11th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 17-19, 2012

66

Abstract—One of the very important factors that may influence and affect the performance of RFID systems and particularly of those using passive tags, consists in the effects of the metallic environments in the proximity of the antenna. For those RFID systems that operate at low frequencies (125 kHz) these effects are less common but, as the frequency increases, interferences increase as well, causing a frequency shift between the antenna and that particular system. In order to alleviate a part of these issues, both the device antennas and their impedance matching units must be optimized. This paper presents a mathematical model that enables the identification of the optimal size of an HF RFID antenna that would encompass the coordinates of a given surface. The antenna has been mathematically modeled, simulated and tested for a specific situation when the metal plate is positioned in its close proximity (approximately 2cm).

Index Terms—RFID, Metal environments, tag, loop antenna

I. INTRODUCTION

RFID systems have certain advantages over bar code identification systems and this is one of the reasons why the former have been increasingly used in medical systems, supply chains, security and monitoring systems, etc. RFID have certain advantages, but there are also a few unsolved issues that are still being tackled. One negative influence for systems operating on a 13.56 MHz frequency consists in the harmful effects of the metal environments in the proximity of antennas and tags.

The effects these environments have on the antenna parameters mainly consist in the reduction of the detection range between the tag and the reader, disturbances in the resonant frequency of both the reader’s antenna and of the tag [1], lowered numbers of read tags, etc. In order to alleviate some of these effects induced by metallic environments, a number of more or less viable suggestions have been made [2] - [3], but they should also take into account the applications that would incorporate them.

For instance, in the case of an application where tags are directly affixed on metal objects (small-sized), efforts should be directed at optimizing the tag that should operate on high frequencies (13.56 MHz). If one wishes to identify the objects stored in a locker or a drawer that is made entirely of metal, research should be directed at finding the best way to improve both the reader’s antenna parameters and the tag.

It is, indeed, quite difficult to design an RFID system that would operate on a 13.56 MHz frequency in various environments where metals are more or less present. Therefore, the antennas used by these systems can be more easily designed for very specific applications.

Several antenna models have been suggested in the past few years [4] - [9] for HF RFID systems that help identify products on the shelves. The suggested configurations have only focused on the detection of the objects, and omitted the possibility of widening the coverage/detection range for those shelves. When the system is implemented in a library, detection is sufficient, as the tags are affixed inside the books according to a specific pattern so that the distance between the tag and the reader is as short as possible. However, when one wishes to exhibit various objects on the entire surface of the shelf, situations occur when the objects can no longer be detected [10] - [12]. Another aspect to be considered is also the way in which the tags are positioned in relation to the reader’s antenna, as well as the shape and size of these tags.

One objective that has been acknowledged throughout the design of this antenna was to keep the system implementation costs at a minimum.

II. MATHEMATICAL MODELING OF THE HF RFIDANTENNA

The design of the antenna has been conducted gradually, following a series of steps that will enable one to identify the optimal size of the desired antenna that would work properly near metallic environments. The suggested antenna model is rectangular, with one or more loops, so that the reading/detection range of this antenna would be as long as possible. The antenna will be mounted on a metallic shelf, at a very short distance from it, and adjusted at the resonant frequency (13.56 MHz).

The design requirements that must be met when optimizing an RFID antenna are further described:

The first criterion met when designing the antenna referred to the length of the antenna that had to be a value of the ratio λ/x, where: λ is the wavelength (22.12 m for 13.56 MHz) and x is a division factor with a multiple of 2.

Since the size of the antenna must fit within the area of the shelf (80 x 30 cm), the constraints above have set the length of the antenna at 184 cm (λ/12 = 184.36 cm the optimal choice). This antenna length allows us to have multiple sizes both for a (length of the antenna) that can range between 12 – 30 cm and for b (width of the antenna) that may range between 62 – 80 cm.

The a and b parameters must be determined in such a way that would enable the antenna to work under the given circumstances. In this respect, the antenna model will have to undergo a mathematical analysis as presented below [13]. The intensity of the magnetic flux generated by a rectangle

The Design and Implementation of HF RFID Loop Antenna for Metallic Environments

Ilie FINIS, Valentin POPA, Alexandru LAVRIC, Adrian-Ioan PETRARIU, Stefan SFICHI"Stefan cel Mare" University of Suceava

str.Universitatii nr.13, RO-720229 Suceavaifinis@eed.usv.ro

11th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 17-19, 2012

67

loop antenna may be calculated by means of the formula (1).

