A Low Cost UHF RFID Dipole Antenna for Metallic

Environments Juvenal Alarcon

1, Thibaut Deleruyelle

2, Matthieu Egels

1, Philippe Pannier

1

1 Institut Matériaux Microélectronique Nanosciences de Provence IM2NP (UMR 7334) Aix-Marseille Université

Technopôle de Château Gombert, 38 rue Joliot Curie, 13013 Marseille CEDEX 20, France 2 IM2NP, ISEN-Toulon

Maison des Technologies, Place George Pompidu, 83000 Toulon, France

{ juvenal.alarcon, thibaut.deleruyelle, matthieu.egels, philippe.pannier }@im2np.fr

Abstract— This paper presents the design of a UHF RFID reader

antenna for metallic environments. The design is based on the

constructive wave phenomenon and on the use of a dipole

antenna which provides a linear polarization parallel to its

ground plane. In addition, the proposed antenna can be placed

on metallic surfaces without impedance mismatch. The overall

antenna size is 160 mm x 80 mm x 61 mm and its gain is 6.5 dBi.

INTRODUCTION I.

The antennas are important components of RFID systems

that ensure the communication between readers and

transponders. The integration of antennas in confined spaces

is a challenge in UHF RFID implementation because of their

size. Besides, the placement and orientation of tags establish

the positioning of the reader antenna. Furthermore, metallic

surfaces close to the antenna affect its behavior by reducing

the radiation efficiency and changing its input impedance.

UHF patch antennas [1] and low profile antennas [2] using

Electromagnetic Band Gap (EBG) or Artificial Magnetic

Conductor (AMC) structures are good candidates to be

installed over metallic structures. They provide linear or

circular polarization parallel to ground plane, however their

overall size is an obstacle to their implementation in confined

spaces. In addition, the cost of the substrates for obtaining

antennas with high gain is usually expensive. Other antennas

like folded dipole [3] and monopole [4] are used near metallic

structures due to the image current. Therefore, these antennas

provide a linear polarization perpendicular to ground plane

and create a non-radiating cone over these antennas.

A solution to obtaining a low cost reader antenna with

linear polarization parallel to its ground plane is proposed in

this work. Its design is based on a dipole antenna which is a

simple radiating element widely used in wireless

communications. The dipole behavior is strongly degraded

when it is near metallic surfaces. To solve this problem, the

constructive wave phenomenon is used by placing a metal

plate at one quarter wavelength that performs a reflector for

the dipole antenna. So, we can achieve a linear polarization

parallel to ground plane with a reduced size at the expense of

the antenna height.

ANTENNA DESIGN II.

The proposed antenna is designed to be used in

environments within metallic objects. The principal point is to

generate a linear polarization parallel to ground plane. A

solution is to place a dipole antenna at a given distance from a

metal plate and feed it by a coaxial cable via a microstrip line.

This antenna is composed of three parts. The first one is a

dipole antenna. The second one is the feed system with a

matching network. The third one is a metallic plate acting as a

reflector which is placed at one quarter wavelength. Figure 1

shows the topology of the antenna.

Fig. 1 Proposed UHF RFID antenna.

The proposed antenna is designed by using the FEM

simulator ANSYS HFSS. The metal plate size, the separation

between the radiating element and metal plate are optimized

as well as the matching network.

A. Radiating element

The radiating element is based on a half wavelength dipole.

It is composed of two metallic conductors on a low cost FR4

substrate (r = 4.2) with a 1.6 mm thickness. The parameters L

(length) and W (width) are optimized to achieve a good

compromise between impedance matching and gain in RFID

ETSI band. Figure 2 shows the reflection coefficient with a

parametric sweep of the dipole length.

IEEE 2012 International Conference on RFID -Technologies and Applications (RFID - TA)

978-1-4673-0328-6/12/$31.00 ©2012 IEEE 122

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L=150mm

L=155mm

L=160mm

L=165mm

L=170mm

L=175mm

L=180mm

ETSI

Fig. 2 Reflection coefficient: parametric sweep of dipole length.

