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870 PIERS Proceedings, Xian, China, March 2226, 2010

Universal UHF RFID Rose Reader Antenna

T. G. Abo-Elnaga1, E. A. F. Abdallah1, and H. El-Hennawy2

1

Electronics Research Institute, Cairo, Egypt2Faculty of Engineering, Ain Shams University, Cairo, Egypt

Abstract Two meandered UHF RFID reader antennas are proposed. Both antenna structuresare composed of a single loop on the top of FR4 substrate, which is meandered in the form of a roseto obtain compact size. The rose antenna is fed directly through microstrip line connecting therose with the RF input. Down the microstrip feeding line a finite ground line placed on the bottomof the substrate. The antenna input impedance is controlled by adjusting the offset length ofthe ground line. Both antennas occupy areas of radius 25 mm and 21 mm, respectively comparedto 72.5 mm for the conventional printed loop one. The measured bandwidths of the designedantennas cover the universal UHF frequency band of 840960 MHz. Other radiation properties

are found to be acceptable for both proposed antennas. Therefore, the proposed fabricatedantennas are cheap, compact and universal for UHF RFID applications reader worldwide.

1. INTRODUCTION

RFID system has been developed for several years owing to its wide range of possible applicationfields. RFID system operating at UHF frequencies has received considerable interests for variouscommercial applications, such as supply chain management or inventory control. In this regard, agreat demand of UHF RFID system is expected to replace the current position of barcode system.Reader antenna is one of the important components in RFID systems, which is used to transmitor receive signal from a tag. Most RFID systems operate at ISM frequencies, such as 13.56 MHz,2.45 GHz and 5.8 GHz, some work at UHF frequencies such as 840.5844.5 and 920.5924.5MHz in

China, 920926 MHz in Australia, 866869 and 920925 MHz in Singapore, 952955 MHz in Japanand 902928 MHz in USA, Canada, Mexico, Puerto Rica, Costa Rica, Latin America, and so on.So, the UHF RFID frequency ranges from 840.5 to 955 MHz [1]. Therefore, a universal readerantenna across the entire UHF RFID band with desired performance would be beneficial for RFIDsystem implementation and configuration, as well as cost reduction. In order to communicate inthe UHF frequency band the antenna size will be larger relative to the wavelength. In this paper,two meandered UHF RFID reader antennas are proposed in order to communicate with the tagsworking at UHF band where tag orientation is known and fixed and low power consumption isrequired in the reader system. Both antenna structures are composed of a single loop on the topof the substrate, which is meandered to obtain compact size, fed directly through microstrip lineconnecting the rose with the RF input. Down the microstrip line, a finite ground line placed on thebottom of the substrate. The antenna input impedance can be controlled by adjusting the offsetlength of the ground line. Both antennas occupy areas of radius 25 mm and 21 mm, respectively

compared to 72.5mm for the conventional printed loop one or other complicated designs withcircular polarization, which may occupy areas of 155 mm 230mm [2] or 250mm 250mm [3].The measured bandwidths of the designed antennas cover the universal UHF frequency band, whichagree well with the computed results. Other radiation properties are found to be acceptable forboth proposed antennas.

2. ANTENNA DESIGN

The Fourier series analysis for the thin-wire circular loop or its image equivalent half-loop antennaexcited with a transverse electromagnetic (TEM) mode assumed in the aperture of the coaxial lineis used for the prototype circular antenna radius prediction in free space. The antenna has radiusR, resonant wavelength o, constructed from a perfectly conducting wire of radius ai, ai R,oai 1 and o = 2/o. MATLAB code based on Equations (1)(5) [5], was built to calculate

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Progress In Electromagnetics Research Symposium Proceedings, Xian, China, March 2226, 2010 871

the input admittance using this theoretical model

Y = G +jB = I(0)/Vo) =

Io + 2

n=1

In

Vo (1)

In =jVo

o

bn

ann = 0, 1, 2, . . . (2)

an =oR

2(Kn+1 + Kn1)

n2

oRKn (3)

Kn =1

B0

ai

R

n2 (oR)

2

H0

ai

R

n2 (oR)

2

+

1

+ ln

4n2 2oR

20.5

2

n1m=0

1

2m + 1

1

2

2oR0

[2n(x) + jJ2n(x)] dx (4)

bn =1

ln (ao/ai)

B0

ai

R

(n + 1)2 (oR)

2

H0

ai

R

(n + 1)2 (oR)

2

H0aoR

(n + 1)

2

(oR)

2+ B0

aiR

(n

1)

2

(oR)

2

H0

ai

R

(n 1)2 (oR)

2

H0

ao

R

(n 1)2 (oR)

2

(5)

In Equations (1)(5), G and B are the input conductance and susceptance respectively, B0 and H0are the modified Bessel functions of the first and second kinds and order zero, 2n is the Lommel-Weber function of order 2n, J2n is the Bessel function of the first kind and order 2n, Eulers

14

0. 2 0.4 0.6 0. 8 1 1.2 1.4

-4

-2

0

2

4

6

8

10

12

R

G,B

(mS)

G

B

-6

Figure 1: Input admittance of loop antenna versusR.

101520 253035 404550 556065 707580 859095100105110-400

-300

-200

-100

0

100

200

300

400

500

600

700

800

Radius R(mm)

R,X()

R

X

Figure 2: Input impedance of loop antenna versusR at F = 900 MHz.

Figure 3: (a) Conventional printed loop antenna. (b) Rose printed loop antenna. (c) Meandered rose printedloop antenna.

