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Design of Flat Spiral Rectangular Loop Antenna for HF-RFID System Abstract — This paper is introduced a flat spiral rectangular loop antenna in walk-through gate installation for improving the distribution of magnetic field in High-Frequency Radio Frequency Identification (HF-RFID) systems. The spiral structure can provide the magnetic field distribution to cover desired region in various directions. This antenna is designed as a walk-through gate for anti-theft application. The prototype antenna was fabricated and measured to confirm the antenna performance. Keywords- HF-RFID, Flat Spiral Rectangular Loop Antenna, Anti-Theft System. Introduction The Radio Frequency Identification (RFID) system is become popular in many applications such as industrial automation and supply chain management because it can provide fast and automation operating. Tags in contactless smart cards used in building-access and anti-theft systems. In practice, these tags are oriented in many directions. Conventionally, the rectangular or circular loop reader antennas are always employed, but they cannot provide sufficient magnetic field intensities in some required directions. Therefore, a large number of publications and patents are available in the literature for loop antenna in various configurations. Most techniques can improve the magnetic field distribution but antenna structure is complicated. In this paper, the flat spiral rectangular loop antenna arranged as a walk-through gate is proposed for the anti-theft system. Anti-theft system is also expected to be applied in the shop or store to protect the thievery. A typical RFID anti-theft system as shown in Fig. 1 consists of three main parts: antenna system, reader and computer. This propose reader antenna is low cost and simple structure, including ability to communicate with various tag orientations. Antenna Analysis The flat spiral rectangular loop antenna is improved from the rectangular loop antenna to provide better magnetic field distributions in various directions a spiral configuration as shown in Fig. 2, where d is the separation distance between each spiral loop In this design, the dimension of antenna are 50 cm of width and 80 cm of height (with d=10 cm). In addition, the matching circuit is designed to match the antenna to the reader 50 Ω input impedance as shown in Fig. 3. The percentage of volume of magnetic field intensity is used as a figure of merit. The percentage of volume for communication achievement is probability of all points in the space that have magnetic field strength stronger than the activation field intensity (Ht) of a tag. This paper considers the percentage of volume in three principal axes (x-axis, y-axis and z-axis) as a antenna performance. where Pvx, Pvy and Pvz are the percentages of volume in x, y and z axes, respectively. The evaluation of percentage of volume starts from considering percentage of line (Pl). It is determined by considering a tag goes through gate along a straight line from the entrance to the exit. If some points in the line (parallel to the x-axis) has H(x,y,z) ≥ Ht, then the percentage

of that line is 100%. However, if every point along the line has H(x,y,z) < Ht, the percentage of line is 0%. Note that H(x,y,z) is the magnetic field intensity simulated by NEC [4] at any point parallel to the x-z plane. (Ht is assumed to be 56 mA/m for the ISO card). The evaluation of the percentage of surface is subsequently calculated by (1). N

Ps ( ym ) 

 P (z ) n 1

l

n

, (1) where N is the total number of lines along the z axis, Ps(ym) is the percentage of surface in the x-z plane, Pl(zn) is the percentage of line along the x axis. Note that Ps(ym)≤100%. The subdivided area is shown in Fig.4a. The percentage of surface is consequently used to evaluate the percentage of volume (Pv) by equation (2). N

