Introduction
Circularly polarized (CP) antennas have been widely utilized in wireless communication systems due to the strong anti-multipath interference and anti-fading ability [Reference Cao and Yu1, Reference Dash, Khan and Kanaujia2]. The traditional CP microstrip antennas have a simple structure, lightweight, and easiness of integration. However, due to the limited of narrow bandwidth, they are not suitable for applications with high transmission rates and high channel capacity. The bandwidth of the CP microstrip antennas can be improved by using thick dielectric substrates [Reference Kovitz and Rahmat-Samii3], stacked patches [Reference Esselle and Verma4], and multi-feed networks [Reference Targonski and Pozar5]. However, the geometric dimension of the antennas will be increased.
Metamaterials have attracted much attention since their excellent characteristics of manipulating electromagnetic waves. As a two-dimensional metamaterial, metasurfaces have opened a new door to improve the performance of the conventional CP microstrip antenna [Reference Juan, Yang and Che6–Reference Pan, Hu, Zhang and Zheng9]. Recently, some CP metasurface antennas have been reported. In papers [Reference Liu, Yang and Pan10, Reference Gao, Tian, Shou and Li11], a square metasurface is excited by a hybrid feed structure to obtain two degenerated orthogonal modes to realize CP radiation, and the axial ratio (AR) bandwidths (ARBWs) reach 14.5% and 20.9%, respectively. In paper [Reference Dicandia and Genovesi12], a nonuniform rectangular metasurface is fed by a slot antenna to radiate CP field. The ARBW is 17.43%. In paper [Reference Supreeyatitikul, Lertwiriyaprapa and Phongcharoenpanich13], an S-shaped metasurface is proposed to efficiently covert linearly polarized into CP wave and the ARBW reaches 22%. In paper [Reference Zhao and Wang14], an H-shaped metasurface is used as a radiator to form CP radiation. The ARBW is 14.3%. In addition, the metasurfaces can be used to improve the AR characteristic. In papers [Reference Yang, Liu and Adisaputra15, Reference Hussain, Jeong, Abbas, Kim and Kim16], the CP radiation is realized through a patch antenna, and the ARBWs are increased up to 22.4% and 20.1% by loading patch and ring metasurfaces, respectively. For the aforementioned antennas, the biggest AR bandwidth is 22.4%, but the profile is 0.063λ 0. Therefore, this work aims to design a low-profile wideband CP metasurface antenna.
In this paper, a low-profile wideband CP metasurface antenna is proposed. According to the characteristic mode analysis (CMA), two degenerate modes of the metasurface are chosen as the operating modes to realize the CP radiation. The metasurface is excited by a slot antenna. Combining the resonance modes of the slot antenna and metasurface to broaden CP bandwidth. The measured result shows that the −10 dB impedance bandwidth (IBW) of the proposed antenna is 3.47–4.76 GHz, and the 3 dB AR bandwidth (ARBW) is 3.5–4.9 GHz.
CMA of metasurface
Figure 1 shows the configuration of the proposed metasurface. It consists of 4 × 4 square patches with Z-shaped slots. The width of the unit cell is w m, and the gap between the adjacent unit cells is g. The commercial simulation software CST 2019 is used to analyze the mode behaviors of the metasurface. The optimized parameters are as follows: L = 55 mm, wm = 11 mm, g = 0.8 mm, l 1 = 2.6 mm, l 2 = 1 mm and w = 0.6 mm, respectively.
Figure 1. Configuration of the metasurface.
The mode significance of the first four modes is given in Fig. 2. It is observed that the fundamental modes J 1/J 2 and high-order modes J 3/J 4 have the same frequency trend. The current distributions and radiation patterns of the first four modes are plotted in Fig. 3. It can be seen that the currents in modes J 1/J 2 is in-phase on the metasurface, but there is a 90° rotation. Hence, they are a pair of degenerate orthogonal modes, and good radiation patterns are obtained. Also, the currents in modes J 3/J 4 are self-symmetrical, and that a radiation null appears due to the out-of-phase current distributions. Figure 4 plots the characteristic angle of the first four modes. It can be seen that the phase difference between modes J 1/J 2 is 0°. According to the CP radiation mechanism, modes J 1/J 2 with equal amplitude are elected as operating modes, and the CP radiation can be obtained by using a feed structure with 90° phase difference.
Figure 2. Mode significance for the first four modes.
Figure 3. Current distributions and radiation patterns for J 1 and J 2 at 4.3 GHz, and J 3 and J 4 at 4.75 GHz.
