce peaks in the

return loss at their resonant

frequencies. By removing

any of the half wavelength

dipoles, the corresponding

resonant peak disappears

without influencing the reso-

nances of the other dipoles.

The main benefit of using the

multiresonant dipole antenna

First

Resonator

Second

Resonator

Third

Resonator

N th

Resonator

UWBMonopole

RxAntenna

UWBMonopole

TxAntenna

Vertical Polarization

Horizontal Polarization

Multiresonator

Figure 10. Chipless RFID tag circuit block diagram.

December 2010 93

is that the size of the entire tag can be reduced and

spatial efficiency is enhanced.

Amplitude-Phase-Backscatter-Modulation-Based Chipless TagsAmplitude/Phase backscatter modulation-based chi-

pless RFID tags are the third type of chipless RFID

tags presented in this article. These tags require less

bandwidth for operation than TDR-based and spectral

signature-based chipless tags. Data encoding is per-

formed by varying the amplitude or phase of the back-

scattered signal based on the loading of the chipless

tag’s antenna. The variation of the loading is not con-

trolled by an on/off switch between two impedances,

but, instead, it is controlled by reactive loading of the

tag’s antenna. The antenna loading influences the RCS

of the antenna [60] in amplitude or phase, which can

be detected by a dedicated RFID reader. The reactance

of the load may vary due to the fact that the antenna

load is an analog sensor or left-handed (LH) delay

line, or that the antenna is terminated by a microstrip-

based stub reflector.

The advantages of this type of chipless tag are

that it operates over narrow bandwidths, and it has a

simple architecture. The disadvantages are the num-

ber of bits that can be detected, and that data encod-

ing is performed by a lumped/chipped component

which increases its cost. Based on the data encoding

antenna loading element we can distinguish between

four types of different backscatter modulation-based

chipless RFID tags.

LH delay line loading of the chipless tags is one of

the most recent developments of chipless tag technol-

ogy. It utilizes analog circuits for phase modulation

and increases the response time of the tag using the

slow-wave effect of LH delay lines [61], which also

minimizes the size of the tag. The operating principle

of the chipless tag is presented in Figure 15.

From Figure 15, it is clear that the chipless tag is

interrogated by a band-limited pulse transmitted from

the RFID reader. The interrogation pulse is received

by the chipless tag antenna and propagates through

a series of cascaded LH delay lines, which represent

periodical discontinuities. The received interrogation

pulse is reflected upon reaching each discontinu-

ity and the information is coded by the phase of the

reflected signal with respect to a reference phase. The

envelope of the reflected signals with encoded data

maintain similar magnitudes (envelopes) while the

phase variation differs due to different G1, G2, and

G3 with phase values w0, w1 and w2, respectively. The

LH delay line-based chipless tag encodes data using

–16

–14

–12

–10

–8

–6

–4

–2

0

3 4 5 6 7Frequency (GHz)

Magnitude o

f S

pectr

al

Sig

natu

re (

dB

)

Figure 12. 35-bit magnitude response of the multiresonator-based chipless RFID tag.

–60

–40

–20

0

20

40

60

80

3 4 5 6 7Frequency (GHz)

Phase o

f S

pectr

al

Sig

natu

re (

°)

Figure 13. 35-bit phase response of the multiresonator-based chipless RFID tag.

Rx UWB

Monopole

Tx Multiresonant

Dipole Antenna

R

Feed

Extension

Spacing

Figure 14. Multiresonant-dipole-based chipless RFID tag [59] (red—top layer, yellow—bottom layer) (© 2009 EuMA, reprinted with permission).

Tag Tx

Antenna

Tag Rx

Antenna

Multiresonator

with 35 Spirals

Figure 11. Photograph of 35-bit chipless RFID tag (length 5 88 mm, width 5 65 mm).

94 December 2010

a higher order modulation scheme, such as quadra-

ture phase shift keying (QPSK), which enables greater

throughput but requires a higher signal-to-noise ratio

for successful tag detection [62]. The QPSK modula-

tor used within the chipless tag is based on a vari-

able reactive element, which minimizes the variation

of the amplitude and maximizes the phase variation.

