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Broadband CPW-FED circularly polarized antenna with an irregular slot for 2.45 GHz RFID reader.

1. INTRODUCTION

Nowadays circular polarization is getting more attention in many applications such as radio-frequency identification (RFID), satellite navigation and wireless communication. Radio-frequency identification (RFID) technology becomes more attractive for many applications such as product tracking, access control, inventory management and telemetry. With a circularly polarized (CP) antenna, a RFID system allows flexible orientation of a reader and a tag. As we know, two orthogonal linearly-polarized modes with equal amplitude and 90[degrees] phase difference can achieve circular polarization. In recent years, various antennas are investigated to achieve circular polarization. Because of the advantages of low profile, design simplicity and robustness, many CP antennas are designed with slots [1-13]. Slot-rings [3-6], square slot [7-11], and U-shaped slot [12,13] are effective methods to achieve circular polarization. In order to simplify the feeding structure and save space, a coplanar waveguide (CPW) approach is adopted in CP antennas [14-21]. In [14, 15], CP antennas with tuning rectangular slot fed by CPW are discussed. The antenna in [15] has an over-lapped bandwidth of 17.39% (2.31 GHz-2.75 GHz) with the size of 54 x 54 [mm.sup.2]; A uniplanar trapezoidal antenna fed by CPW is presented in [16] with an impedance bandwidth of 80% and a 3 dB AR bandwidth of 8% in the size of 55 x 50 [mm.sup.2]. In [20], a regular-hexagonal slot antenna with L-shape monopole is presented. The antenna in [20] comprises an L-monopole and a regular hexagonal ring combined with a pair of inverted-L strips. The side length of the regular-hexagonal substrate is 31 mm, and the size is bigger than 2 * (31x31) [mm.sup.2]. The measured 3-dB AR bandwidth is about 50% (2.25-3.75 GHz) at 3 GHz. A circularly polarized square slot antenna is designed in [21]. The antenna consists of a square slot ground and a fork shaped feed line with an additional strip attached to the right side. To optimize the performance of the antenna, a fork shape strip, two L-shape strips, two inverted L-shape strips, two rectangle chips and two horizontally loaded strips are added. The antenna in [21] has a size of 40 x 40 [mm.sup.2] and obtains a 3 dB AR bandwidth of 36% (2112-3056 MHz) for lower band I and 9% (5292-5837 MHz) for band II. Although the performances of the antennas in [20, 21] are quite good, the structures of the antennas might be complex.

A novel wideband CP slot antenna fed by CPW is proposed in this paper. This antenna consists of an irregularly slot and an L-shaped feed line. A stair-shaped edge of the slot could improve the circular polarization of the antenna. The planar structure of the antenna makes it easy to fabricate. The impedance bandwidth of the antenna is 42% at 2.47 GHz and the 3dB AR bandwidth is 25.8% at 2.48 GHz. The RHCP gain of the antenna is about 3 dBi over the frequency band of operation.

2. ANTENNA DESIGN AND ANALYSIS

2.1. Antenna Design

The structure of the proposed antenna in this paper is shown in Fig. 1. This CP antenna is printed on a FR4 substrate with a dielectric constant of 4.4, a loss tangent of 0.02, and a thickness of 1 mm. The size of the proposed antenna is 52 x 52 [mm.sup.2], which is designed at 0.42A0 (A0 is the wavelength of 2.45 GHz in free space). The detailed dimensions of the proposed circularly polarized antenna are listed in Table 1. The antenna has an irregularly slot etched on the ground plane. An L-shaped feed line is against one edge of the slot (Fig. 1(b)). The 50-Q CPW has a strip width W = 2.2 mm and gaps width 9 = 0.8 mm between the strip and the ground plane. The stair-shaped edge of the slot could optimize the circular polarization characteristics (Table 2).

For clarifying the improvement process, three prototypes of the antenna are defined as follows (Fig. 2): Ant. I is the original antenna with an I-shaped slot and an L-shaped feed line; Ant. II has an I-shaped slot with changed dimensions; Ant. III is the final contracture which is added a stair-shaped edge of the slot on the basis of Ant. II. These three prototypes are simulated by a high frequency structure simulator (HFSS 13.0) and the detailed performances are presented in Table 2: Ant. I has no circularly polarized characteristic, while the circular polarization of Ant. II is good. Further, Ant. III has a 3 dB AR bandwidth of 650 MHz, which is much wider than that of Ant. II. It is observed that the stair-shaped edge has positive influence on circular polarization of the antenna.

