Design of Circular Polarization Microstrip Antenna in RFID Reader

for 5.8GHz Electronic Toll Collection Application

Bin Lua, Peng Li

IInstitute of Inforrnation Engineering, Nanchang University, Jiangxi Province, P.R.China 2Institute of Computer Engineering, Nanchang University, Jiangxi Province, P.R.China

In this paper, the design indices of the antenna used in Electronic Toll Collection submitted to the national specifications of China

have been drawn out and then a right-hand circular polarization microstrip array antenna subject to the specification is presented.

The simulation result obtained by ADS software show that the performances of the presented antenna meet the needs of system very

well.

Index Terms-ETC, RHCP, Microstrip Antenna, ADS Software

1. INTRODUCTION

Electronic Toll Collection(ETC) system is a kind of modem traffic toll collection means based on RFID technology[ 1]. By using the microwave dedicated short range communication (DSRC) operating at 5.8GHz between the tag or so-called on board unit(OBU), which are installed on the front window of vehicles and the reader or road side unit(RSU) on the ETC road of toll collection station, the passing vehicles' toll could be collected automatically without stopping. The time spent on each vehicle is less than two second and the ability for vehicles' passing of ETC passage is five to ten times of manual toll collection passage. For these reasons, ETC system has been developed and spread rapidly in recent years at home and abroad.

On March 2007, the national specifications of China on ETC&DSRC, GB/T20851-2007[2], had been published. Some needs on the reader's antenna have been defined in it. According to the specifications, the microstrip antenna array is the leading choice for the RSU because of its low cost, low profile and ease of fabrication[3] as well as the gain of the array is easy to meet the demands.

In this paper, the design indices about the reader's antenna are listed and discussed, then a microstrip array antenna

resonating at 5.8GHz and providing a peak gain of about 15dBi at center frequency is developed. It radiates a right­

hand circular polarization beam with half-power lobe-width

18° in the horizon plane and 34° in the vertical plane. And the

lOdB return 10ss(RL) impedance bandwidth and 3dB axial

ratio bandwidth of the antenna is greater than 200MHz.

II. DESIGN INDICES AND METHOD

A. Design Indices

In accordance to the national specifications, the same right­hand circular polarization(RHCP) antenna is used in the RSU for either transmitting or receiving signal. Some concerned parameters of the antenna are listed in Table 1.

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TABLE I SOME CONCERNED PARAMETERS ON READER'S ANTENNA OF PHYSICAL LAYER IN THE ETC-DSRC NATIONAL SPECIFICATIONS OF CHINA

Parameter Downlink Carrier Cl Frequency C2 Occupied bandwidth

Antenna's Horizon half-power lobe width Vertical

Polarization

XPD I Peak gain direction

I -3dB domain Uplink Carrier Cl Frequency C2 Occupied bandwidth

Type A TypeB

5.83GHz 5.83GHz 5.84GHz 5.84GHz <5MHz <5MHz 8<38° 8<38°

\0<45° \0<45°

Right-hand CP Right-hand CP RSUt>15dB RSUt>15dB RSUt>IOdB RSUt>IOdB

5.79GHz 5.79GHz 5.80GHz 5.80GHz <5MHz <5MHz

TABLE II DESIGN INDICES OF THE READER'S ANTENNA Parameter Requirement Type Microstrip Array Antenna Center Frequency 5.82GHz

Bandwidth Cover the range of 5.785GHz-5.845GHz

Gain >14dBi Antenna's I horizon <38° half-power I vertical <45° lobe width Polarization Right-hand CP

XPD I Peak gain direction RSUt>15dB I -3dB domain RSUCIOdB

According to the Table, (), the transmlttmg-recelvmg antenna's half-power lobe width in the horizon plane is less

than 38° and rp that in the vertical plane less than 45° , and so

the minimum gain of the antenna could be estimated by the following forrnula[ 4].

Gain � 41253 = 41253 = 13.8dBi (kp 38x45

(1)

From Eq.l and Table I, the design indices of the antenna are drawn out and listed in detail as Table II.

B. Single Element

Figure I. The geometry of single element

In this paper, single-feed square patch is chosen as radiating element to generate right-hand circular polarization wave. There are several structures to attain the goal[5], such as the truncated comer structure applied in this paper as shown in Fig.1, where the middle point L at the side denotes the feed point location to generate LHCP wave and R the feed point of RHCP wave. And a and l1a are the dimension parameters of the patch shown in the figure. To obtain them, the total quality factor Q should be found firstly. The total quality factor is usually defined according to the quality factors related with various kinds of losses in the patch antenna.

