A Slotted Circularly-polarized Patch Antenna for Near-field and Far-field UHF RFID Applications

Chean Khan Goh1, Xianming Qing1, and Zhi Ning Chen1,2

1Institute for Infocomm Research, Singapore, 1 Fusionopolis Way, #21-01 Connexis, South Tower, Singapore 138632 Email: {ckgoh, qingxm, chenzn}@i2r.a-star.edu.sg

2Department of Electrical and Computer, National University of Singapore, 4 Engineering Drive 3, Singapore117583 Email: eleczn@nus.edu.sg

Abstract—A slotted patch antenna is proposed for near-field and far-field ultra high frequency (UHF) radio frequency identification (RFID) applications. By slotting the patch, the antenna is able to produce strong and uniform magnetic field in the near field and generate the circularly-polarized radiation in the far field. The fabricated antenna (90 mm × 90 mm × 4.5 mm) with 10-dB return loss and 3-dBic gain from 913 MHz to 924 MHz, achieves a 100% reading rate for arbitrary oriented near-field tag over a distance of 35 mm and offers a far-field tag reading up to 3.6 m. The antenna is suitable for portable near-field and far-field UHF RFID readers.

I. INTRODUCTION

The requirements of reader antenna for the near-field radio frequency identification (RFID) systems are very different from those of the far-field RFID systems. In a near-field RFID system, the reader antenna should provide strong and uniform magnetic/electric field across an interrogation zone [1]. An antenna in the far-field RFID system, however, should generate circularly-polarized radiation to detect arbitrary-placed RFID tags [2]. From a system point of view, an antenna that features uniform magnetic field near-field distribution and yet exhibits circularly-polarized far-field radiation is preferable. Such an antenna offers more configuration freedom, simplifies system complexity and reduces production cost. Some near-field/far-field ultra high frequency (UHF) RFID antennas have been reported while the performance is not desired, which are unable to achieve circularly polarization radiation in the far field and produce strong and uniform magnetic field distribution in the near-field zone simultaneously [3-5].

In this paper, a slotted patch antenna is proposed for both near- and far-field RFID applications. The antenna is able to generate strong and uniform magnetic field in all directions (x, y, and z) in the near-field zone and produces circularly polarized radiation in the far-field zone.

II. ANTENNA CONFIGURATION

Fig. 1 shows the configuration of the proposed antenna. It consists of a square patch with a side length of a1 = 75 mm and a square ground plane with a side length of a2 = 90 mm. The antenna is designed on an RO4003C substrate (r = 3.38 and tan = 0.0027) with a thickness of h = 4.572 mm. The antenna is oriented in such a way that the upper surface of

Fig.1. Configuration of the proposed antenna, (a) top view and (b) cross-

sectional view.

the square patch lies in the xy-plane and with the center of the patch located at origin of the Cartesian coordinate. The coaxial feed is placed at the location of F1 (x1 = -9 mm, y1= 0 mm). A pair of cross-slot S1, with dimensions of l1 = 31 mm and w = 5 mm, is located along the diagonal axis D1 of the square patch. Another pair of cross-slot, with slightly smaller dimensions of l2 = 21 mm and w = 5 mm, is located at the other diagonal axis, D2, of the square patch. The S1 and S2 slots are placed in such a way that each slot’s center is at a distance d1 = 27.5 mm from the origin. A fifth cross-slot S3, with dimensions of l3 = 21 mm and l4 = 17 mm, is tilted 45 degrees from the main axis (the x- and y-axes), and located at the center of the patch. Four circular slots, C1, with radius of r1 = 3 mm, are placed on the x- and y-axes with a distance of d2 = 27 mm form the origin. Another four circular slots, C2, with radius of r2 = 4mm, are placed on the diagonal axes (the D1- and D2-axes) with a distance of d3 = 42.5 mm form the origin. A pair of slits P1 (t1 = 2.5 mm and q = 1.5 mm) and P2 (t2 = 4 mm and q = 1.5 mm) are etched at the edge of the square patch on the x- and y-axes, respectively. The slots S1 and S2 are mainly designed for

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circularly-polarized radiation and the others are designed for achieving uniform near-field distribution.

