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Performance of RFID Tag Antennas

Printed on Flexible Substrates

Kamil Janeczek, Tele and Radio Research Institute

Abstract

Radio Frequency Identification is becoming more and more popular. It has found application in many various fields, like person identification or supply chain management. Nowadays unit price of RFID tag is too high comparing to bar codes. It causes that widespreading of RFID techniques is still restrained. The decrease in RFID tag’s unit price is possible with printed electronics which allows to produce low cost devices on large scale.

In this study, screen printed UHF RFID antennas were investigated. A semi-automatic screen printer with polyester mesh screen (68 T/cm) was applied. As conductive materials silver pastes were used. They were printed on photo paper and foils to provide flexibility of produced RFID antennas. After printing process antennas were cured in 120°C for 15 min in exception of a paste with silver nanopowder which was dried in 120°C for 15 min and then cured in 300°C for 1 h.

To evaluate properties of printed layers their thickness and resistance were measured using contact profilometer and 4-probe method, relatively. Microwave properties of printed antennas were measured. In this order network analyzer was applied which allows to measure reflection coefficient of performed antenna. Then, it was possible to evaluate an influence of used substrate and conductive materials on the received results.

Tested UHF antennas were simulated in CST Microwave Studio. Reflection coefficient was taken into consideration.

1. Basics

Recently a rapid progress has been made in the field of printed RFID tags. Elaborated investigations are focused especially on printed RFID antennas which work in HF (13,56 MHz) [1, 2] or UHF (860 – 960 MHz) [3, 4] frequency range. To print them, pastes based on silver particles, carbon nanotubes or conductive polymer PEDOT:PSS are used. These materials are deposited on flexible and low-cost substrates like paper and foil using various printing techniques (for example flexography, screen printing, inkjet) [5].

Printing of RFID antennas is the first step to produce completely printed tag. It should have comparable parameters with traditional tags whose antennas are made from copper or alumina. Performed investigations show that it is possible to achieve. However, an efficiency of such antennas is lower comparing to its equivalents made from copper or alumina [6].

This paper presents the results of investigations on printed UHF RFID tag antennas. Two pastes based on silver flakes or silver nanopowder were applied. It makes possible to compare properties of layer printed with pastes containing silver particles in two different form. As substrates photo paper and foil were used. After printing process thickness and resistance of printed layer were measured using a contact profilometer and digital multimeter, relatively. To evaluate microwave properties of conducted antennas their reflection coeffiecient was measured using network analyzer. In order to compare obtained results a simulation of tested antenna was performed in CST Microwave Studio. Reflection coefficient was considered.

2. Experimental

In this section, material and antenna pattern used in performed investigations are described.

2.1 Materials

To conduct tested antenna two pastes were applied. One was consisted of silver flakes (SF) and the second one of silver nanopowder (SP). These two materials were dispersed in PMMA resin. As a solvent a buthyl carbitol acetate was used. After printing process layers made with these pastes were

dried in 120°C for 15 min. Additionally, the layer

printed with the paste SP was cured in 300°C for 1 h.

The paste SF was printed on two types of flexible substrates: photo paper and PET foil with

a thickness of 150 µm and 125 µm, relatively. Regarding to required high curing temperature the paste SP was deposited on Kapton foil (a thickness

of 125 µm).

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To deposit described above materials a semi-automatic screen printer with a mesh screen 68T/cm was applied.

2.2 Antenna pattern

In the described experiments an antenna pattern presented in figure 1 was used. It corresponds with an antenna of commercial available RFID tag [7].

Fig.1. A pattern of tested antenna.

The antenna has dimensions of 20 x 20 mm. Its assumed basic resonant frequency is 915 MHz.

3. Antenna simulation

Before practical experiments a simulation of tested antenna was conducted in CST Microwave Studio. This makes possible to verify if the antenna works properly in UHF frequency range. Moreover, the results of simulation are a reference to performed measurements.

Reflection coefficient was simulated

in the frequency range of 0,5 ÷ 1,5 GHz. Its characteristic is presented in figure 2.

-25

-20

-15

-10

-5

0

0,5 0,7 0,9 1,1 1,3 1,5

f [GHz]

|S11

| [d

B]

Fig.2. A characteristic of reflection coefficient

for tested UHF antenna.