22

22

222

2

1

2

1

224 x

bx

ax

ba

baIH

where H is the intensity of the magnetic field generated by a rectangle loop antenna, I is the current passing through the antenna, a is the length of the antenna, b is the width and x is positioned at the center of the antenna, on the z axis (3 coordinates system) Figure 1.

Fig.1.Determining the magnetic field in point H(0,0,z) for a rectangular antenna.

The formula enables the measuring of the magnetic field generated by a rectangle loop antenna in an environment with no disturbances. In order to singularize this approach for a specific circumstance when a metal plate is positioned in the vicinity of the antenna, one must also take into account the effects and influence of the metallic environment on the parameters of the antenna. Starting from formula (1), we will introduce a new parameter that measures the distance between the antenna and the metal plate, as well as the interferences it generates [14] - [15]. Thus, the intensity of the magnetic field can be rewritten by using formula (2).

22

22

222

22

22

222

22

1

22

1

222

4

2

1

2

1

224

dxb

dxa

dxba

baI

xb

xa

xba

baIH

where d is the distance between the antenna and the metal plate, provided that the antenna is placed parallel to the metal plate.

Since the metal plate induces the so-called “eddy currents” onto the loop antenna and damps its magnetic field, we can approximate the effects of the metal plate by means of another antenna that is identical with the loop antenna and positioned at a distance 2d from it. The current induced in the virtual antenna is directed in the opposite way from the one induced in the main loop antenna, representing the negative effects of the metal plate.

Figure2 presents, the intensity of the magnetic field H(x). Here we can observe the variation of the magnetic field H(x) when the antenna is positioned in an environment without any interference and at a distance of 2 cm from the metal plate. The simulated antenna will operate in

compliance with standard ISO15693 where the intensity of the magnetic field needed to activate a tag is of 0.07 A/m[16].

Fig.2.The magnetic field variation for a rectangle loop antenna in free space and at 2 cm from a metal plate.

These formulas allow the measurement of the detection range of the RFID antenna along the z axis at which the tag can be read. If one wishes to determine the detection range of an antenna, the formula must be generalized from a coordination point H(0,0,z) to a three-dimensional point with the coordinates H(x,y,z). We will further present a mathematical approach based on which the detection range of a rectangle loop antenna can be measured in a three-dimensional space.

Starting from Weber’s formula [17], we can determine the components of the vector potentials Ax (3) and Ay (4), respectively, in the xOy space for a rectangle loop antenna of the size made of copper wire (Figure 3).

Fig.3.Determining the magnetic field in point H(x,y,z) for a rectangular antenna.

'2

2

2

24=

R

ds

R

dsIA

a

a

a

ax

'2

2

2

24=

R

ds

R

dsIA

a

a

a

ax

where

222'

222

=

=

zbyaxR

zbyaxR

Thus, the presence of the vector potentials in the xOy plane enables the measurement of the magnetic field intensity in the P(x,y,z) point where, according to equation (6), we can determine the magnetic field for each coordinate (x,y,z) of the three-dimensional plan.

(1)

(2)

(3)

(4)

(5)

11th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 17-19, 2012

68

z

A

z

AH

z

AH

z

AH xy

zx

yy

x

0000

11=,

1=,

1=

If we use the equation (3) in (4) and (5), we get the equations (7) and (8), respectively:

xar

xar

xar

xarln

IAx /2

/2*

/2

/2

4=

4

3

2

10

xbr

xbr

xbr

xbrln

IAy /2

/2*

/2

/2

4=

1

4

3

20

Thus, if we substitute the results arrived at for the vector potentials Ax and Ay, respectively, in (6), we will arrive at the general shape of the magnetic field at each of the three coordinates (x,y,z) according to (9).

Drr

zIH y 1

14

1= 1

1

4=

drr

zIH x

14

1=

1

4=

drr

D

Drr

dIH z 1

4

1= 1

1

4=

The equations (10) have been used in order to reduce the complexity of the formula when measuring the magnetic field for each coordinate.