Remote complex impedance-based chipless RFID

tags comprise a printable antenna, which is loaded/

terminated with a lossless reactance. The tag

antenna is chosen to be a scattering antenna (such

as a patch antenna) instead of a typically used mini-

mum scattering antenna (such as a dipole) [63]. The

difference between scattering and minimum scat-

tering antennas is that, when terminated with an

open or short, the scattering antenna should scatter

back the same power, irrespective of the type of loss-

less termination (including open and short), while

the minimum scattering antenna will scatter almost

no power back in open circuit conditions [64], [65].

This property of scattering antennas is reported by

Mukherjee et al. in [66] to encode data by means

of loading a scattering antenna with microstrip

stubs, which represent different inductances, and

therefore manipulating the phase component of the

antennas RCS and backscattered signal. The chi-

pless RFID system based on remote measurement of

complex impedance can be modeled as a two-port

network where the reader is considered to be the

source while the reactive impedance is considered

to be the load. Figure 16 shows the model of the chi-

pless RFID system. The transmitted interrogation

signal is defined by the S21 parameter while the S12

parameter is the backscattered chipless tag response

signal with phase signature.

By having chipless RFID tags with different induc-

tive loadings of their antennas, it is possible to cre-

ate different phase signatures in the backscattered

signal, which can be used to identify each tag at the

reader end [67]. The reactive loadings are designed

to be microstrip stubs in order to make the tag fully

printable and low-cost. Figure 17 shows the phase sig-

natures of different chipless RFID tags with different

inductive loadings.

Stub-loaded-patch-antenna (SLPA)-based chi-

pless RFID tags reported by Balbin et al. in [68] are

a newer generation backscatter phase signature tags

similar to the remote complex impedance based tag

presented earlier. However, the SLPA-based tags are

more robust and industry-suited since they incor-

porate another degree of diversity, such as cross-

polarization diversity (besides the phase variation

of the backscattered signal due to reactance loading)

and multiple tag antennas. The operating principle

of the SLPA chipless RFID tag is based on basic prin-

ciples of vector backscattered signals from multiple

planar reflectors. The SLPA-based tag is shown in

Figure 18.

The chipless tag antennas are multiple patch

antennas, which are suited due to their scattering

antenna properties as described earlier. The planar

reflectors are in the form of meander stubs in order to

minimize area and cost. The numbers of bits that can

be encoded by the tag vary depending on the number

of patches (n) and the available meander line induc-

tances. The chipless tag is interrogated by transmit-

ting n different continuous wave (CW) signals from

the reader at n frequencies corresponding to the oper-

ating frequencies of each patch antenna. When the tag

Reflection

SectionΓ1 Γ2 Γ3

ϕ1

e jϕ1

ϑ1

ϕ2

ϑ2

ϕ3

ϑ3

T ⋅ e jϕ0 T ⋅ e jϕ

0 Delay Line

Section

Carrier Phase

Carrier

Envelopet

ϕ1 + 2ϕ0

ϕ2 + 2ϑ1 + 4ϕ0

ϕ3 + 2(ϑ1 + ϑ2)+ 6ϕ0

Figure 15. Operating principle of left-hand-delay-line-based chipless RFID tag [61].

RFID Reader

Free Space

Loss

Scattering

Antenna

Inductive/

Reactance

Load

Zfreespace

Zfreespace

S21

S12

S11

S22 Γ

Z0

Figure 16. 2-port model of chipless RFID system based on remote measurement of complex impedance.

60

40

20

0

Pha

se R

ippl

e (°

)

–20–40

–606.9 7.1 7.3 7.5 7.7 7.9

Frequency (GHz)

Figure 17. Variation of the chipless tag’s phase signature with inductance loading [67] (© 2007 EuMA, reprinted with permission).

