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PROCESS-DEPENDENCE OF INKJET PRINTED

FOLDED DIPOLE ANTENNA FOR 2.45 GHZ

RFID TAGS

Botao Shao #1, Qiang Chen #2, Yasar Amin #3, Julius Hllstedt #4, Ran Liu 5, Hannu Tenhunen #6, Li-Rong Zheng #7

#iPack Vinn Excellence Center, School of Information and Communication Technology,

KTH (Royal Institute of Technology), Forum 120, 164 40 Stockholm-Kista, Sweden.1,7botao,lirong@kth.se

Department of Microelectronics, Fudan University

200433, Shanghai, China

Abstract This paper focuses on the process dependence ofan inkjet printed folded dipole antenna based on practicalparameters in a typical inkjet printing process. We present theeffect of width variations and number of overprinting times onthe antenna properties such as gain, radiation efficiency andinput impedance. Furthermore we investigate the read rangedegradation of the tag on which the antenna is attached, dueto width or thickness variations. In addition, an comparisonbetween an inkjet printed antenna on a regular paper substrateand a copper antenna on Printed Circuit Board (PCB) wasmade, manifesting the strong competitiveness of the printed silverantenna as a low cost solution.

I. INTRODUCTION

With the vast demands for inexpensive, flexible, high-quality

RFID tags, printable antenna has attracted a lot of attention in

RFID community. Compared with a common PCB process

employing subtractive process and mask plates, the inkjet

printed process has advantages such as [1][2]:

Implementing additive process where the valuable mate-

rial are deposited only on a desired location

No requirement for the mask manufacturing and the

easiness of switching to the patterns to be realized

Inkjet printing, moreover, has a potential to be combined

with reel-to-reel process for greatly reduced cost of mass-

production.

Some papers have been published dealing with printed

antennas for RFID tags, e.g. [3][4]. These studies focus on

the printed antenna performance itself instead of the process-

dependence thereof. However, it is essential to understand the

effect of width and thickness variations on antenna properties

because there exist huge discrepancies between an inkjet

printing process and other processes such as PCB and silicon-

based process. For instance, the thickness of an inkjet printed

silver film roughly varies within the range of 0.3 m to 4 m[5][6], compared to 18 or 35 m for a copper antenna in aconventional PCB process.

In practice, the orientation of tags on which antennas are

attached is uncontrollable, thus a trade-off must be made be-

tween the use of circularly polarized read antennas, sacrificing

read range, and the use of polarization-diverse tag antennas,

adding cost and size to the antenna structure. However, for

the tag antennas aiming to ultra low cost, an omni-directional

radiation pattern is preferred, promoting the choice of the

dipole antenna and its derivatives, such as the folded dipole

antenna, the meander-line antenna etc.

Folded dipole antennas are commonly employed on RFID

tags when greater bandwidth and higher impedance matching

are required. Additionally, 2.4 GHz tags are in general smaller

than 900 MHz tags making them more convenient to use atlower cost. As a result, the combination of these two concerns

allows the incentive to study the process-dependence of an

folded dipole antenna operating at 2.45 GHz .

II . ANTENNA CHARACTERISTICS

Fig. 1 shows the schematic of the analyzed folded dipole

antenna operating at 2.45 GHz with geometries described inTab. I. The selection of these parameters in this table was

based on practical experiments either from our own experience

or from others published contributions. The conductivity of

nano-silver film printed on paper substrate was supposed to

be 21

10

6

S/m [1][7][8]. The thickness of conducting filmon a paper was assumed 0.9 m from our trial printing, andpaper properties were obtained from [9]. It is assumed that,

in addition, the counterpart copper antenna was realized by

using a standard FR-4 Copper Clad Laminate with 18mcopper thickness. On the basis of these assumptions, the

structure of these two classes antennas, one is the nano-silver

based antenna and the other is a typical copper antenna on

PCB as a reference, were constructed and analyzed in Ansoft

HFSS 3-D simulation tool. For the convenience of consequent

comparisons, all the characteristics responses versus process

variations have been normalized to a nano-silver based stan-

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TABLE I

GEOMETRIES OF THE SIMULATED FOLDED DIPOLE ANTENNA AND

SUBSTRATE

Parameters Value Specification

l 54.96 mm Length of the antennaw 1 mm Width of the antennas 1 mm Space between two parallel armst 0.9 m Thickness of the metal film

21 106 S/m Conductivity of the metalH 0.2 mm Height of the paper substrate 3.2 Dielectric Constant of the substrate

tan 0.077 Loss Tangent of the substrate

dard antenna without variations on width and thickness, that

is, with a nominal width and a single layer thickness.

l

s

X

Y

w

Fig. 1. Folded dipole antenna operating at 2.45 GHz

The considerations of some antenna characteristics are

frequently made such as gain, radiation efficiency, port im-

pedance, and read range. In this paper these four feature

properties were detailedly discussed.