The proposed antenna is designed to realize right-hand circular polarization (RHCP) in +Z direction. Fig. 3 illustrates the surface current distribution of the antenna presented in this paper. The orientation of surface currents on the ground plane is shown at 2.45 GHz as the phase changes from 0[degrees] to 270[degrees]. The current amplitude at phase = 0[degrees] is almost as same as that at phase = 180[degrees], however, their directions are opposite. It is obvious that the current flows from the a>axis to the y-axis, generating a RHCP radiation for the presented antenna.

2.2. Parameter Study

In previous section, the geometry of the proposed antenna is presented. It is of practical interest to investigate the antenna performance when some of the geometrical parameters are changed. Throughout the studies presented in this section, all other parameters that have not been mentioned are fixed to the dimensions which are listed in Table 1.

Figures 4 and 5 show the studies of ss and [DELTA] = wb - wa of the proposed antenna in this paper. When ss is reduced, the minimum frequency of VSWR is shifted downwards as the circular polarization is seen to get worse as shown in Fig. 4(b). The variation of [DELTA] in Fig. 5 shows that the circular polarization is greatly influenced by the difference between wa and wb.

As depicted in Fig. 6, the AR curves are sensitive to the variations of Gand fx. It is shown that the minimum frequency of AR is shifted downwards as G is increased. Changing the value of fx would shift the minimum frequency of AR and has a little effect on the value of the AR. The fx could be modified to make the frequency band of AR matches the frequency band of VSWR.

3. SIMULATED AND MEASURED RESULTS

The performance of the proposed antenna is simulated by a high frequency structure simulator (HFSS 13.0), and measured in anechoic chamber. The simulated and measured VSWR and AR values for the proposed antenna are given in Fig. 7. The measured impedance bandwidth (VSWR < 2) is about 42% (2.15-3.19 GHz) at 2.47 GHz. The measured 3dB axial ratio bandwidth is about 25.8% (2.072.71 GHz) at 2.48 GHz. There has been a good agreement between the simulated and measured results.

The RHCP and LHCP radiation patterns are measured in the XZ (phi = 0[degrees]) and YZ planes (phi = 90[degrees]) at the frequency of 2.45 GHz in Fig. 8. A left-hand circular polarization is considered to be the cross polarization in +Z direction. Reasonably good patterns are found both in the XZ-plane and the FZ-plane. In Fig. 9, it can be observed that the simulated radiation efficiency is quite good. The measured and simulated RHCP gain of the proposed antenna is shown in Fig. 10. The measured result varies from 2.3 dBi to 3.8 dBi.

4. CONCLUSION

In this paper, a novel wideband CP slot antenna is proposed for 2.45 GHz RFID reader. The antenna is fed by CPW with an irregular slot. A stair-shaped edge of the slot can improve the circular polarization of the antenna. The overlap bandwidth (VSWR < 2 and AR < 3 dB) of the antenna is 22.4% (2.15-2.71 GHz). And the RHCP gain in main radiation direction is over 2.3 dBi.

REFERENCES

[1.] Wang, C.-J. and C.-H. Chen, "CPW-fed stair-shaped slot antennas with circular polarization," IEEE Transactions on Antennas and Propagation, Vol. 57, No. 8, 2483-2486, August 2009.

[2.] Joseph, R. and T. Fukusako, "Circularly polarized broadband antenna with circular slot on circular ground plane," Progress In Electromagnetics Research C, Vol. 26, 205-217, 2012.

[3.] Chang, T.-N. and J.-M. Lin, "Circularly polarized antenna having two linked slot-rings," IEEE Transactions on Antennas and Propagation, Vol. 58, No. 8, 3057-3060, August 2011.

[4.] Wang, M.-Z. and F.-S. Zhang, "A circularly polarized elliptical-ring slot antenna using an L-shaped coupling strip," Progress In Electromagnetics Research Letters, Vol. 35, 29-35, 2012.

[5.] Wang, C.-C., T.-Y. Lee, and J.-S. Row, "A simple design for circularly polarized ring-slot antennas," Microwave and Optical Technology Letters, Vol. 53, No. 10, October 2011.

[6.] Zhou, S.-W., P.-H. Li, and Y. Wang, "A dual-band dual-sense circularly polarized square-ring slot antenna," Microwave and Millimeter Wave Technology (ICMMT), 1-4, May 5-8, 2012.

[7.] Lu, J.-H. and S.-F. Wang, "Planar broadband circularly polarized antenna with square slot for UHF RFID reader," IEEE Transactions on Antennas and Propagation, Vol. 61, No. 1, 45-53, January 2013.

[8.] Rezaeieh, S. A. and M. Kartal, "A new triple band circularly polarized square slot antenna design with crooked T and F-shape strips for wireless applications," Progress In Electromagnetics Research, Vol. 121, 1-18, 2011.