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872 PIERS Proceedings, Xian, China, March 2226, 2010

constant = 0.57722 and o =

o/o = 120 with o and o are free space permeability andpermittivity, respectively. The input admittance of an antenna has R = 40.36mm, 2ai = 3.25mmand outer to inner coaxial line feed radius ratio ao/ai = 2.174 was calculated with the MATLABcode. The result in Figure 1 agreed well with the results published in [4]. The code was modified tocalculate different radii with different input impedances at the same frequency which is chosen to be900 MHz as shown in Figure 2, with ao = 3.5mm and ai = 1.52 mm for coaxial line characteristicimpedance Zc = 50 . The thin-wire circular loop with R = 96.09 mm and input impedance

Z = R + jX = 52.96 j3.73 (R and X are the input resistance and reactance, respectively) istransformed to a printed circular loop with width w = 4ai [5]. FR4 material with relative dielectricconstant r = 4.65 and thickness h = 1.5 mm is used as substrate. Microstrip line on a finite groundline is used for feeding purpose. The whole structure was optimized using the method of momentsbased IE3D Zeland software. Figure 3 shows the proposed RFID reader antennas which uses thesame feeding scheme in which a microstrip feeding line connected directly with the radiating elementon the top, beneath lied a finite ground line. Figure 3(a) shows an optimized Conventional PrintedLoop Antenna with width w = 2.73 mm, ground line with width wg = 1.46w is chosen, Lg = 75mmand radius R = 72.5 mm. Figure 3(b) shows the Rose Antenna (RA) in which the radiating loopelement is meandered to a rose figure with L1 = 19.66mm, L2 = 16.94mm, R = 25mm, andLg = 27.5 mm. Figure 3(c) shows the Meandered Rose Antenna (MRA) with L1 = 12.09mm,R = 21 mm, and Lg = 26.86mm.

3. SIMULATED AND MEASURED RESULTS

The fabricated Rose Antenna (RA) and Meandered Rose Antenna (MRA) are shown in Figures 4and 5, respectively. At the beginning, we design RA and MRA with radii 29 mm and 25 mm, re-spectively. Figure 6 shows that the measured antenna return loss departs the interested bandwidth,again RA and MRA designed with radii 25 mm and 21 mm, respectively with other dimensions asgiven in the previous section. The measured impedance bandwidth of the RA at the UHF bandreaches 22.47% (0.784370.970 GHz) at resonant frequency of 0.82625 GHz, which is broader than

(a) (b)

Figure 4: RA (a) Bottom view. (b) Top view.

(a) (b)

Figure 5: MRA (a) Bottom view. (b) Top view.

0.6 0.8 1.0 1.2 1.4

-50

-40

-30

-20

-10

0

10

Measured RA S11

Simulated RA S11

Measured MRA S11

Simulated MRA S11

S11

(dB)

Frequency (GHz)

Figure 6: Reflection coefficient S11 versus frequencyfor both RA and MRA with radii 29 mm and 25mm,respectively.

0.6 0.8 1.0 1.2 1.4-40

-30

-20

-10

0

10

S11

(dB)

Frequency (GHz )

Measured RA S11

Simulated RA S11

Measured MRA S11

Simulated MRA S11

Figure 7: Reflection coefficient S11 versus frequencyfor both RA and MRA with radii 25 mm and 21mm,respectively.

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Progress In Electromagnetics Research Symposium Proceedings, Xian, China, March 2226, 2010 873

0.8 1.0 1.2 1.4

-30

-20

-10

0

S

11(dB)

Frequency (GHz)

Lg=27.85 mm

Lg=22.85 mm

Lg=17.85 mm

Lg=12.85 mm

Lg=7.85 mm

Figure 8: RA offset ground line effect.

0.81 .0 1.2 1.4

-30

-20

-10

0

S11

(dB)

Frequency (GHz)

Lg=30.36 mm

Lg=25.36 mm

Lg=20.36 mm

Lg=15.36 mm

Lg=10.36 mm

Figure 9: MRA offset ground line effect.

Figure 10: RA radiation pattern. Figure 11: MRA radiation pattern.

the 5.35% (1.0771.136 GHz) predicted during simulation at 1.06 GHz and cover the universal UHFRFID band. For the MRA, the measured bandwidth reaches 25.25% (0.7981 GHz0.9675 GHz) atresonant frequency of 0.83625 GHz, which is broader than the 4.47% (1.0521.1 GHz) predictedduring simulation at 1.074 GHz and cover also the universal UHF RFID band as shown in Figure 7.The discrepancies are mainly because of the fabrication difficulty in precise alignment between thefinite ground line and the radiated element and the SMA connector soldering before measurement.Figures 8 and 9 show the effect of ground line on the tuning of the input impedance for both RAand MRA, respectively. The MRA radiation efficiency was found to be 73.6% with gain of 1.21 dBiand directivity of 2.54 dBi. For the RA, radiation efficiency was found to be 74.1% with gain of

1.136 dBi and directivity of 2.5527 dBi. RA and MRA radiation patterns are shown in Figures 10and 11, respectively.

4. CONCLUSIONS

A Rose Antenna (RA) and Meandered Rose Antenna (MRA) for UHF RFID reader are designedand fabricated. The measured input impedances could be controlled by the ground line length.The measured bandwidths cover the universal UHF frequency band of 840960 MHz. Therefore,the proposed fabricated antennas are simple, cheap, compact and suitable for the universal UHFRFID applications.

REFERENCES

1. Barthel, H., Regulatory status for RFID in the UHF Spectrum, EPC Global, Brussels,Belgium, March 2009.

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