In this paper, we present a low-cost compact Radio Frequency Identification (RFID) tag antenna for the toll collection system in the ultra-high frequency (UHF) bands. The antenna is a printed dipole antenna, which is built on the top-side of a low-cost FR-4 substrate (εr = 4.4 and a thickness of 0.8 mm).
Trang 112 Thi Ngoc Hien-Doan, Van Khang-Nguyen, Ngoc Chien-Dao
A LOW-COST COMPACT RFID TAG ANTENNA FOR TOLL-GATES
ĂNG TEN THẺ RFID GIÁ THÀNH THẤP DÀNH CHO TRẠM THU PHÍ
Thi Ngoc Hien-Doan 1 , Van Khang-Nguyen 1 , Ngoc Chien-Dao 2
1 School of Electronics and Telecommunications Hanoi University of Science and Technology;
hien.doanthingoc@hust.vn, khang.nguyen@hust.edu.vn
2Ministry of Science and Technology; dnchien@most.gov.vn
Abstract - In this paper, we present a low-cost compact Radio
Frequency Identification (RFID) tag antenna for the toll collection
system in the ultra-high frequency (UHF) bands The antenna is a
printed dipole antenna, which is built on the top-side of a low-cost
FR-4 substrate (εr = 4.4 and a thickness of 0.8 mm) A
meander-line with E-shaped ending is inserted into each-arm of dipole for
the compactness The antenna is fed by a modified T-matching
network for impedance matching with the UCODE G2XM chip The
final design with overall size of 70 mm x 20 mm x 0.8 mm yields a
|S11| < –10-dB bandwidth of 250 MHz (800 – 1050 MHz), which
covers entire UHF RFID bands (860 – 960 MHz) Also, the antenna
yields an omnidirectional radiation pattern with a directivity of
1.3 dB across its operational bandwidth
Tóm tắt - Trong bài báo này, chúng tôi giới thiệu một ăng ten dành
cho thẻ sử dụng công nghệ nhận dạng tần số vô tuyến RFID giá thành thấp ở dải tần số siêu cao UHF Đây là loại ăng ten lưỡng cực được in trên đế FR-4 có giá thành thấp (εr = 4,4 và có độ dày 0,8mm) Một đường gấp khúc có dạng chữ E được thêm vào hai nhánh của ăng ten lưỡng cực nhằm làm nhỏ kích thước của ăng ten Ăng ten được cấp nguồn bởi một mạng phối hợp trở kháng chữ T với chip UCODE G2XM Ăng ten thu được có kích thước
70 mm x 20 mm x 0,8mm, hệ số phản xạ |S11| < –10-dB, băng thông 250 MHz (800 – 1050 MHz), phủ toàn bộ băng tần UHF RFID (860 – 960 MHz) Thêm nữa, ăng ten có đồ thị bức xạ đẳng hướng với độ định hướng 1,3dB
Key words - Radio Frequency Identification; toll collection system;
antenna; T-matching network; meander-line; E-shape
Từ khóa - nhận dạng tần số vô tuyến; hệ thống trạm thu phí; ăng ten;
mạng phối hợp trở kháng chữ T; đường gấp khúc; dạng chữ E
1 Introduction
The intelligent transportation system (ITS) with
electronic toll collection (ETC) enables the electronic
collection of toll payments These systems allow reducing
road congestion and increasing road safety Manual toll
collection causes vehicles to pile up in queues at collection
stations, hampering free flow of vehicles [1] The ETC
systems are usually developed based on RFID
technologies RFID systems are composed of at least three
core components: RFID tags, RFID readers, and databases
that associate arbitrary records with tag identifying data It
is obvious that a tag antenna plays a key role in overall
RFID system performance factors because passive tags
obtain energy from the incoming radio frequency
communication signal Therefore, the tag antenna has
substantial effects on the reading distance, the overall size,
and the compatibility with the tagged object of RFID
systems Especially, for the ETC systems, RFID tags can
be detected with high accuracy even when vehicles are
moving at high speed and drivers do not want to waste time
waiting in a long queue to pay their toll Therefore the tag
antenna of ETC system needs to satisfy some requirements
as follows [2]:
• Large bandwidth to get enough data transferring velocity
• High gain, the tags can be read by the reader from big
distance
• High directivity, the antenna should have proper main
lobe width and low side-lobe level
To date, several RFID-tag antennas, e.g [3] – [5], have
been reported for the ETC systems Nuttaka Homsup
(2016) developed an ETC system by using semi-passive
RFID at frequency of 5.8 GHz in order to communicate
between the transceiver and the transponder The