M  Ps ( ym ) Pv  m1 M

, (2) where M is the total number of surface parallel to the x-z plane between two gate as shown in Fig.4b. Note that Pv≤100%. In addition, the considered volume must cover all regions between two antennas. Therefore, this paper uses the following dimension to obtain the percentage of volume: x-axis from -0.25 m to 0.25 m, y-axis from 0 to 1 m and z-axis from -0.4 m to 0.4 m. The simulation result of the percentage of volume in principle axes equal to 100%, 97.01% and 90.06%. Measured Results The prototype antenna was fabricated by using enamel wire 16 AWG, and connected to RFID reader (ACENTECH 13.56 MHz) with a matching circuit as shown in Fig. 5. The tag under test is the ISO card tag. The measure result magnitude of S11 of antenna with matching circuit shows in Fig. 6. It is found that |S11| equal to -17 dB at 13.56 MHz bandwidth of 500 kHz. The antenna performance is found in term of the percentage of volume in principal directions, that is the probability of communication between RFID reader and tag as shown in Fig. 8. It has the percentage of volume are 100%, 95.23% and 89.62% in x-, y- and z-direction, respectively. From Fig. 7 shows the communication performance between reader and tag of the spiral rectangular loop antenna, it improved the communication performance in x and z axes. The gray slot represents the activated fields whereas the black slot represents the null fields. Conclusions The flat spiral rectangular loop antennas in walk-through gate is found to provide better magnetic field distributions in various directions. Resulting is the enhancement of communication performance over the rectangular. It is found that the percentages of volume for this antenna are Pv , H = 100%, Pv , H = x

y

95.23% and Pv , H = 89.62%, respectively. Thus, this antenna can be efficiently used in the HF–RFID anti-theft system. z

References [1] [2]

[3] [4]

Dominique Paret, RFID and Contactless Smart Card Applications, John wiley&Sons, 2005. Hassan M. Elkamchouchi and Mona N. Abd El-Salam, “Square Loop Antenna Miniaturization using Fractal Geometry,” Antennas and Propagation Society International Symposium IEEE, Vol.4, June 2003, pp.254-257. Klaus Finkenzeller, RFID-Handbook, John wiley&Sons, 1999. G. J. Burke and A. J. Poggio. Numerical Electromagnetics Code (NEC) Method of Moment, Part I-III. Lawrence LiverMore Nat. Lab., Livermore, CA, 1981. Figures Antenna System

Reader

Computer

z Walk-through

x

y

Figure 1 The RFID anti-theft system.

D

50 cm z

z

80 cm

d

x

x

y

y

walk-through

Connect to matching circuit

Figure 2 The flat spiral rectangular loop antenna in walk-through gate. Antenna 1 Spiral Rectangular Loop Antenna

Matching Circuit 50 Ω

Power Splitter

Antenna 2 Spiral Rectangular Loop Antenna

RFID Reader

Matching Circuit

Figure 3 The flat spiral rectangular loop in walk-through gate of RFID system.

z

z x1 x2 x3

xK

z

z1 z2 z3

y

x

x

y zN

Ps (y1 ) Ps(y2 ) Ps (y3 )

Ps (yM)

(a) (b) Figure 4 The evaluation of percentage of volume: (a) Consideration of the percentage of line parallel to the x-z plane. (b) Consideration of the percentage of volume.

Figure 5 Fabricated flat spiral rectangular loop antenna. 0 -2 -4

|S11| (dB)

-6 -8 -10 -12 -14

13.56 MHz -17 dB

-16 -18 13

13.1

13.2

13.3

13 .4

13.5

13.6

13.7

13.8

13.9

14

Frequency (MHz)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Distance from antenna (cm)

81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0

81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0

0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5

10 15 20 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10 15 20

25 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 25

30 35 40 45 50 55 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1 1 0 30 35 40 45 50 55

60 65 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 60 65

Distance from antenna (cm)

(b) Pl , H y

70 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 70

75 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 75

80 85 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 80 85

90 95 100 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 90 95 100

81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0

A n t e n n a H e ig h t ( c m )

81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0

A n te n n a H e ig h t (c m )

A n te n n a H e ig h t ( c m )

Figure 6 Measure result magnitude of |S11| of antenna with matching circuit. 81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0

0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 1 1 1 1 1 1 Distance 1 1 from 1 1antenna 1 1(cm)1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Distance from antenna (cm)

(a) Pl , H (c) Pl , H Figure 7 The communication performance between RFID reader and tag in the straight line between antennas in principal directions of flat spiral rectangular loop antennas. x

z

81 78 75 72 69 66 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 12 9 6 3 0