Figure 4. Characteristic angle for the first four modes.
Wideband CP metasurface antenna
To obtain CP radiation, an appropriate feed structure with a 90°phase difference should be employed to simultaneously excite J 1/J 2. As shown in Fig. 3, the maximum currents of modes J 1/J 2 concentrate at the central patches and the minor currents distribute at the surrounding patches. On the contrary, the maximum currents of modes J3/J 4 are located at the four corners patches of the metasurface. Based on the above analysis, the feed structure should be positioned under the center patches of the metasurface, so that modes J 1/J 2 can be excited efficiently, while modes J 3/J 4 are difficult to be excited. A slot antenna is employed as the feed structure in this paper.
Figure 5 shows the configuration of the proposed antenna. It is composed of a metasurface aforementioned on the top substrate and a slot antenna on the bottom substrate. A stepped microstrip feedline is used to feed a tilted cross-slot on the ground plane. Both substrates have a relative dielectric constant of 4.4 and a loss tangent of 0.03. Numerical simulations are carried out using CST Microwave Studio. Following are the final optimized parameters of the antenna: ls = 24.4 mm, ls 1 = 14.4 mm, ws = 1.4 mm, lf = 16 mm, wf = 1.5 mm, lf 1 = 16 mm, wf 1 = 1 mm, h 1 = 3.2 mm, and h 2 = 0.8 mm, respectively. The S 11 and AR of the antenna with/without the metasurface are depicted in Fig. 6. It is observed that the slot antenna without the metasurface has a −10 dB IBW of 6.79% (4.51–4.83 GHz), and a 3 dB ARBW of 3.44% (4.57–4.73 GHz). When the metasurface is loaded, the IBW of the antenna is 30.54% (3.44–4.68 GHz), and the ARBW is 28.43% (3.56–4.74 GHz). By combining the resonance modes of the metasurface and slot antenna, the wideband CP radiation is achieved.
Figure 5. Configuration of the antenna, (a) side view; (b) top view; (c) bottom view.
Figure 6. Simulated results of the antenna with or without the metasurface, (a) S 11; (b) AR.
To illustrate the CP mechanism, the surface current distribution of the metasurface at 4.3 GHz and the slot antenna at 4.65 GHz are illustrated in Fig. 7. It can be seen that the current at both 4.3 and 4.65 GHz rotates in the clockwise direction for different phases. As a result, the left-hand CP radiation is formed.
Figure 7. Surface current distributions on the metasurface and the slot antenna, (a) metasurface at 4.3 GHz; (b) slot antenna at 4.65 GHz.
The influences of the structural parameters on the antenna performance are also conducted. It is found that the metasurface element length wm and the slot length ls 1 play an important role. The S 11 and AR for different wm are given in Fig. 8. It can be seen that wm mainly affects the AR while the S 11 are insensitive to wm. When wm increases from 10 to 11 mm, the ARBW is expanded from 4.55% (4.3–4.5 GHz) to 27.54% (3.6–4.75 GHz). The CP performance deteriorates when wm reaches up to 12 mm. Figure 9 shows the S 11 and AR with different ls 1. It is observed that, when ls 1 increases from 13.4 to 14.4 mm, both the IBW and ARBW increase. When ls 1 reaches up to 15.4 mm, the IBW increases while the ARBW decreases.
Figure 8. S 11 and AR for different w m, (a) S 11, (b) AR.
Figure 9. S 11 and AR for different l s1, (a) S 11, (b) AR.
Measurement results and discussions
To demonstrate the performance of the proposed antenna, a prototype of the antenna is fabricated and measured, as shown in Fig. 10. The S 11 is measured by an Agilent N5221A vector network analyzer, and the far-field radiation performance, which contains the AR, gain, and radiation patterns, is measured by using a Lab-Volt 8092 antenna training and measuring system in a microwave anechoic chamber.
Figure 10. Photograph of the antenna, (a) top view; (b) back view.
Figure 11 plots the measured and simulated S 11 and AR of the antenna. It can be seen that reasonable agreement between the simulation and measurement results is obtained. The simulated and measured −10 dB IBW is 30.5% (3.44–4.68 GHz) and 31.3% (3.47–4.76 GHz), respectively. The simulated and measured 3 dB ARBW is 28.4% (3.56–4.74 GHz) and 33.3% (3.5–4.9 GHz), respectively. The discrepancy is mainly caused by the deviation of the dielectric constant and the effect of the subminiature version A (SMA) connector.
