RFID Tag Antennas Designed by Fractal Features and

Manufactured by Printing Technology

Chi-Fang Huang

, Jing-Qing Zhan

and Tsung-Yu Hao

Graduate Institute of Communication Engineering

Tatung University, Taiwan

Dept. of Graphic Communications and Publishing

Shih Hsin University, Taiwan

Abstract. Based on the fractal features, this work is to design RFID Tag anten-

nas with minimum area needed as a tag. The designed passive tag is to be re-

sponsible for the EM wave of 915MHz from the reader. The concept of com-

plex conjugated matching is used in designing the antenna. Electromagnetic

Simulation tool is also used to help this design as well. The techniques of

measuring the material parameters necessary in the design procedure are also

mentioned in this paper. Offset printing technology is employed to manufac-

ture these tag antennas which is assumed as a kind of low-cost tags. Real per-

formance of these tags is also shown in this work.

1 Introduction

Even invented and applied initially during the World War II, RFID (Radio Frequency

Identifi-cation) [1] has attracted much attention recently. Precisely speaking, RFID

has been applied very widely in some proprietary or closed systems, for example,

animal control, portal control (access badges), etc. in last decades. The main advan-

tages of RFID application are, storing item data in an electronic way, data access by

electromagnetic wave, and allowing multiple accesses to RFID tags. Based on the

diverse applications, different spectrum bands are allocated, for example, LF (125 -

134.2 kHz and 140 - 148.5 kHz) for animal control, HF (13.56MHz) for electronic

ticket, and UHF (868 MHz-928 MHz) for logistics, etc. Most of the frequencies are

located in the ISM (Industrial, Scientific and Medical) bands [1].

However, RFID was emphasized again mainly because of the need of supply chain

[2]. By proposing a standard for the format of electronic data used for goods items,

of which EPC (Electronic Product Code) [3] is an example, the products can be regis-

tered at once when they are shipped out from the factories, and be released when they

are checked out at the counter of a supermarket. This is called “product tracking” and

is to be carried out in an “Internet of Thing”.

When the RFID tags are used in the logistics, they are not supposed to be recycled

after being used. Therefore, the cost of tag will be a key factor to determine if this

technology can be widely applied or not. There are three parts composing the tag

Huang C., Zhan J. and Hao T. (2007).

RFID Tag Antennas Designed by Fractal Features and Manufactured by Printing Technology.

In Proceedings of the 1st International Workshop on RFID Technology - Concepts, Applications, Challenges, pages 93-99

DOI: 10.5220/0002418400930099

 SciTePress

cost, namely, antenna, chip and assembly for them. It has been believed that, apply-

ing the traditional printing technologies [4] to produce the antennas will lower the

cost of the antenna part. One of the major efforts of this present work is to produce

the tag antenna by traditional offset printing method to demonstrate the possibilities

of making low-cost tags in high-volume.

Based on the fractal features [5], this work is to design RFID Tag antenna with

minimum area used as a tag. Usually the fractal antennas own the characteristic fea-

tures of minimization, wideband and multi-band. As a matter of fact, design by frac-

tals is also able to give an engineering path to follow in the simulation procedure

when searching for the target. As a RFID application in UHF band [6], the designed

passive tag is to be responsible for the EM wave of 915MHz from the reader.

Referring to Figure 1, RFID tag antenna is a kind of planar antenna, in which the

antenna metal layer is laminated on a dielectric substrate. Usually, even they look

diverse in shape in RFID Tag industry; the type of dipole antenna is used for the tags

operating at frequency for UHF band and for higher bands. In designing such a kind

of tag, the material parameters, for example, the conductivity

of the antenna metal

and the dielectric constant

, are necessary to be given in the simulation phase. In

this work, the measurement techniques of material parameters are also mentioned.

Performance evaluation both by simulation and measurement are shown in this paper.

Fig. 1. The physical structure of a RFID tag.

Fig. 2. Situation of complex conjugated impedance matching on the Smith Chart.

2 Design of Tag Antenna Using Fractal Features

The operation in a tag is that, the antenna receives the incoming EM energy and trans-

fers into the chip; and chip sends back the data-modulated EM wave to the RFID

reader. As depicted in Figure 1, to ensure the efficiency of energy transfer in between

chip and antenna, they should be in a “match” condition. In ordinary antenna indus-

try, the antenna is designed with a standard input impedance, for instance,

Ω50

Ω75

, to have impedance match with transceivers or other RF devices. However, in

the RFID Tag industry, for the purpose of cost-down, usually the match network

inside the chip is not offered. Consequently, it needs a complex conjugated matching

to ensure highest power transfer in between the chip and antenna, namely, to maxi-

mize the tag performance. Those two “X” marks show the input impedance positions

of the chip and antenna on the Smith Chart in Figure 2. Most of the cases, chip’s is

the lower “X”, and antenna’s is the other one. That means, usually the chip is capaci-

tive; and the antenna for being designed should be inductive. The present tag antenna

is developed based on this theory.

As an EM design tool, CST [7] is employed to help design antenna prototype.