performance of transponder was improved by adding a top and a bottom parasitic element with the loop antenna [3] Shunbo Zhang (2008) proposed a Slot-coupled circularly polarized square patch antenna for electronic toll collection system Wide bandwidth is obtained by using two suitable coupling slots in the ground plane of the stripline structure [4] Jae Su Jang (2013) presented a planar array antenna with special beam shaping for ETCS-RSE (Electronic Toll Collection System-Road Side Equipment) in an operating band of 5.79 – 5.85 GHz The proposed 10 x 10 array antenna achieved flat-topped radiation pattern by using Woodward and Lawson pattern synthesis [5] Also, various different types of passive RFID tag antennas have been developed including designs based on planar circular patch antennas [6], square microstrip patch antennas [7]–[9] However, the above antennas have some limitations such
as large size, complex structure and high cost
This paper presents a low-cost RFID tag antenna for the toll collection systems in UHF bands The antenna is a printed dipole, which employs meander-line with E-shaped
d ending in each-arm to achieve a compact size A modified T-matching network is used to match the impedance between the antenna and UCODE G2XM chip, and consequently, achieves a broad operational bandwidth The antenna is characterized by using the commercially available electromagnetic simulation software Ansoft High-Frequency Structure Simulator (HFSS)
2 Antenna design and characteristics
2.1 Antenna Geometry
Figure 1 illustrates the geometry of the passive RFID tag antenna, which is built on a FR4 substrate with a dielectric constant of 4.4, a loss tangent of 0.025, and a thickness of 0.8 mm The proposed tag antenna is
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constructed by two major below sections:
• Two capacitive loadings at the end of antenna arms
for the antenna miniaturization
• A T-matching network in the feeding structure for the
impedance matching with the complex impedance of the
tag chip
Lf
Wb1
Lb1 Ws1
g1
Wb2 Ls1 Rt1
Lsub
Wsub
Lb2
Lt1
Wt Lb1
Lt
Wff
Wf Lf
Lm
Chip Sm Wm Ht
Figure 1 The geometry of the proposed antenna with
T-match network
After the optimization, the designed antenna has the
dimensions as follows: W sub = 20 mm, L sub = 70 mm,
W b1 = W b2 = 3 mm, L b1 = L b2 = 18 mm, L s1 = L s2 = 10 mm,
W s1 = W s2 = 0.4 mm, g 1 = g 2 =1 mm, R t1 = 4 mm,
W t = 1 mm, L t1 = 6 mm, W f = 0.4 mm, L f = 15 mm,
L t = 2.5 mm, W m = 0.5 mm, S m = 0.6 mm, and W ff = 0.6mm
2.2 Antenna Miniaturization
As mentioned above, the proposed antenna employs
meander-line with E-shaped d ending in each-arm to
achieve a compact size For better understanding this issue,
this subsection undertakes an analysis of the current on the
dipole with meander-line with E-shaped ending, which can
be considered as the capacitive loading The Figure 2
illustrates a dipole antenna with two capacitive loadings at
the end of each arm
I0
l/2 L/2
ltd
~
Figure 2 Dipole antenna with capacitive load at
the end of two arms
Because of this capacitive load, the terminal impedance
has finite value, the terminal current is not zero, which
means that 3the current distribution will be similar to the
case where the terminal is extended by one segment Then
the current distribution function is determined by the
following formula [10]:
I(z) = Iccoskz + iUc
I c , U c: Current, voltage at the end of the arm
ρ: Wave impedance
z: The distance from the point of view from the end of
the arm
With,
𝑈𝑐= 𝐼𝑐
𝑖𝜔𝐶
hen,
C: Capacitance of the load
If we put:
𝜔𝐶𝜌
Then
The 𝑙
2 of an arm with capacitive load is replaced by an arm without load having the length as follows:
2= l
2+θ
k
So the greater the capacitance of the load, the longer the arm length is The geometry of the proposed antenna without T-matching network is shown in Figure 3 Two capacitive loadings have the shape of T letter
Figure 3 The geometry of the proposed antenna without
T-match network
2.3 T-Matching Network
Because of the complex impedance of the tag chip, a T-matching network loaded with a meander line is used in the feed to obtain the impedance matching The main role
of an impedance matching is to force load impedance being nearly equal the complex conjugate of the source impedance and maximum power can be transferred to the load The reactance between source impedance and load impedance reduces the current and dissipates the power in the load To restore the dissipation to the maximum that occurs when Rs is equal to RL, the reactance of the loop must be zero It means that the load and source are made to