Figure 11. Simulated and measured results of the antenna, (a) S 11; (b) AR.
The simulated and measured far-field radiation patterns in xoz and yoz planes are plotted in Fig. 12. It is obvious that the proposed antenna realizes the left-hand CP radiation along +z-direction in the operating bandwidth. Figure 13 shows the simulated and measured antenna gains varying as frequency. The measured peak gain in the CP bandwidth reaches 6.9 dBic. It is noted that the gain after 4.25 GHz decreases, which is caused by the increasing right-hand CP component.
Figure 12. Normalized radiation patterns in xoz plane (left side) and yoz plane (right side), (a) at 3.75 GHz; (b) at 4.2 GHz; (c) at 4.65 GHz.
Figure 13. Gains of the antenna.
Finally, a performance comparison of the reported and proposed CP metasurface antennas is provided in Table 1. It is obvious that the size of proposed antenna is the smallest except [Reference Supreeyatitikul, Lertwiriyaprapa and Phongcharoenpanich13]. Compared to the antenna in literature [Reference Supreeyatitikul, Lertwiriyaprapa and Phongcharoenpanich13], the antenna in this work has a wider AR bandwidth.
Table 1. Performance of CP metasurface antennas
Conclusion
A low-profile wideband CP metasurface antenna has been demonstrated experimentally. The modes behavior of the metasurface is investigated by the CMA, and two required degenerate modes of the metasurface are excited by the slot antenna to generate CP radiation. Combined with the resonance mode of the slot antenna and metasurface, broadband CP radiation is realized. The fabricated antenna with a low profile of 0.046λ 0 exhibits a measured wide −10 dB IBW of 31.3% (3.47–4.76 GHz) and a 3 dB ARBW of 33.3% (3.5–4.9 GHz), respectively. The overlapping CP bandwidth can cover the C-band downlink spectrum, and the simple design can be easily integrated into the communication devices.
Acknowledgements
Kun Chai is the first author, Professor Liping Han is the corresponding author. Thanks to Zhi Li, Guorui Han, and Yufeng Liu for their suggestions and guidance on this research.
Financial support
This work is supported by the National Science Foundation of China (62071282) and in part by the Natural Science Foundation of Shanxi Province (201901D111026).
Competing interest
The authors report that they have no known conflict of interests or personal relationships that could have appeared to influence the work reported in this paper.
Author contributions
Kun Chai and Zhi Li performed the simulations, Professor Liping Han guided the design, Yajuan Zhao, Guorui Han, and Yufeng Liu provided support during the experiment. All authors contributed equally to analyzing data and reaching conclusions, and in writing the paper.
Kun Chai received the B.S. degree in electronic and Information Engineering from Shanxi University, Taiyuan, China, in 2019. Currently, he is a graduate in Shanxi University, Taiyuan, China. His research interests include design and analysis of circularly polarized antenna.
Zhi Li received the B.S. degree in electronic and Information Engineering from Taiyuan Normal University, Taiyuan, China, in 2019. Currently, he is a graduate in Shanxi University, Taiyuan, China. His research interests include design and analysis of metasurface antenna.
Yajuan Zhao received the M.S. degrees in communication engineering from Shanxi University, Taiyuan, China, in 2014. Currently, she is an engineer at the No. 33 Research Institute of China Electronics Technology Group Corporation. Her research interests include electromagnetic shielding composite materials, transparent stealth metamaterials, and metamaterials structure function integration.
Liping Han received the B.S., M.S., and Ph.D. degrees in electronic engineering from Shanxi University, Taiyuan, China, in 1993, 2002, and 2010, respectively. Currently, she is working as an associate professor with the School of Physics and Electronic Engineering, Shanxi University. Her research interests include microwave and millimeter-wave integrated circuits and microstrip antenna.
Guorui Han was born in Shanxi Taiyuan, China, in 1977. He received the B.S. and M.S. degrees in Applied Mechanics from Peking University, Beijing, China, in 2000 and 2004, respectively. He received the Ph.D. degree in Radio Physics from Shanxi University, Taiyuan, China, in 2013. Currently, he is working as an associate professor in School of Physics and Electronic Engineering, Shanxi University. His research interests include microwave and millimeter-wave antenna and MIMO antenna.
Yufeng Liu was born in Shanxi Lv liang, China, in 1986. He received the Ph.D. degree in radio physics from Sichuan University, Sichuan, China, in 2014. In 2015, he joined the College of Physics and Electronic Engineering, Shanxi University. His research is mainly focused on computational electromagnetics and antenna design.