On the basis of simulated characteristic of reflection coefficient it can be stated that a basic resonant frequency of tested antenna is 915 MHz what is consistent with initial assumption.

4. Antenna measurements

4.1 Thickness and resistance of tested

pattern

After printing process a thickness of layers printed with mentioned above silver pastes was measured using contact profilometer. In the figure 3 a profile of a layer printed with the paste SF on PET foil is presented.

Fig.3. A profile of a layer printed with the paste SF

on PET foil.

The average thickness of mentioned above layer

is about 18 µm. On paper substrates it equals 24 µm

so 6 µm more than for the layer on PET foil. This difference can be caused by physicochemical properties of applied substrates.

The thickness of a layer printed with the paste SP

equals about 1 µm. It is several dozen times lower than for the paste SF. In the consequence, antennas made from the paste SP should probably characterize with higher bending resistance comparing to its equivalents printed with the paste SF.

The resistance of tested antenna pattern was measured using a 4-probe method. Despite the lower thickness resistance is lower for pattern printed with

the paste SP. It equals 0,733 Ω. The resistance of antenna pattern made with the second paste

is 8,954 Ω. It causes that losses resistance of antenna printed with the paste SP is smaller than with the paste SF.

4.2 Reflection coefficient of tested antenna

In order to verify microwave properties of tested antenna its reflection coefficient was measured. It was done with a network analyzer N5242A PNA-X Network Analyzer 10M-26,5GHz Agilent Technologies. To connect the analyzer to a tested antenna a probe (figure 4) was constructed.

It consists of a waveguide in the form of a 50 Ω non-symmetrical strip line, SMA connector, output produced as a pad connected to the ground and the end of the strip line. Before measurements a calibration was done using a short section

of conductor (short), free output (open), 51 Ω SMD resistor (load). Their accuracy is limited but it is sufficient to state if tested antenna works properly in UHF frequency range.

Fig.4. A probe used for reflection coefficient measurements.

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The obtained characteristics of reflection coefficient were analyzed regarding to an influence of substrate, conductor and number of printed layers.

An influence of a substrate type on obtained characteristics of reflection coefficient (figure 5) was investigated for the paste SF.

0.5 0.7 0.9 1.1 1.3 1.5-25

-20

-15

-10

-5

0

f [GHz]

|S11

| [dB

]

paperPETCuCu

Fig.5. Characteristics of reflection coefficient for the

paste SF.

A shift in basic resonant frequency was observed for antennas on two applied substrates: paper and foil. It equals 957 MHz and 1,145 MHz, relatively. In this connection it can be affirmed that a type of substrates does not have a significant influence on characteristics of reflection coefficient.

Furthermore, properties of investigated antennas should depend on a used conductor. In the described experiments pastes based on two different forms of silver were applied. The particles differ in size. This causes that mechanical properties of antennas printed with these pastes can be different. It also may influence on microwave properties of tested antennas. In the figure 6 characteristics of reflection coefficient for two investigated conductive materials are presented.

0.5 0.7 0.9 1.1 1.3 1.5-25

-20

-15

-10

-5

0

f [GHz]

|S11

| [dB

]

SFSPCuCu

Fig.6. Characteristics of reflection coefficient for two

tested pastes.

Presented above characteristics have also other basic resonant frequency what can be caused by structure of printed layers.

Moreover, the minimum of S11 factor is different for these antennas. For antennas made with the paste SF it is -17 dB and with the paste SP -12 dB.

This can be combined with skin depth. It can be calculated from the following equation [8]:

ff rroµµµµ

ρρρρµµµµ

ρρρρπµπµπµπµ

δδδδ 5031 ≈≈≈≈==== (1)

For silver conductor µr = 1 and ρ = 3,4⋅10-7 Ω⋅m. Assuming basic resonant frequency 915 MHz

the skin depth equals δ = 9,7 µm. The highest antenna efficiency is achieved when its thickness

is two times bigger than skin depth so it should equal

19,4 µm. If the antenna has another thickness its efficiency decreases.

In the performed investigations the antenna made with the paste SF has the thickness comparable with its optimal value resulting from skin depth. It causes that the antenna printed with this conductor characterizes with lower minimum of S11 factor comparing its equivalent printed with the paste SP.

