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Broadband Circularly Polarized Microstrip Antenna for RFID Reader

Applications

Yan Shan Boo#1, Nasimuddin*2, Z. N. Chen*3, and A. Alphones#4 #

School of EEE, Nanyang Technological University, Singapore*

RF and Optical Department, Institute for Infocomm Research1 Fusionopolis Way, #21-01 Singapore

[email protected]

[email protected]

[email protected]

[email protected]

Abstract — A broadband single-feed compact circularly polarized

stacked microstrip antenna for radio frequency identification

(RFID) reader applications is proposed and investigated. The

proposed antenna achieved a gain of more than 5.0 dBic,impedance bandwidth with VSWR less than 2 and a wide 3-dB

axial-ratio (AR) bandwidth of more than 12% over ultra-high

frequency (UHF) band (840 – 940 MHz). The overall antenna

size is 0.48λo × 0.48λo × 0.0984λo at 900 MHz. The proposed

antenna is attractive for RFID reader applications which require

compact size and good circular polarization operation.

Index terms – Circular polarization, RFID, microstrip

antenna, UHF

I. I NTRODUCTION

RFID technology utilizes the characteristics of microstrip

antennas and is used extensively in service industries,

distribution logistics and even transport systems [1, 2].

However, there is a standard UHF range accepted

internationally for RFID applications and different countriesuses different frequency ranges [3, 4]: 865.5 – 867.6 MHz in

Europe, 920 – 926 MHz in Australia, 866 – 869 MHz, 923 –

925 MHz in Singapore, 902 – 928 MHz in US/Canada, 910 –

914 MHz in Korea and 952 – 955 MHz in Japan. The total

frequency span of UHF band used for RFID systems is 840 -

960 MHz. A RFID reader that is able to accommodate theentire RFID frequency range provides better flexibility and

interoperability between devices and a reader antenna with a

smaller size reduces the manufacturing cost. However, these

two properties are dependent on each other. The drawback to

a reduced antenna size is limited bandwidth and gain. Hence,

it is necessary to make a trade off between size and bandwidth.In this paper, a single-feed compact broadband circularly

polarized stacked microstrip antenna for RFID reader

applications is presented. The main patch is fabricated on

thick FR4 substrate to enhance the axial-ratio (AR) and

impedance bandwidths of the antenna. A broadband circularly

polarized stacked microstrip antenna is designed, fabricatedand tested. The measured and simulated results are in good

agreement. The proposed antenna design and optimization is

carried out with IE3D EM simulator.

II. A NTENNA GEOMETRY AND DESIGN The geometry of the proposed antenna is as shown in Fig. 1.

A coaxial-probe is used as the feeding structure of the antenna.

To achieve a wide bandwidth, the upper patch (114.0 mm ×

104.0 mm) on thin FR4 substrate with foam and the lower

patch (86.0 mm × 77.5 mm) is fabricated on thick FR4

substrate (εr = 4.3, tanδ = 0.02). The patch is fed at an

optimum feed location to radiate wide and good CP waves.The antenna designed dimensions are given in Table I, and all

are in mm.

Fig. 1(a). Cross-section view of the proposed antenna.

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Table I. Antenna designed dimensions (in mm)

Fig. 1(b). Fabricated prototype antenna.

The challenges of the proposed RFID reader antenna require

good impedance matching, wide axial-ratio bandwidth and

high gain with compact size and low cost. There are many

ways of implementing circular polarization. In this proposed

design, circular polarization is achieved by introducing anoffset feed in the antenna at an angle around of 45º using a

coaxial probe [5]. The exact location of the probe is

determined by varying the probe along an arc to locate the

optimum point between x- y axes.

The coaxial-probe location is first determined as one fourth

of the upper patch length. The coaxial-probe location is then

simulated along the x-axis to obtain the best impedance

matching with the load. Thereafter, the probe location is

simulated along the xy-axis with varying angles to find the

optimum circularly polarized performance [5] at which the

good impedance matching of the antenna within the operating

frequency.

Table II: Effects of the probe location on AR and 3dB AR bandwidth.

The x-coordinate of the probe location is determined as

41.0 mm from the origin. The probe location at 35o

from the

x-axis is chosen as it provided a broader bandwidth compared to the others. The 3-dB AR bandwidth is also wide over the

frequency band from 850 MHz to 950 MHz. The final probe

location (origin at centre of the patch) achieved is at xo =

33.58 mm, and yo = 23.51 mm for wide circular polarization

radiation as shown in Table II.

