A low-profile wideband circularly polarized metasurface antenna based on characteristic mode analysis

Abstract

A low-profile wideband circularly polarized (CP) metasurface antenna is demonstrated for C-band applications. The metasurface consists of 4 × 4 square patches with Z-shaped slots. Characteristic mode analysis is used to investigate the modal behavior of the metasurface, and a pair of degenerate modes is chosen as the operating modes. The CP radiation is realized by exciting a pair of degenerate modes of the metasurface through a slot antenna, which is used as a feed structure with a 90° phase difference. The CP bandwidth is further widened by combining the resonance modes of the metasurface and slot antenna. The measured results show that the −10 dB impedance bandwidth of the antenna is 3.47–4.76 GHz, and the 3 dB axial ratio bandwidth is 3.5–4.9 GHz with a peak gain of 6.9 dBic. Moreover, the antenna exhibits well left-hand CP radiation performances with a low profile of 0.046λ₀.

Introduction

Circularly polarized (CP) antennas have been widely utilized in wireless communication systems due to the strong anti-multipath interference and anti-fading ability [1, 2]. 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 [3], stacked patches [4], and multi-feed networks [5]. 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 [6–9]. Recently, some CP metasurface antennas have been reported. In papers [10, 11], 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 [12], a nonuniform rectangular metasurface is fed by a slot antenna to radiate CP field. The ARBW is 17.43%. In paper [13], an S-shaped metasurface is proposed to efficiently covert linearly polarized into CP wave and the ARBW reaches 22%. In paper [14], 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 [15, 16], 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, w_m = 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: l_s = 24.4 mm, l_{s 1} = 14.4 mm, w_s = 1.4 mm, l_f = 16 mm, w_f = 1.5 mm, l_{f 1} = 16 mm, w_{f 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 w_m and the slot length l_{s 1} play an important role. The S ₁₁ and AR for different w_m are given in Fig. 8. It can be seen that w_m mainly affects the AR while the S ₁₁ are insensitive to w_m. When w_m 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 w_m reaches up to 12 mm. Figure 9 shows the S ₁₁ and AR with different l_{s 1}. It is observed that, when l_{s 1} increases from 13.4 to 14.4 mm, both the IBW and ARBW increase. When l_{s 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 [13]. Compared to the antenna in literature [13], 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.