Integrated compact circular polarization annular ring slot antenna design for RFID reader.
Radio frequency identification (RFID) mainly consists of a reader and a tag. For the reader antenna, circular polarization (CP) is preferred because it can reduce the loss caused by the multi-path effects between the reader and the tag antenna [1-3]. In addition, a compact size is often required because of the limited space in the portable devices.
CP can be realized by the perturbation method or the orthogonal feed technique. CP is formed by the perturbation method in [4-6]. By introducing asymmetry in the ring slot and arranging the feeding point in , CP is realized for the annular and rectangular ring slot antennas. The CP is realized by selecting a proper length of the shorted section in the ring slot in . In , CP is realized by placing a metal strip across the diagonal line of the aperture. However, the axial ratio bandwidth is very narrow using this method. To address this issue, power divider or coupler, which generates the same amplitude but 90[degrees] phase differences at the two output port, is used to feed the antenna to obtain wide AR bandwidth in [1,7-9]. Wilkson power divider is place inside the rectangular ring antenna to obtain CP in  and an additional matching network is needed for this method. Dual-Fed method is used to form CP in . Circular ring slot antenna is fed by stripline hybrid coupler in . Since multilayer structure is used in , it is not easy to fabricate. Patch hybrid coupler is integrated with the rectangular ring antenna in  to achieve a compact circular polarized antenna, but the radiation bandwidth is narrow for this design. In addition, because of the compact size of the ring and ring slot antenna, it has been widely used for different antenna designs such as UWB  and dual band applications [11-13].
In this paper, annular ring slot antenna is integrated with the hybrid patch coupler to radiate the circular polarized wave. Meanwhile, because hybrid patch coupler is at the bottom of the annular ring slot antenna, the inner ground of the ring slot antenna can be considered as the ground plane of the hybrid patch coupler. With this novel configuration, CP is realized with broad bandwidth. In addition, the antenna size is not affected by the additional patch coupler and a very compact structure is obtained. Both the impedance bandwidth and axial ratio bandwidth of the proposed antenna almost cover the entire UHF band for RFID system.
2. THE PROPOSED ANTENNA DESIGN
As shown in Figure 1, the proposed antenna consists of an annular ring slot radiator on the top layer and a hybrid patch coupler on the bottom layer. When the annular ring slot antenna operates at its fundamental mode [TM.sub.11], circumference of the annular ring should be one free space wavelength , which means the radius of the annular ring slot antenna is only about 0.32 free space wavelength. To match the antenna to the 50-ohms feed line, the coupled open-ended microstrip line is used here to feed the slot ring antenna, as shown in Figure 2. By tuning [l.sub.f] and [w.sub.f], the input impedance of this antenna can be changed easily, and a good matching condition can be achieved.
Following the design guideline in , a miniaturized hybrid patch coupler is designed to feed the antenna. It is employed here to provide power assignment with the same amplitude but 90[degrees] phase difference and is placed on the bottom of the FR4 substrate. By this arrangement, the inner ground plane on the top layer can be considered as the ground plane of the hybrid coupler. As a result, the proposed antenna can radiate circular polarized wave with an integrated compact configuration.
The proposed antenna is simulated with the commercial full wave solver HFSS, and the final dimensions of the proposed antenna are shown in Table 1. Figure 3 shows the isolation between the two input ports of the annular ring slot antenna. As can be seen, isolation between the two ports is almost lower than -10 dB within the operating bandwidth. The performances of the hybrid patch coupler are depicted in Figure 4 too. It is found that there is a good amplitude balance with 90[degrees] phase difference at two output ports. Then the annular ring slot antenna and the hybrid patch coupler are integrated together to radiate the circular polarized wave with compact size. The simulated [absolute value of [S.sub.11]] of the integrated antenna is shown in Figure 5. It is found the proposed antenna operated from 830 MHz-950 MHz ([absolute value of [S.sub.11]] < -10dB) and almost covers the entire UHF band. As shown in Figure 6, a bidirectional radiation pattern is obtained. Gain of the proposed antenna at the operating band is shown in Figure 7. The peak gain over the entire UHF band is from 1 dBi to 2.2 dBi. Figure 8 shows the simulated axial ratio of the proposed antenna, the bandwidth of axial ratio (AR < 3dB) is 870MHz-960MHz. Based on the simulated results, the proposed antenna has a good performance in the entire UHF band and is a good candidate for the RFID reader antenna.
3. EXPERIMENTAL RESULTS AND DISCUSSION
To verify the proposed antenna, a prototype is fabricated and measured. Photograph of the fabricated antenna is shown in Figure 9.
Measure [absolute value of [S.sub.11]] is shown in Figure 5. It agrees well with the simulated results except a slightly frequency shift, which is mainly caused by the poor characteristics of the cheap FR4 substrate. The radiation pattern of this antenna is measured in a microwave chamber, and a good symmetry of the bidirectional radiation pattern is observed, as shown in Figure 6.
Figure 7 shows the measured gain of the proposed antenna, good agreement is obtained between the simulated and measured results. The measured gain is larger than 1 dB in the entire UHF RFID band with small fluctuation. However, there is a discrepancy between the simulated axial ratio and measured axial ratio. It is because that axial ratio is sensitive to the amplitude balance and phase differences, and both of them are related to the unstable FR4 substrate. The axial ratio bandwidth with AR < 3 dB for the simulated result covers from 870MHz-960MHz, while the measured one is 885MHz-960MHz.
A compact bidirectional radiated circular polarized annular ring slot antenna is designed, fabricated and measured in this paper. The measured results show that the proposed antenna not only has a wide impedance bandwidth, but also has a wide axial ratio bandwidth. It almost covers the entire UHF band of the RFID system. In addition, the proposed antenna has the features of easy fabrication and low cost and is suitable for mass production.
This work supported by the National Natural Science Foundation of China (No. 61001019) and Major State Basic Research Development Program (2013CB328900 and 2013CB328905).
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Jun Lin Zhang and Xiao Qing Yang *
College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, P. R. China
Received 11 March 2013, Accepted 15 April 2013, Scheduled 18 April 2013
* Corresponding author: Xiao Qing Yang (firstname.lastname@example.org).
Table 1. Final dimensions of the proposed antenna (unit: mm). Parameters value a 36 b 40.5 [l.sub.g] 150 [w.sub.0] 3 [l.sub.0] 10 [l.sub.1] 40 Parameters value s 0.5 [l.sub.s] 10 [l.sub.t1] 5.25 [l.sub.t2] 6.25 h 1.6
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|Title Annotation:||Radio frequency identification|
|Author:||Zhang, Jun Lin; Yang, Xiao Qing|
|Publication:||Progress In Electromagnetics Research Letters|
|Date:||Apr 1, 2013|
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