[9.] Sim, C.-Y.-D. and C.-J. Chi, "A slot loaded circularly polarized patch antenna for UHF RFID reader," IEEE Transactions on Antennas and Propagation, Vol. 60, No. 10, 4516-4521, October 2012.

[10.] Sze, J.-Y. and C.-C. Chang, "Circularly polarized square slot antenna with a pair of inverted-L grounded strips," IEEE Antennas and Wireless Propagation Letters, Vol. 7, 149-151, 2008.

[11.] Sze, J.-Y. and S.-P. Pan, "Design of broadband circularly polarized square slot antenna with a compact size," Progress In Electromagnetics Research, Vol. 120, 513-533, 2011.

[12.] Nayeri, P., K.-F. Lee, A. Z. Elsherbeni, and F. Yang, "Dual-band circularly polarized antennas using stacked patches with asymmetric U-slots," IEEE Antennas and Wireless Propagation Letters, Vol. 10, 492-495, 2011.

[13.] Tong, K.-F. and T.-P. Wong, "Circularly polarized U-slot antenna," IEEE Transactions on Antennas and Propagation, Vol. 55, No. 8, 2382-2385, August 2007.

[14.] Wu, F.-X., W.-M. Li, and S.-M. Zhang, "Dual-band CPW-FED circularly-polarized slot antenna for DMB/WiMAX application," Progress In Electromagnetics Research Letters, Vol. 30, 185-193, 2012.

[15.] Gosalvitr, J., C. Mahatthanajatuphat, and P. Akkaraekthalin, "A wideband circular polarization antenna with tuning rectangular slot fed by CPW," Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON), 1-4, May 2012.

[16.] Augustin, G. and T. A. Denidni, "Coplanar waveguide fed uniplanar trapezoidal antenna with linear and circular polarization," IEEE Transactions on Antennas and Propagation, Vol. 60, No. 5, 2522-2526, May 2012.

[17.] Ahmad Mashaal, O., S. K. A. Rahim, A. Y. Abdulrahman, M. I. Sabran, M. S. A. Rani, and P. S. Hall, "A coplanar waveguide fed two arm archimedean spiral slot antenna with improved bandwidth," IEEE Transactions on Antennas and Propagation, Vol. 61, No. 2, 939-943, February 2013.

[18.] Liao, W. and Q.-X. Chu, "CPW-fed square slot antenna with lightening-shaped feedline for broadband circularly polarized radiation," Progress In Electromagnetics Research Letters, Vol. 18, 61-69, 2010.

[19.] Li, W.-M., Y.-C. Jiao, and J. Huang, "A novel compact printed antenna with circularly polarized characteristic," Progress In Electromagnetics Research Letters, Vol. 28, 83-90, 2012.

[20.] Zhou, S. W., P. H. Li, Y. Wang, W. H. Feng, and Z. Q. Liu, "A CPW-fed broadband circularly polarized regular-hexagonal slot antenna with L-shape monopole," IEEE Antennas and Wireless Propagation Letters, Vol. 10, 1182-1185, 2011.

[21.] Rezaeieh, S. A., M. Abbak, and I. Akduman, "A circularly polarized square slot antenna for wireless applications," 19th Telecommunications Forum TELFOR, 955-957, Belgrade, Serbia, November 2011.

Lu Chen *, Xueshi Ren, Yingzeng Yin, and Zedong Wang

National Laboratory of Science and Technology on Antennas and Microwaves, Xidian University, Xi'an, Shaanxi 710071, People's Republic of China

Received 20 May 2013, Accepted 22 June 2013, Scheduled 27 June 2013

* Corresponding author: Lu Chen (lucksea001@163.com).

Table 1. Dimensions of the proposed antenna (Unit: mm).

G      L      W     9    t    fx   fy

52    24.1   2.2   0.8   1    20   25
fw     wa    wb     s    ss   sx   sy
1      2      6     5    3    2    7

Table 2. Comparison of characteristics of Ant. I-Ant. III.

Antenna       Bandwidth          3dB        BW (VSWR<2
             (VSWR < 2)          AR          and AR <
                                              3 dB)

Ant. I      2.11-2.25 GHz        --             --
Ant. II     2.19-2.95 GHz   2.02-2.61 GHz    420 MHz
Ant. III    2.16-3.19 GHz   2.06-2.71 GHz    550 MHz
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Article Details
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Title Annotation:coplanar waveguide; radio frequency identification
Author:Chen, Lu; Ren, Xueshi; Yin, Yingzeng; Wang, Zedong
Publication:Progress In Electromagnetics Research Letters
Article Type:Abstract
Geographic Code:9CHIN
Date:Jun 1, 2013
Words:2137
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