1 1 1 1 1 - = - + - + - + --

Q Qr Qd Qc Qsw (2)

where Qn Qd, Qc and Qsw are quality factors relative to radiation loss, dielectric loss, conductor loss and surface wave loss, respectively. In general, a kind of substrate material with low permittivity, low loss tangent and thin thickness is chosen so that Qr is much lesser than Qd, Qc and Qsw for a good radiation efficiency. Therefore, the quality factor Q is approximately equal to radiation quality factor, Qn and Qr could be estimated by (3) [5].

c.j;;: Q;::: Qr;:::

4hfo (3)

where Cr and h denotes relative permittivity and thickness of the substrate, and fo denotes the resonant frequency.

Having obtained the quality factor, the geometry of square patch with the truncated comer could be calculated by (4) and (5).

c (4)

a l1a = �2

Q (5)

Another important parameter, the input impedance at the feed point location could be obtained by Cavity Model Method[3][6], or using simulation software such as Agilent ADS software.

C. Sequential Feeding Network/or the Array

To improve the performance of the antenna such as gain and AR bandwidth, sequential feeding network technology[7] is applied for the application with the four same CP patch antennas arranged as shown in Fig.2(a), where all patched are placed at same spacing at vertical or horizon direction, FJ, Fz, F 3 and F 4 are feed point where each of the elements is excited in the same amplitude and 900 phase delay in an anti-clockwise sequence. There are several concrete implementations such as parallel feeding network, series feeding network and hybrid ring parallel feeding network[8]. Among these, the parallel feeding network is applied in this paper for the simplicity and ease of implementation shown as Fig 2(b), where Zp is the input impedance of the element and Zin the impedance of the feeder. Meanwhile AgO is the waveguide wavelength of the micros trip transmission line and h,lz are the adjustable lengths to fit the position of the feed points.

180·

270·

(a)

'n F_"_�:--_.....,...--: __ ro F! I I

90·

Zp,i.gol4+i, 7 ... ,1,

JZpoZ .. /Z,i.gr/4 I (0 Feeder

'0 F..;,, -:-_"'-_--:-__ to Fl

1 Zp.J.gr/4+"

(b)

Figure 2. The sequential feeding network for the truncated comer single-feed microstrip patch array antenna (a)the feed locations of the sub-array composed by the four elements; (b) the parallel sequential feeding network applied here.

III. EXAMPLE AND SIMULATION RESULTS

A practical design, an eight-elements microstrip array antenna was implemented on the PTFE substrate with relative permittivity Cr =2.55, thickness h =lmm and loss tangent tane) =0.0015. The realistic geometry of array and the feed network is shown in Fig.3.The geometry of the single patch element a and l1a are set to 15.5mm and 2mm, respectively, calculated by (2)�(5) and optimized using ADS software. The array could be divided into two sub-arrays that each of them is fed by the sequential feeding network described in Section II and there is 1800 out of phase between the two sub-arrays. The input impedance of the array is carefully matched to 50n at the center frequency.

FigA shows the amplitude of the RL at the range of 5.10Hz to 6.50Hz simulated by ADS software. The minimum

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RL, -29.4dB, is reached at the center frequency 5.S2GHz and the return loss are both lower than -20 dB at the edge frequencies 5.7SGHz and 5.S5GHz. The lOdB return loss impedance bandwidth is about 400MHz ranged from 5.6GHz to 6.0GHz. The axial ratio(AR) and the gain of the array at different frequencies are shown in Fig.5. Hence, the gain is greater than 14dBi ranged from 5.72GHz to 5.91GHz and 3dB AR bandwidth is approximately 2S0MHz ranged from to 5.7GHz to 5.9SGHz.

50Q

Figure 3. The geometry of the 8-elements microstrip array antenna and the feeding network

511

-5

P ·10 ;;;-; ·15 "

tj -lll �

-25

-JO 5 .0 52 5 .' 5a 58 &.'