Finally, it was verified if a number of antenna layers plays an important role. This experiments was conducted using the paste SP printed on Kapton. The obtained characteristics of reflection coefficient (figure 7) exhibit that a second layer changes properties of investigated antenna. Its basic resonant frequency was shifted from 859 MHz to 1,082 GHz. This can be combined with changes in layer structure what can be observed in figure 8.

0.5 0.7 0.9 1.1 1.3 1.5-25

-20

-15

-10

-5

0

f [GHz]

|S11

| [dB

]

1 layer2 layers

Fig.7. Characteristics of reflection coefficient for the

paste SP for one and two layers.

a)

b) Fig.8. A surface of antenna printed with the paste SP,

a) one layer, b) two layers.

The structure of layers printed with the paste SP changes significantly when it is deposited once or twice. For two layers printing the surface does not contain holes and is less rough. Moreover, two layers

antenna has a thickness of 3 µm what results in the lower minimum of reflection coefficient then for one layer printing.

Furthermore, the second layer caused a decrease

in antenna pattern resistance from 0,607 Ω to 0,255

Ω what leads to decline in antenna losses resistance. For comparison a surface of antenna printed with

the paste SF on paper and PET foil are presented

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in figure 9. There is no holes like in the above presented layer but unevenness resulting from applied printing techniques can be seen.

a)

b)

Fig.8. A surface of antenna printed with the paste SF on a) PET, b) paper.

5. Summary

In this paper, investigations on printed UHF antennas were described. Antennas were made with screen printing techniques and two paste based on silver flakes or silver nanopowder. As substrate materials photo paper and PET foil for the paste SF or Kapton for the paste SP were applied.

Tested antenna pattern was measured with respect to its thickness and resistance. Microwave properties of performed antenna was checked using a network analyzer. Apart from practical measurements a simulation was done in CST Microwave Studio.

The characteristic of reflection coefficient obtained in simulation has comparable basic resonant frequency with measurements. Observed differences can be a result of layer structure or type of applied paste.

Measurements of reflection coefficient exhibit that a type of substrate does not have a significant influence on properties of tested antenna. However, applied conductor and number of antenna layer play an important role. It was observed that from these two factors depends a basic resonant frequency of tested antenna.

In the next investigations, environmental tests of performed antenna will be carried out. This provides information about its resistance for bending and thermal exposure.

Bibliography

[1] Stanley Y. Y. Leung, David C. C. Lam, Geometric and Compaction Dependence of Printed Polymer-Based RFID Tag Antenna Performance, Electronics

Packaging Manufacturing, IEEE Transactions April 2008, Volume: 31, Issue: 2, p.: 120 – 125

[2] Liu Caifeng, Wang Zhongyu, Du Yubao, Liu Sainan, Analysis and Improvement of Conductive Performance of Ink in RFID antenna, Communication Technology, 2008. ICCT 2008. 11th IEEE International Conference on Issue Date: 10-12 Nov. 2008, p.: 264 – 267

[3] Afzal Syed, Kenneth Demarest, Daniel D. Deavours, Effects of antenna material on the performance of UHF RFID tags, IEEE International Conference on RFID, pp. 57–62, Grapevine, Tex, USA, March 2007

[4] Amin Rida, Li Yang, Rushi Vyas, Manos M. Tentzeris, Conductive Inkjet-Printed Antennas on Flexible Low-Cost Paper-Based Substrates for RFID and WSN Applications, Antennas and Propagation Magazine, IEEE Volume 51, Issue 3, June 2009, p.:13 – 23

[5] Anne Blayo, Bernard Pineaux, Printing Processes and their Potential for RFID Printing, Joint sOc-EUSAI conference, Grenoble, 2005

[6] Dong-Youn Shina, Yongshik Leeb, Chung Hwan Kim, Performance characterization of screen printed radio frequency identification antennas with silver nanopaste, Thin Solid Films Vol. 517, Issue: 21, September 1, 2009, pp. 6112-6118

[7] http://www.rfid.averydennison.com/online_spec.php?id=41

[8] Bo Gao Matthew M.F. Yuen, Optimization of Silver Paste Printed passive UHF RFID Tags, Electronic Packaging Technology & High Density Packaging, p.:512 – 515, 2009

Authors:

Kamil Janeczek Tele & Radio Research Institute ul. Ratuszowa 11 03-450 Warszawa tel. (022) 619 01 64 fax (022) 619 29 47

email: kamil.janeczek@itr.org.pl