December 2010 95

is read by directive reader antennas, a bit sequence

can be detected using the relative phase difference of

the backscattered signals. The relative phase refers to

the phase difference between the E-plane and H-plane

signals at the reader, adding another degree of dif-

ferentiation. It is important to notice that this type of

chipless RFID tag requires interrogation and reading

with a directional dual polarized reader antenna and

not circularly polarized due to the tag’s operating

principles. The SLPA-based chipless tag is suitable for

conveyor belt applications due to the cascaded place-

ment of its antennas.

Carbon-nanotube-loaded (CNL) chipless tags

are a novel and unique example of RFID technol-

ogy and nanotechnology combining to create a

novel RFID tag and sensor module. The CNL chi-

pless RFID tag comprises a conformal UHF RFID

antenna and a single-walled carbon nanotube

(SWCNT) designed for toxic gas detection [69].

The CNL chipless RFID tag is shown in Figure 19.

It is important to note that both the antenna and

SWCNT were printed using inkjet printing technol-

ogy for the first time. The chipless tag antenna is a

bowtie meander-line dipole antenna. The SWCNT

is placed between at the input port of the antenna

in order to enable data encoding.

The SWCNT is highly sensitive to the presence

of ammonia (NH3), and its impedance characteris-

tics when placed in air and NH3 are shown in Fig-

ure 20. From Figure 20, it is clear that the impedance

of the SWCNT varies depending on the presence or

absence of NH3 in the environment. The CNL chi-

pless RFID tag operates by varying the amplitude

of the backscattered signal, depending on the con-

centration of NH3, as shown in Figure 21. Ampli-

tude variation of the backscattered signal is due

to the RCS variation influenced by the change of

the impedance of SWCNT. The amplitude varia-

tion of the backscattered power from the tag can be

detected at the reader end and decoded to estimate

the level of NH3.

ConclusionAn overview of reported chipless RFID tags in open

literature and on the market has been presented. As

the requirement for cheaper RFID tags for various

Meandering O/C Stubs

L1 L2 L3

Inset

LengthInset

Width

Element 1Spacing Element 2 Element 3

E-Plane

H-Plane

Figure 18. Stub-loaded-patch-antenna-based chipless RFID tag comprising three patch antennas loaded with meander line stubs [68].

15 m

m

27 m

m

25 mm36 mm

SWCNT

SWCNT

(a)

(b)

118 mm

Figure 19. Carbon-nanotube-loaded chipless RFID tag on flexible laminate with (a) dimensions and (b) actual photograph [69].

0

–5

–10

–15

–200.6 0.7 0.8 0.9 1

Frequency (GHz)

Pow

er

Reflection C

oeffic

ient (d

B)

AirNH3 Flow

Figure 21. Power reflection coefficient of the carbon-nanotube-loaded chipless RFID tag before and after gas flow [69].

125

100

75

50

25

0

75

50

25

0

–25

–50

Resis

tance (

Ω)

Resis

tance (

Ω)

0 0.2 0.4 0.6Frequency (GHz)

0.8 1

Resistance in NH3 Resistance in NH3

Resistance in Air Resistance in Air

Figure 20. Measured impedance characteristics of single-walled carbon nanotube in air and ammonia [69].

96 December 2010

applications grows, there are a greater number of dif-

ferent chipless RFID tags that can be classified in a

wide range of different types. This article reports the

first classification of chipless RFID tags, which classi-

fies 14 different chipless tags in three main categories.

The main classification of chipless tags is based on

modulation techniques, which are TDR-based, spec-

tral signature-based and amplitude/phase backscatter

modulation-based chipless RFID tags. All three types

of tags can be either printable or nonprintable, which

determines their eligibility for certain applications,

robustness and cost.

Although the majority of chipless tags are still in

prototyping stage it remains to be seen whether they

will make it into the mainstream market. However, the

progress of chipless RFID technology in recent years

enthusiastically suggests that the best of chipless RFID

is yet to come.

AcknowledgmentThis work was supported in part by the Australian

Research Council under Discovery Grant DP0665523:

Chipless RFID for Bar code Replacement.

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