Broadband CPW-FED circularly polarized antenna with an irregular slot for 2.45 GHz RFID reader.
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  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  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 , a regular-hexagonal slot antenna with L-shape monopole is presented. The antenna in  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 . 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  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.
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.
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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 (email@example.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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|Title Annotation:||coplanar waveguide; radio frequency identification|
|Author:||Chen, Lu; Ren, Xueshi; Yin, Yingzeng; Wang, Zedong|
|Publication:||Progress In Electromagnetics Research Letters|
|Date:||Jun 1, 2013|
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