222

4

222

3

222

2

222

1

22

22

22

22

zyb

xa

r

zyb

xa

r

zyb

xa

r

zyb

xa

r

2==

2==

2==

2==

43

21

32

41

bydd

bydd

xa

DD

xa

DD

(10)

Formula (12) is employed when measuring the magnetic field generated by a rectangle loop antenna in the three-dimensional space H(x,y,z):

222),,( zyx HHHzyxH

After these measurements have been conducted forrectangular loop antenna, they can be applied in the simulation environment MATLAB, in order to establish the longest detection range [18].

Figure 4 presents the detection range of a rectangle loop antenna with the dimensions 22 x 64 cm through which a 0.1A current will be induced. Cross sections of the three plans can be noted as displaying a value of 0.07A/m needed to activate an HF RFID tag available on the market. Mention must be made of the fact that the tag is positioned parallel to the loop antenna.

a) b)

c) d)

Fig.4.The detection range of an HF RFID antenna (22 x 64 cm) a) 3D cross-section of the detection range b) plan x – y, magnetic field strength c)

plan x – z, magnetic field strength d) plan y – z magnetic field strength.

Considering the infinite size metallic plan approximation as a virtual antenna, we can determine the magnetic field generated by an antenna at a certain distance from a metal plane. Figure 5 presents, the detection range and the magnetic field strength for the antenna located at a distance of 2 cm from the metallic plane. The antenna is parallel position above the metal plane.

a) b)

c) d)

Fig.5.The detection range of an HF RFID antenna (22 x 64 cm) at a distance of 2 cm from the metallic plane a) 3D cross-section of the detection range b) plan x – y, magnetic field strength c) plan x – z,

magnetic field strength d) plan y – z magnetic field strength.

III. TESTS AND RESULTS

This section presents the efficiency evaluation of the antenna that had been mathematically modeled in the previous section. The dimensions of the antenna are detailed in Table 1. The antenna was made of copper.

TABLE I. THE ANTENNA PARAMETERSParameter Dimension

Length a 64 cmWidth b 22 cm

Wire diameter 1 mm

Figure 6 presents the measured Return Loss parameter |S11| arrived at after the simulations vs. the results measured by means of a FieldFox RF Analyzer from Agilent. The Ansoft HFSS application was used when simulating the |S11| return loss parameter.

(6)

(7)

(8)

(9)

(12)

11th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 17-19, 2012

69

Fig.6. The simulated return losses |S11| vs. the measured return losses

The detection range of the loop antenna positioned in the metallic environment was measured through various tests where the range of the reader was varied, as can be noted in the figure below.

Fig.7. Variation between the readers power and the detection range of the antenna

An analysis of Figure 7 reveals that the 25 cm detection range corresponds to a 3W power, which is considered as accurate since the same result was obtained after the simulations, i.e. the mathematical modeling. The distance between the antenna and the metal plate has been the same in all cases, i.e. 2 cm.

TABLE II. READING DISTANCE FOR LOOP ANTENNA

Antenna output

power [W]

Free space[cm]

Metal(tag parallel with

antenna)[cm]

Metal(tag

perpendicularwith antenna)

[cm]1 40 14 82 48 17 103 56 25 154 64 23 12

IV. CONCLUSION

The present paper describes a method used to determinethe magnetic field generated by a rectangle loop antenna when it is positioned in the proximity of a metallic environment. The results arrived at by means of this method were compared with the data collected during a simulation for the same type of antenna, by means of the HFSS application.

The distance between the rectangle loop antenna and the metal plate was reduced by up to 2 cm, without using any absorbent material (ferrite) that would also increase the implementation costs. The identification of objects does not depend on the position of the tags opposite the reader’s antenna.

The implemented antenna provides a maximum detection range of up to 25 cm and proves to be very efficient when it is positioned in the vicinity of metallic environments.