The optimized dimensions are L = 155 mm (0.45 0) and

W = 5 mm (0.008 0). These values give an impedance

matching better than -15 dB, a gain of 2.1 dBi and rigid

radiating elements.

B. Feeding system

The dipole is fed by a microstrip line because the antenna

and feeding system are made in the same FR4 substrate. In

addition, the microstrip line enables a 50 unbalanced

coaxial to be connected with the balanced dipole impedance.

To connect the dipole antenna to the microstrip line, one of

the radiating elements is made on the top layer of the substrate

and connected to the microstrip line. The other one is made on

the bottom layer and connected to the ground plane as shown

in Fig.1.

For a microstrip line made on a FR4 substrate (r = 4.2)

with 1.6 mm thickness, the microstrip line width at 866 MHz

is W = 3 mm.

To minimize the ground plane of the microstrip line, a

parametric sweep of ground plane width is performed. Figure

3 shows the normalized electric field on the cross section of

the microstrip line.

0,0 0,5 1,0 1,5 2,0 2,5 3,00,0

0,1

0,2

0,3

0,4

0,5

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1,0

No

rma

lize

d E

lec

tric

fie

ld

Distance from the symmetry plane (W)

11%

3,4%

Fig. 3 Normalized electric field on the microstrip line cross section at

866 MHz.

Considering a distance W from the symmetry plane, the

electric field represents about 11 % of maximum value. At

1.5 W, the electric field becomes 3.4 % of the maximum value.

That means, for a ground plane width larger than 2 W (6 mm),

the electric field becomes less than 11 %. For ground plane

width larger than 3 W (9 mm) the electric field decreases less

than 3.4 %.

The reflection coefficient normalized for an impedance of

50 is better than -23 dB for substrate width of 2 W and

better than -27 dB for 3 W as shown in Fig.4.

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Substrate width (W) Fig. 4 Microstrip line reflection coefficient.

Then, the substrate and ground plane widths of the

microstrip line of the feed system are set to 13 mm (4.3 W)

(0.037 0). The minimal distance from microstrip line edge for

the feed system is set to 5 mm (1.6 W) (0.008 0).

Modifying the topology of the dipole as mentioned below

and placing the feed system, the new dipole is optimized.

Figure 5 depicts the reflection coefficient with a parametric

sweep of the dipole length. The new dipole length is

L = 160 mm (0.46 0) which enables the impedance matching

in the RFID ETSI band to be better than -15 dB.

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L=150mm

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L=160mm

ETSI

Fig. 5 Reflection coefficient: length dipole parametric sweep.

W/2

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C. Antenna height

To take benefit from the constructive wave phenomenon,

the dipole is placed at 0.25 on a metal plane of

x 0.5 Then, the spacing between dipole and metal plate

is adjusted. The criterion to set its height is to assume the gain

to be 5 % less than the maximum gain. Figure 6 shows the

antenna gain and HPBW in function of the antenna height.

Finally, the antenna height is set to 61 mm (0.17 0).

6,0

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Ga

in (d

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Gain

0,125 0,150 0,175 0,200 0,225 0,25060

65

70

75

80

HP

BW

(°)

Antenna height (0)

HPBW in E Plane

Fig. 6 Gain and half power beamwidth in function of the antenna height at

866 MHz.

D. Ground plane

The ground plane dimensions are reduced in order to have

minimum metallic area under the antenna. Figure 7 shows the

antenna gain in function of the ground plane size. Considering

the antenna gain to be 13 % less than the maximum gain, the

ground plane width is set to 80 mm (0.23 0).

0,0 0,2 0,4 0,6 0,8 1,05,0

5,5

6,0

6,5

7,0

7,5

8,0

5,0

5,5

6,0

6,5

7,0

7,5

8,0

Ga

in (

dB

i)

Ground plane size (0)

Gain

Fig. 7 Ground plane effect on the antenna gain at 866 MHz.