Absolute gain of an antenna is defined as the ratio of the

intensity, in a given direction, to the radiation intensity that

would be obtained if the power accepted by the antenna were

radiated isotropically, and connected with the accepted power,

Pacc and the radiation intensity, U(, ):

Gain = 4U(, )

Pacc(1)

Peak gain, in turn, is the maximum gain over all the user-

specified directions of the far-field infinite sphere. It is no-

ticeable that according to the IEEE Standard Definitions of

Terms for Antenna [10], gain does not include losses arising

from impedance mismatch and polarization mismatches.

The radiation efficiency e is the ratio of the radiated powerto the accepted power (excluding the reflected power), given

by the analytical formula:

e =

Prad

Pacc(2)

where Prad is the radiated power, Pacc is the accepted power.It is noteworthy that radiation efficiency, which is exclusively

concerned with the performance for the interior of a antenna,

is different from antenna efficiency, which is defined as the

ratio of the radiated power to the incident power (including

the reflected power) and consequently used to take into account

losses both at the input terminals and within the structure of

the antenna.

Read Range, as one of the most important characteristics,

defines the maximum distance at which RFID reader can detect

the backscattered signal from the tag. To obtain the read range

expression, we can start from well-known Friis formula Eq.3:

Pr =(Pt Gt) Gr

2

(4R)2(3)

where R is the distance between an RFID tag and reader,Pt Gt is the Effective Radiated Power (ERP) transmitted by

the reader. Gt is the gain of tag antenna. With simple algebraicmanipulation, the expression of the read range normalized to

the standard antenna is shown in Eq. 4:

R

R=

G

G(4)

where R is the read range of the antenna with process variant,R is the read range of the standard antenna claimed above, G

is the gain with process varied, and G the gain of the standardantenna.

III. PROCESS DEPENDENCE ANALYSIS

There exist two main considerations involved in the inkjet

printing process of an antenna, width and thickness variations

of a conducting film. The cross-sectional area of the film is

changed in response to these variations, resulting in the devi-

ations of nominal features such as gain, radiation efficiency,

input impedance and read range of the antenna.

A. Width Variation Effect

The process standardization of inkjet printing for electronics

has not matured so far, and adding to this problem is that

its feature size depends on various factors such as spacing

between two ink drops, printing nozzle size, substrate material

and rheology, heating platform temperature etc. For simplifica-

tion and convenience of the investigation, the width variationsof a antenna are described as percentage ranging from 20%to +20%. In Fig. 2, the antenna radiation patterns at thehorizontal plan ( = 90o) present little alterations with widthvariation increasing from 20% to 0 then to +20%, especiallyin the maximum gain direction where in practice tags are

frequently placed, so that it can be concluded that the width

variations almost have no impact on the directional depedance

of radiaton.

By using Eq. 1 and Eq. 2 respectively, the gain and the

radiation efficiency of the antenna were derived as shown

in Fig. 3, where with the increase of the wire width the

gain rises from 1.73 dB to 1.77 dB, while the radiationefficiency approximately maintain its original value across thewhole width variations. It can be concluded that width increase

might improve gain but takes no effect to the enhancement of

radiation gain. Moreover, as the width rises the gain of the

silver antenna seemingly approaches the gain of the copper

antenna.

B. Overprinting Effect

Owing to the restriction of thin thickness of a conducting

film, a printed antenna might not be comparable with copper

counterparts on PCB. In order to enhance the competitiveness,

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-23.00

-16.00

-9.00

-2.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120

Fig. 2. 2D radiation pattern at = 90 degree, 3 radiation plots superposevs. width variations with percent increment from 20% to 0 then +20%

1.4

1.5

1.6

1.7

1.8

1.9

2

-20 -15 -10 -5 0 5 10 15 20

Varied Width (%)

Gain

(dB)

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Radiation

Efficiency

copper

Fig. 3. Gain and radiation efficiency vs. varied width percent from 20%to +20%.

overprinting technique is commonly employed to increase

the thickness of a film, leading to an inescapable trade-offbetween inferior performances with single layer and superior

performances with multilayer. Although the average thickness

of overprinted film is dependent on the previous surface

topology and substrate temperature factors and so on, the

multilayer thickness is roughly proportional to the overprinting

times [2]. Based on these reasonable assumptions, the effect

of overprinting on gain and radiation efficiency was examined,

shown in Fig. 4. The behavior of the antenna gain and the

radiation efficiency versus the overprinted times gives that the

efficiency retains constant while the gain increases slightly

although the cross-sectional area of the antenna rise up by

as much as 5 times. This phenomena stems from the skineffect where most of the electromagnetic wave are constrainedwithin some extent known as the skin depth. At 2.45 GHz, theskin depth for copper antennas is around 1.3 m compared to2.2 m, less than the three layers thickness, for nano-silverink based film.