ACKNOWLEDGMENTS

This work was supported by the project "Knowledge provocation and development through doctoral research PRO-DOCT - Contract no. POSDRU/88/1.5/S/52946 ", project co-funded from European Social Fund through Sectorial Operational Program Human Resources 2007-2013, and by European Framework Program 7 under the contract no. PIRG02-GA-2007-224904

REFERENCES[1] K. D’Hoe et al., “Influence of Different Types of Metal Plates on a

High Frequency RFID Loop Antenna: Study and Design,” Advances in Electrical and Computer Engineering, vol. 9, no. 2, pp. 3-8, 2009.

[2] D’hoe, K., Van Nieuwenhuyse, A., Ottoy, G., Goemaere, J.-P., & De Strycker, L. (2009). A New Low-Cost HF RFID Loop Antenna Concept for Metallic Environments. 2009 16th International Conference on Systems, Signals and Image Processing, (2), 1-5. Ieee. doi:10.1109/IWSSIP.2009.5367785;

[3] H. Zhu, S. Lai, and H. Dai, “Solutions of Metal Surface Effect for HF RFID Systems,” 2007 International Conference on Wireless Communications, Networking and Mobile Computing, no. 1, pp. 2089-2092, Sep. 2007;

[4] X. Qing and Z. N. Chen, “Characteristics of a Metal-Backed Loop Antenna and its Application to a High-Frequency RFID Smart Shelf,” IEEE Antennas and Propagation Magazine, vol. 51, no. 2, pp. 26-38, Apr. 2009;

[5] P.-yi Lau, K. Kin, O. Yung, E. Kai, and N. Yung, “A Smart Bookshelf for Library RFID System,” Electronics, pp. 3-6, 2008;

[6] P. Li, Y. L. Lu, and W. Liu, “RFID high frequency 3-dimensional loop antenna analysis and design,” 2009 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, pp. 1119-1123, Jul. 2009;

[7] X. Qing, Z. N. Chen, and A. Cai, “Multi-loop Antenna for High Frequency RFID Smart Shelf Application,” Science, pp. 5467-5470, 2007.

[8] X. Wang, H. Wang, and G. Wang, “Distributed High-Frequency RFID Antennas for Smart Storage Racks,” 2010 Second International Conference on Networks Security, Wireless Communications and Trusted Computing, pp. 472-474, 2010;

[9] P. Li, Y. L. Lu, and W. Liu, “RFID high frequency 3-dimensional loop antenna analysis and design,” 2009 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, pp. 1119-1123, Jul. 2009;

[10] A.-I. PETRARIU, V. POPA, V.-G. GAITAN, I. FINIS, "Test results for HF RFID antenna system tuning in metal environment", Carpathian Control Conference (ICCC), 2012 13th International, (unpublished).

[11] Petrariu Adrian-Ioan, PopaValentin, GaitanVasile-Gheorghita, Finis Ilie, Lavric Alexandru, 13.56 MHz RFID multi-turn antenna formetallic environments, European Conference on the Use of Modern Information and Communication Technologies 5th Edition” ECUMICT 2012, Gent, Belgia;

[12] Finis Ilie, Popa Valentin, Petrariu Adrian-Ioan, Lavric Alexandru, Gaitan Adrian-Mihai Smart shelves architecture for warehousemanagement using HF RFID, European Conference on the Use of Modern Information and Communication Technologies 5th Edition” ECUMICT 2012, Gent, Belgia;

[13] Klaus Finkenzeller, “RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, Second Edition”, John Wiley & Sons, Ltd., 2003;

[14] W. M. Frixand G. G. Karady, “A Circuital Approach to Estimate the Magnetic Field Reduction of Nonferrous Metal Shields,” vol. 39, no. 1, pp. 24-32, 1997.

[15] Design of an RFID loop antenna in non-ideal conditions, D'hoe Kevin, Stevens Nobby, Goemaere Jean-Pierre, De Strycker Lieven, Nauwelaers Bart, ECUMICT edition:4 location: Ghent, Belgium 2010;

[16] Texas Instruments, “RI-I02-112A-03– HF-I Plus Transponder”, 2010;[17] E. Weber, “Electromagnetic Theory”, Dover, p.131-135, 1965[18] I. Finis, V. Popa, A. Lavric, A.-I. Petrariu and C. Males, An

Analytical Determination of the Reading Volume for an HF RFID Antenna, Conference on Future Internet Communications BCFIC 2012, Vilnius, Lithuania (unpublished).