be complex conjugates one another, so they have the same real parts and opposite type reactive parts The input impedance of the antenna can be adjusted by using the modified T-matching network in the feeding structure The dimension of T-matching network is reduced by using the meander lines
IC Model
Rchip
Cchip Antenna
Antenna
Resonant Inductor
Figure 4 The UCODE G2XM application model
The UCODE G2XM chip, which has an input impedance of 24 –j195 (Ω) at 915 MHz is selected to attach
to the proposed antenna So, the input impedance of the proposed antenna must be approximately 24 + j195 (Ω) for
Trang 314 Thi Ngoc Hien-Doan, Van Khang-Nguyen, Ngoc Chien-Dao
the complex conjugate matching between the tag chip and
the antenna The UCODE G2XM application model is
shown in Figure 4, the input capacitance and resistance are
in parallel [11]
The input impedance of the antenna can be adjusted by
using the modified T-matching network in the feeding
structure
3 Simulated results
The simulated input impedances of the antenna are
illustrated in Figure 5 Good conjugate matching between
the input impedances of the antenna and the UCODE
G2XM chip is received The resistance and reactance
components of the input impedance are fairly close to those
of the chip in the 890–925 MHz range It has value of
22+j202 (Ω) at 915MHz
0.70 0.75 0.80 0.85 0.90 0.95
0
40
80
120
160
200
240
280
Frequency (MHz)
Tag chip reactance Simulated reactance Tag chip resistance Simulated resistance
Figure 5 The simulated input impedances
of the proposed antenna
Figure 6 illustrates the geometry of T-matching
network As mentioned above, the input impedance of the
antenna is easily adjusted by using the modified
T-matching network in the feeding structure This is
observed in Figure 7, which shows the input impedance of
the antenna for different values of L m
Lt
Wff
Wf Lf
Lm Chip Sm
Wm Ht
Figure 6 The geometry of T-matching network
As shown in Figure 7, the input reactance significantly
increases with an increment of Lm of the meander line
whereas the input resistance is changed insignificantly
The |S11| value is determined by using the basic
formula as follows:
S11= −20log |Za −Zc∗
Za+Zc|
where Za and Zc are the input impedances of the antenna
and the tag chip, respectively
The simulated input impedance of the antenna is shown
in Figure 8 The simulated results yield a –10-dB reflection coefficient bandwidth of 250 MHz (800-1050 MHz)
0 20 40 60 80 100 120
Frequency (MHz)
Lm=2.3 mm Lm=3.0 mm Lm=3.5 mm
100 120 140 160 180 200 220 240 260 280 300
Frequency (MHz)
Lm= 2.3 mm Lm= 3.0 mm Lm= 3.5 mm
Figure 7 Simulated input impedances of the proposed antenna
with different value of L m
0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 -18
-16 -14 -12 -10 -8 -6 -4 -2 0 2
Frequency (MHz)
Figure 8 The simulated reflection coefficients
of the proposed antenna
Figure 9 shows the simulated radiation patterns of the proposed antenna in the E-plane and H-plane It is observed that the antenna yields a nearly perfect omnidirectional pattern This is further confirmed in Figure 10, which illustrates the 3D radiation pattern of the antenna It has a gain of 1.3 dB at the frequency of 915MHz
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-30
-25
-20
-15
-10
-5
0
5
0 30
60
90
120
150 180
210 240
270
300
330
-30
-25
-20
-15
-10
-5
0
5
(dBic)
(a) E plane
-30
-25
-20
-15
-10
-5
0
5
0 30
60
90
120
150 180
210 240
270
300
330
-30
-25
-20
-15
-10
-5
0
5
(dBic)
(b) H plane
Figure 9 Simulated radiation pattern at 915 MHz:
(a) E plane and (b) H plane
Figure 10 The 3D radiation patterns of the proposed antenna
4 Conclusions
A compact, UHF–RFID tag antenna has been designed and simulated Two capacitive loadings are applied at the end of two arms of proposed antenna for the antenna miniaturization A meander-line T-match has been used to conjugate matching for the desired input resistance and reactance by tuning design parameters The optimized structure achieves a good antenna characteristic; its impedance bandwidth is 250 MHz (800–1050 MHz) Due
to flow-cost, compact size, broadband, easy fabrication, the proposed antenna can be widely used in RFID applications This antenna structure solves the disadvantages of the previously introduced antennas in section 1
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[11] UCODE G2XM Datasheet
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(The Board of Editors received the paper on 11/09/2017, its review was completed on 13/10/2017)