"'''''''OCY

Figure 4. The return loss of the array

The radiation patterns are shown in Fig.6 at <p=0° corresponding to vertical plane and at <p=90° corresponding to horizon plane at center frequency 5.S2GHz. As the result, the half-power lobe width of the vertical plane is about 34° and that of horizon plane is ISo thus both lobe widths are less than the values listed in Table II.

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AA(dO( GainldB]

FleqlGHz] Fleq[GHzl

Figure 5. The axial ratio and gain of the array

The radiation patterns are shown in Fig.6 at ip=O° corresponding to vertical plane and at ip=90° corresponding to horizon plane at center frequency 5.S2GHz. As the result, the half-power lobe width of the vertical plane is about 34° and that of horizon plane is ISo thus both lobe widths are less than the values listed in Table II.

The graph in Fig.7 illustrates that the array radiates RHCP beam with the cross-polarization discrimination(XPD) about 19.6dB (approximately equal to 19.797-0.245) at maximum gain direction and 19.1dB at -3dB gain direction, greater than 15dB for peak gain direction and lOdB for -3dB gain direction demanded in Table II.

"-.;, :::

·10 ·20 ·30 AD ·50 ·60

g

lHETA Ma< THETA�1. oo0 1 O'I og1 O(real(Gain)); 14.983

L THETA�16.000 10'1 0\I 10(real(Gai n)); 12 .715

R THETA ;14.000 1 O'Iog1 O(real(Ga i n));12.702

(a)

Ge j n atF8 � �,-------���--�-.

lH ETA Ma< THETA ;2. 000 1 0'log10( real(Gain));1 5 . 147

L THETA�7. oo0 10' l og10(real(Gain));11.419

R THETA;11.oo0 1 O' i og 1 O(real(Gain)); 11. 777

(b)

Figure 6. The radiation pattern at center frequency 5.82GHz (a) <p=0° (b) <p=90°

ig g � � 0 � 0

(a) (b)

Figure 7. The cross-polarization discrimination at frequency 5.82GHz (a) <p=0° (b) <p=90°

In summary, a comparison between the design indices and the simulation results of the example is held and the comparative results are listed in Table III and it implies clearly the design satisfies the needs in the national specifications of China on ETC&DSRC.

TABLE III COMPARISON OF DESIGN INDICES AND THE EXAMPLE The result of

Parameter Requirement the example meets the demands?

Type Microstrip Array Yes Antenna

Center Frequency 5.82GHz Yes

Bandwidth Cover the range from Yes 5.785GHz to 5.845GHz

Gain 2>14dBi Yes Antenna's I horizon <380 180 half-power

I vertical <450 340 lobe width Polarization RHCP RHCP

Peak gain RSUt::>15dB 19.6dB at center

XPD direction frequency

RSUt::>IOdB 19.1dB at center -3dB domain

frequency

IV. CONCLUSION

In this paper, the design indices on the 5.80Hz ETC reader's antenna are drawn out from the national specifications of China on ETC&DSRC and then an eight elements microstrip array antenna fed by sequential feeding network is developed here which the perfermance meets the requirements of the specifications.

REFERENCES

(I] Andrew T. W. Pickford, and Philip T. Blythe, Road User Charging and

Electronic Toll Collection, Artech House, Inc. Oct. 2006 [2] The national specifications of P.R. China on ETC&DSRC, GBIT2085/-

2007, Part I [3] Kai Chang, Encyclopedia of RF and Microwace Engineering. A John

Wiley and Sons, Inc. pp. 2580-2602, 2005. [4] John D. Kraus, and Ronald J. Marhefka, Antenna for all application, 3rd

ed. , McGraw-Hill Company, 2002 [5] R. Garg, P.Bhartia, I. Bahl, and A. Ittipiboon, Microstrip Antenna Design

Handbook, Artech House, Inc. pp. 493-530, 2001 [6] Wanzhen Lu, Antenna theory and techniques, Xidian University Press,

May 2004 [7] P.S.Hall, "Application of sequential feeding to wide bandwidth,

circularly polarised microstrip patch arrays", lEE Proceedings,Vo1.l36,

October 1989 [8] Shing-Iung StevenYang, RickyChair, AhmedA.Kishk, Kai-FongLee and

Kwai-ManLuk, "Study on Sequential Feeding Networks for Subarrays of Circularly Polarized Elliptical Dielectric Resonator Antenna", IEEE

Trans. Antennas Propag., Vo1.55, NO.2, pp.321-333, Feb.2007.

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