Since there two kinds of material are involved in the tag, and since this tag antenna is

to be printed on paper by offset printing technology, before beginning the design, the

conductivity

of the conductive ink, the paper’s dielectric constant

and its asso-

ciated loss tangent

tan

should be given. The lithographic conductive ink used in

this work is CLO-101A purchased from Precisia LLC [8], and its corresponding con-

ductivity

was measured based on the technique described in the literature [9][10].

The metal cylindrical cavity shown in the Figure 3 is used to measure the ink’s con-

ductivity according to that theory. The ink is coated on the inner side of the right and

left planar walls of this cavity. The measured conductivity is

1085.3 ×

, which

is only 6.6% or so of the copper’s

108.5 ×

. As what expected, such a kind of

ink is not as good as ordinary conductors to be antenna radiating material. This

should be seriously taken into account when the tag performance is emphasized and

they are produced by printing technologies.

Fig. 3. Metal cylindrical cavity for measuring the ink’s conductivity.

Fig. 4. Rectangular cavity for measuring the dielectric constant of coated paper.

On the other hand, the dielectric constant

of the coated paper, which is used as

the substrate of the tag, can be obtained by applying the technique of cavity resonator

[11][12]. By this method, referring to Figure 4, the frequency response is measured

two times at least for this rectangular cavity with different height. The paper layers

are contained in this cavity in measurement. The resultant dielectric constant

paper used is 2.83 and its

tan

is 0.046 around the frequency 915MHz. This shows

that the paper is of a little loss and should be carefully considered.

To have a path to follow for designing the antenna shape that is of complex conju-

gate matching with the chip, the fractal is used to generate it. Among the fractal ge-

ometries, the Sierpinski gasket fractal is adopted to develop, see Figure 5.

The chip used in this project is the XRA00 of Alien Technology [13], which has

an input impedance

− 4.1977.6 j

. Hence, the target impedance for the antenna is

+ 4.1977.6 j

in theory. Fig. 6 is the simulation model established in the CST

package for a tag antenna. It is based on the Sierpinski gasket fractals. In addition to

generating fractals through different stages, the rectangular dimension of this tag is

also under adjustment to search for the target input impedance of the antenna. Table

1 shows the simulated input impedance of this fractal antenna due to different dimen-

sion. Some of the candidates in this table are to be printed and analyzed later.

Fig. 5. Sierpinski gasket fractals.

Fig. 6. Simulation model of a tag antenna in the EM package CST

Table 1. Simulated input impedance of the tag antenna.

Tag dimension

mm x mm

R (

) jX (

)

10 x 120 9.211 31.86j

10 x 130 9.614 51.86j

10 x 140 10.32 75.71j

10 x 150 10.08 91.83j

10 x 160 9.728 119.4j

10 x 170 12.95 145.6j

10 x 180 15.04 178.5j

10 x 190 18.31 221.8j

10 x 200 20.15 281.3j

3 Printing Tag Antenna and Performance Analysis

Figure 7 shows a demonstration of the printed paper sheets on which many tag anten-

nas had been printed by offset printing technology. This antenna is based on the

simulation results mentioned above and operating in UHF band.

Fig. 7. Demonstration of high-speed production of RFID tag by offset printing technology.

Figure 8 shows the details of this printed tag antenna with a dimension 10mm x

180mm. In an antenna anechoic chamber, by using a UHF reader in measurement,

this tag’s responsible distance is about 1m. For the purpose of comparison, around

the same dimension, this antenna shape have also been made by screen printing and

by ordinary PCB process, the former can reach to 1.5 m and the latter can reach up to

3.4m. These antennas are supposed not perfectly matched with the chip. The farther

distance they can make is because the screen printing can print thicker film than off-

set printing does; and the PCB process is to provide 0.018mm thick metal layer of

copper. Less conductive ink, thinner printed ink’s layer and substrate with higher

loss (coated paper) indeed make the tag antennas produced by offset printing technol-

ogy less efficiency. Anyway, its unique advantage is to able produce tags in high-

speed and in high volume, yet being low-cost.

Fig. 8. A single RFID tag of UHF band made by offset printing technology.

4 Conclusion

This paper has outlined and demonstrated a complete procedure by which the offset

printing technology is applied to produce high volume and low-cost RFID tags.

Based on the concept of complex conjugated matching, the design for tag antenna is

carried out firstly by the help of the EM simulation package. To precisely design the

antenna by computer simulation, the techniques of measuring material parameters

given in literatures are also applied to obtain those parameters of conductive ink and

paper. By the up-to-date offset printing machine, the tag antenna had been printed

out by a high-speed manner to demonstrate its possibility to be a low-cost product.

Acknowledgements

This project was granted by Tatung Company [14], Taipei, TAIWAN, who plays the

main role offering long-term support for the academic-industrial projects being car-

ried on in Tatung University. Sun Sui Print Co., Ltd, Taipei County, TAIWAN, is

appreciated for their kind support to provide the offset machines in printing the RFID

tags designed in the present work. Moreover, we want to specially thank Mr. Wen-

Ho Wu, the factory manager of this company. Without his professional guide in the

printing procedure, this present work would not be done completely.

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