E. Matching network

In order to achieve 50 impedance bandwidth, the

matching network is composed of a microstrip feed line and a

short circuit stub. The short circuit is made on the ground

plane which is connected to the microstrip ground plane in

order to avoid the use of a metallic “via” interlayer.

Figure 8 shows the matching network topology used in the

design. This matching network is included in the design and

then it is optimized.

Fig. 8 Matching network.

MEASUREMENTS III.

To validate the proposed antenna characteristics, a

prototype is made on FR4 substrate as shown in Fig.1. A

comparison between measured and simulated reflection

coefficient is depicted in Fig.9. The measured values concord

with the simulated ones. Furthermore, the simulated

bandwidth is 48 MHz whereas the measured one is 55 MHz.

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Simulated

Measured

Measured over metal plate

ETSI

BW

Fig. 9 Reflection coefficient.

The maximum simulated gain is 6.4 dBi and the measured

one is 6.5 dBi. The half power beam width (HPBW) is 106 °

in simulation whereas in measurements it is 90 °. In addition,

the front to back ratio is about 5.1 dB. So, there is no

significant variation between simulations and measurements.

When a metal plate (1.2 0 x 1.2 0) is placed in parallel to

dipole and orthogonal to ground plane, the main effect appears

on the radiation pattern as shown in Fig.10. The measured

gain along the main direction decreases about 1.5 dB for a

metal plate placed at 100 mm from the radiating element. For

distances of 80 mm and 60 mm the gain reduction is

respectively 3 dB and 5 dB, due to the tilt of the antenna beam.

The beam tilt is not really important for this specific

application because it can be replaced to have a higher

coverage zone.

Another effect of the metal plate is when the antenna is

mounted on it. For instance, on a metal plate of 1.2 0 x 1.2 0

Short circuited stub

To SMA Connector

To dipole

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the antenna gain increases about 2.5 dBi and the reflection

coefficient is slightly affected in the RFID ETSI band as

shown in Fig.9.

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Measured d=60mm

Measured d=80mm

Measured d=100mm

Simulated d=60mm

Simulated d=80mm

Simulated d=100mm

Simulated H plane

Measured H plane

Ga

in p

att

ern

(d

Bi)

Fig. 10 Antenna gain pattern.

Table I summarizes the electrical characteristics of the

antenna prototype and compares the measured values with the

simulated ones.

TABLE I

READING RANGE ON DIFFERENT OBJECTS.

Simulations Measurements Units

Gain 6.4 6.5 dBi

BW (-10 dB) 48 55 MHz

HPBW 106 90 °

Front/Back 5.5 5.1 dB

CONCLUSIONS IV.

In this paper the design of a low cost antenna for metallic

environments has been presented. A traditional dipole antenna

as radiating element was used and the constructive wave

phenomenon was applied. The proposed antenna performs

well in the ETSI band. It has an impedance bandwidth of

55 MHz, a HPBW of 90 °, a gain of 6.5 dBi and a linear

polarization parallel to the ground plane. According to the

measurements, the metal plate effect under the antenna is

positive when increasing the antenna gain. The proposed

antenna could be tuned to broaden the impedance bandwidth

and to cover the ETSI and the FCC UHF RFID frequency

bands.

REFERENCES

[1] J.R. Sanford, A Novel RFID Reader Antenna for Extreme

Environments, IEEE Antennas and Propagation Society International

Symposium (2008), 1-4. [2] J. Alarcon, M. Egels, P. Pannier, A Low Profile Circularly Polarized

Antenna for UHF RFID Readers, IEEE RFID-TA (2011), 469-472.

[3] B. Josephson, The Quarter-Wave Dipole, WESCON (1957), 77-90. [4] S. Esfandiarpour, H.R. Hassani, A. Frotanpour, A Dual-Band

Circularly Polarized Monopole Antenna for WLAN Applications,

IEEE EUCAP (2011), 346-349.

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