C. Input Impedance Response

Since the geometry of the antenna was changed because

of width and thickness variations, the input impedance was

correspondingly altered consisting of the real part, resistance

and the imaginary part, reactance, leading to the reconsider-

1.4

1.5

1.6

1.7

1.8

1.9

2

1 2 3 4 5

Number of Layers

Gain(dB)

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Radiation

Efficiency

copper

Fig. 4. Gain and radiation efficiency as the function of number of overprintedtimes

ation of antenna port matching. Given in Fig. 5 is the plot

of real part of antenna input impedance at 2.45 GHz whenthe constraints are laid down of the wire width varied within

20% and the number of overprinting times from 1 to 5. Aswith this plot, we can observe that the tendancy goes high

when the overprinting times increasing up to 3, and then goesflat when the times above 3. This happens as a result of theheavy influence of skin effect, where the skin depth is 2.22mas stated previously. Although the maxium value in this plot

is 226 Ohm when width reducing to 20% and overprinting4 times, the values at 3 and 5 times printing with the identicalwidth are 225 Ohm and 221 Ohm, respectively, providing theconclusion that these values are in good agreement with com-

mensense. Secondly, under the environment with width upto

20% and twice-printing, achived is the minimum magnitudeof real part of the impedance, 166 Ohm, occuring for 2 timesprinting and 20% width increment. Seen in Fig. 5, furthermore,a series of valley values are merely distributed along the line

where twice-printing employed, which is in accordance with

the general knowledge that when the thickness of a wire rises,

this wire resistance decreases until the skin effect becomes

dominant in determining the magnitue of impedance. The

imaginary part of the input impedance for the changed width

and thickness are plotted in Fig. 6, where the tendancy similar

as the real part is observed, as well as the maxium and the

minimum value are 2.3 Ohm and 1.5 Ohm respectively.

D. Read Range Degradation

For a tag antenna which is perfectly matched with full-wave

rectification, the read range is primarily determined from the

antenna gain. As a result, with the gain alteration the read

range of the antenna would be modified, which is proven

by Eq. 4. The corresponding degradations of read range are

presented in Fig. 7, where the read range rises from 0.99 to1.01 with the film width increasing from 20% to +20%while for the overprinting, the magnitude of read range of the

antenna appears not to change. It can be concluded that at

2.45 GHz using overprinting method plays a minor role inimproving the read range of the antenna.

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0.2

0.1

0

0.1

0.2 12

34

5

160

170

180

190

200

210

220

230

Overprinting timesWidth variation(from 20% to 20%)

Re(Impedance)(Ohm)

Fig. 5. Input resistance for process variations at 2.45 GHz

0.2

0.1

0

0.1

0.2 12

34

5

1.5

2

2.5

Overprinting timesWidth variation(from 20% to 20% )

Im(impedance)(Ohm)

Fig. 6. Input reactance for process variations at 2.45 GHz

Moreover, in comparison with the read range, set to 1, of

the standard antenna, the regular copper antenna has a weaklysuperior read range of 1.02, leading to that inkjet printedantennas with a lower cost have the competent capability with

the copper counterparts.

IV. CONCLUSIONS

Firstly, with the width variation of the silver based antenna

increasing from 20% to +20%, its radiation pattern andradiation efficiency approximately remain unchanged, and the

gain rises slightly, leading to the slight enhancement of the

read range of the tag on which the antenna are attached.

Secondly, although using overprinting technique to increase

the thickness leads to a limited improvement on the gain and

the radiation efficiency of the antenna owing to the skin effect

at 2.45 GHz , the influence on the input impedance of theantenna imposed by overprinting are fairly considerable, as

much as 50 Ohm.The enhancement, furthermore, of read range by overprint-

ing process seems not effective as thought, informing that at

2.45 GHz the solution of performance improvement of anantenna by increasing the thickness of the conducting film

requires to be treated cautiously since the expenditure on this

point may not be cost efficient. It can also be concluded

that the antenna from nano-silver ink has a performance

0.9

1

1.1

-20 -10 0 10 20

Varied Width (%)

Read

RangeDegradation

0.85

0.95

1.05

1 2 3 4 5

Number of Layers

Read

RangeDegradation

copper

Fig. 7. Process dependence of read range vs. width and thickness variations.

comparable with the antenna from copper on PCB but at lower

cost and with flexible property.

ACKNOWLEDGMENT

This work was financially supported by Vinnova (TheSwedish Governmental Agency for Innovation Systems)

through the Vinn Excellence centers program, and we are

grateful as well to China Scholarship Council(CSC) for its

co-funding.

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[3] P.V. Nikitin, S. Lam, and K.V.S. Rao. Low cost silver ink RFIDtag antennas. In Antennas and Propagation Society InternationalSymposium, 2005 IEEE, volume 2, 2005.

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