III. R ESULTS AND DISCUSSIONS

Fig. 2 shows the measured and simulated VSWR with

frequency of the antenna. The measured bandwidth with

VSWR of 2 is around 141 MHz (803 – 944 MHz). The

simulated VSWR bandwidth from IE3D simulator is 138 MHz

(805 – 943 MHz). The measured return loss of the antenna isin good agreement with the return loss results obtained from

the simulation.

0.80 0.85 0.90 0.95 1.001.0

1.5

2.0

2.5

3.0

3.5

4.0

V S W R

Frequency, GHz

Measured

Simulated

Fig. 2. Simulated and measured return loss.

0.80 0.85 0.90 0.95 1.00

0

5

10

15

20

25

30

A x i a l - r a t i o ,

d B

Frequency, GHz

Measured

Simulated

Fig. 3. Simulated and measured axial-ratio at boresight

The AR is measured using spinning linear method where a

rotating linearly polarized transmit horn antenna is used to

measure the CP performance of the antenna. The measured

data is then stored electronically and post-processed to

determine the axial-ratio and gain of the antenna. From Fig. 3,

the measured 3-dB AR bandwidth is more than 12%. The AR

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bandwidth of the antenna is able to cover total UHF RFID

band. However, impedance bandwidth (2.0 VSWR) is not ableto cover total UHF RFID band. The measured axial-ratio is

achieved better than the simulated axial-ratio results from the

frequency range of 843 MHz to 950 MHz. There is a slight

shift in frequency between the measured and simulated results

which could be due to fabrication tolerances.

0.80 0.85 0.90 0.95 1.000

1

2

3

4

5

6

7

8

9

10

G a i n ( d B i c )

Frequency, GHz

Measured

Simulated

Fig. 4. Simulated and measured gain at boresight.

Fig. 4 shows the measured and simulated gain at boresight.

From the results, the measured maximum gain is around 7.2

dBic at 912 MHz. The measured gain is more than 5 dBic

over the frequency range from 840 MHz to 960 MHz. A goodagreement is achieved between measured and simulated gain

values. The normalized radiation patterns of the antenna at

865 MHz, 900 MHz, 920 MHz and 960 MHz are shown in

Figs. 5(a)-5(d), respectively for both x-z and y-z planes. The

3-dB AR beamwidth is more than 100o

over the 3-dB axial-

ratio bandwidth.

(a)

(b)

627




(c)

(d)

Fig. 5. Measured patterns of the antenna; (a) 865 MHz, (b) 900 MHz,(c) 920 MHz and (d) 960 MHz for both planes ( x-z and y-z planes).

IV. CONCLUSION

A broadband circularly polarized compact stacked

microstrip antenna has been investigated for UHF RFIDapplications. The proposed antenna is able to cover UHF band

of 840 – 850 MHz with VSWR of 2 and less than 3-dB axial-

ratio. The simulated and measured results match well except

at few points which are due to the fabrication errors. The

antenna can be re-optimized to improve the impedance

matching for full UHF RFID frequency band.

ACKNOWLEDGMENT The authors wish to thank Mr Tat Meng, for fabrication of

the prototype antenna.

R EFERENCES

[1] R. Waterhouse , ‘Microstrip Patch Antennas’, Handbook of Antennas in Wireless Communication, edited by Lal ChandGodara, CRC Press, 2002, section 6.1.

[2] K. Finkenzeller, ‘RFID Handbook’, Wiley, New York, 2003,

2nd edition.[3] H. L. Chung, X. Qing, and Z. N. Chen, “A Broadband

Circularly Polarized Stacked Probe-Fed Patch Antenna for UHF RFID Applications,” International Journal of Antennas

and Propagation, 2007, ID76793, 8 pages.[4] Z. N. Chen, X. Qing, and H. L. Chung, “A universal UHF

RFID reader antenna,” IEEE Trans. Microw. Theory Tech., Vol.

57, No. 5, May 2009, pp. 1275-1282.

[5] Nasimuddin, “Design of wideband circularly polarized stackedmicrostrip antennas with dielectric cover using single coaxialfeed”, Microwave and Optical Technology Letters, 49, 3027– 3033, 2007.

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