III. RESUTLS AND DISCUSSION

Fig. 2 exhibits the simulated and measured reflection coefficients of the antenna prototype. The measured |S11| is less than -10 dB from 913 MHz to 924 MHz. The 10-MHz frequency shift is resulted from the in-house fabrication and assembly tolerance.

Fig. 3 compares the simulated magnetic field distribution of a conventional circularly polarized truncated patch antenna and the proposed antenna along the x-, y-, and z-directions at 915 MHz. It is observed that the magnetic field intensity of the conventional circularly polarized patch is very weak in the z-direction compared to those in the x- and y-directions. Such weak field intensity hinders the antenna from providing reliable tag detection in a RFID system. Fig. 3b shows the near-field distribution of the proposed antenna operating at the same frequency. The introduction of slots on the square patch improves the magnetic field distribution in x- and y-directions and enhances the magnetic field intensity in the z-direction significantly. Fig. 4 shows the simulated and measured axial ratio and gain of the antenna. The proposed antenna is with less than 3-dB axial ratio from 915 MHz to 921 MHz. It features more than 3-dBic gain over above frequency band.

The proposed antenna is used as the reader antenna in a UHF RFID system to detect near-field and far-field RFID tags respectively. Using the Impinj Speedway Reader (865 – 956 MHz, 30 dBm output power), the proposed antenna is able to detect the NXP UHF near-field tags (diameter: 18 mm) up to 35 mm with a 100% reading rate in all directions. For far-field tag test, an Impinj Monza IN-22 tag (80 mm × 40 mm) is detected at a maximum distance of 3.6 m.

875 900 925 950

-20

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S11(

dB)

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Fig. 2. Simulated and measured reflection coefficients.

Fig. 3. Simulated magnetic field distribution at 915 MHz in the xy-plane, z= 0.5mm above the patch with an area of 90 mm × 90 mm, (a) conventional truncated circularly polarized patch antenna (i) Hx, (ii) Hy, and (iii) Hz; (b) the proposed antenna (i) Hx, (ii) Hy, and (iii) Hz.

850 860 870 880 890 900 910 920 930 940 950-8

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n (d

BiC

)Frequency (MHz)

Simulated Measured

0

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Axi

al R

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Fig. 4. Simulated and measured axial ratio and gain.

IV. CONCLUSION

Design of an antenna with desired near-field characteristics as well as good far-field radiation is challenging. Adding slots onto the patch antenna has been validated to be an efficient way to control near-field distribution. The proposed slotted patch antenna has demonstrated the capability for near-field and far-field UHF RFID tags detection; it is suitable for portable near-field and far-field UHF RFID systems.

REFERENCES [1] X. Qing, C. K. Goh, and Z. N. Chen, “A Broadband UHF Near-Field

RFID Antenna,” IEEE Transactions on Antennas and Propagation, vol. 58, pp. 38293838, 2010.

[2] Nasimuddin, Z. N. Chen, and X. Qing, “A Compact Circularly Polarized Cross-Shaped Slotted Microstrip Antenna,” IEEE Transactions on Antennas and Propagation, vol. 60, pp. 15841588, 2012.

[3] X. Qing, Z. N. Chen, and C. K. Goh, “A UHF near-field/far-field RFID metamaterial-inspired loop antenna,” IEEE Antennas and Propagation Society International Symposium (APSURSI), 2012 , vol., no., pp.1,2, 814 July 2012.

[4] A. L. Borja, A. Belenguer, J. Cascon, and J. R. Kelly, “A Reconfigurable Passive UHF Reader Loop Antenna for Near-Field and Far-Field RFID Applications,” IEEE Antennas and Wireless Propagation Letters, vol. 11, pp. 580583, 2012.

[5] B. Shrestha, A. Elsherbeni, and L. Ukkonen, “UHF RFID Reader Antenna for Near-Field and Far-Field Operations,” IEEE Antennas and Wireless Propagation Letters, vol. 10, pp. 12741277, 2011.

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