† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 61372034).
A new method to design an ultra-thin high-gain circularly-polarized antenna system with high efficiency is proposed based on the geometrical phase gradient metasurface (GPGM). With an accuracy control of the transmission phase and also the high transmission amplitude, the GPGM is capable of manipulating an electromagnetic wave arbitrarily. A focusing transmission lens working at Ku band is well optimized with the F/D of 0.32. A good focusing effect is demonstrated clearly by theoretical calculation and electromagnetic simulation. For further application, an ultra-thin single-layer transmissive lens antenna based on the proposed focusing metasurface operating at 13 GHz is implemented and launched by an original patch antenna from the perspective of high integration, simple structure, and low cost. Numerical and experimental results coincide well, indicating the advantages of the antenna system, such as a high gain of 17.6 dB, the axis ratio better than 2 dB, a high aperture efficiency of 41%, and also a simple fabrication process based on the convenient print circuit board technology. The good performance of the proposed antenna indicates promising applications in portable communication systems.
In recent years, phase gradient metasurfaces (PGMs), proposed by Yu et al.,[1] have induced a great impetus and been applied widely. Completely distinct from the phase accumulation in conventional metamaterials,[2] the PGM presents a strong control over the phase by suitably tailoring phase discontinuities on the dielectric surface, providing a promising route to construct ultra-thin planar devices. With the unprecedented control of electromagnetic (EM) wavefronts, PGMs have found wide applications in cloaks,[3,4] focusing lenses,[5–7] holography,[8,9] polarization beam splitter,[10,11] beam scanning,[12,13] polarization keeping,[14] polarization conversion,[15] to name a few. However, the relatively low transmission efficiency for the transmissive metasurface has limited its further applications. Multilayer transmissive PGMs improve the transmission efficiency significantly.[16,17] However, the multilayer technology brings a great challenge for the fabrication and a high cost. Geometrical phase gradient metasurface (GPGM), with full control of transmission phase and high transmission amplitude, provides a good way to solve all issues aforementioned.
Circularly polarized (CP) antennas have been applied to numerous wireless communication systems for their attractive features, such as light weight, low cost, ease of fabrication, especially the stable date transmission rate without reference to the polarization orientation between the transmitter and the receiver.[18] However, conventional CP antennas suffer from low radiation gain, large cross polarization, and complex feeding system. Moreover, the CP antenna based on GPGM has been reported rarely. The insufficient technology makes the attempt of employing GPGM in high gain CP antenna design a challenging and pressing task.
In this paper, a single-layer transmissive GPGM is proposed for the first time. The phase gradient on the metasurface is obtained by rotating the designed element. The transmission coefficient Tx for x polarization and Ty for y polarization reach about 0.9 for the element, and the transmission phase difference between them keeps π. Both aspects play an essential role in polarization and phase control. The rest of the paper is arranged as follows. Section 2 shows the element structure, the characterization of the unit cell and also the working principle of GPGM. Furthermore, the focusing lens based on the GPGM is calculated, EM simulated and further analyzed. Section 3 discusses the simulated and measured performances of the high-gain antenna based on the focusing lens. Finally, the paper is summarized.
We utilize the spin element to compensate the phase deviation of a circularly polarized wave. Figure
It is obvious that the transmissive wave only has left-hand rotation component which is opposite to the incident wave and the shift of transmissive wave phase is equal to twice the angle of rotation (θ), and the condition is the same as the left-hand circularly polarized (LCP) transmitted wave. Here, the working principle of the element can be obtained clearly. For one thing, |ϕx − ϕy| = π is necessary to ensure a pure RCP or LCP. For another, Tx and Ty should be equal and as close as 1 to improve the transmissive efficiency.
According to the design principle, the structure of a well-optimized cell is shown in Fig.
Figure
To demonstrate the characterization of the geometrical phase, the transmitted RCP plane wave along the z direction is used to illuminate the optimized unit cell with the simulation setup shown in Fig.
The demonstration of geometrical phase indicates that the proposed transparent element is promising to achieve anomalous phenomenon, such as focusing, deflection, and hologram. More importantly, these applications are capable of high efficiency based on the high transmission coefficient of the element. Here, in order to demonstrate the good performance of the element, a focusing lens is designed. To achieve the planar focusing lens, the refracted phase should satisfy the parabolic profile. The required transmission phase profile on the GPGM is calculated by
In the above section, we have verified that the designed focusing lens is capable of focusing a circularly polarized plane wave. According to the reversibility principle of EM wave propagation, a spherical-like wave that radiates from the point source on a focal point can be transformed into a plane wave by the lens. Then we put an RCP patch antenna with a peak gain of 6.4 dB working at 13 GHz as reference. We can observe the distribution of electric field in near field without and with the GPGM in Fig.
Since the patch antenna is an unideal point source, the phase center is not strictly located at 23 mm, and should be optimized to achieve a good performance. Here, the length is well designed at 25 mm. Figure
In order to verify the simulation, the GPGM lens and patch antenna are fabricated with the photograph shown in Fig.
In conclusion, a new method to design a CP lens is proposed based on the GPGM. An RCP lens is designed for demonstration of the proposed design method. The lens is single-layered with a thickness of 1.5 mm (λ0/15) and a total size of 77 mm× 77 mm (3.3λ0 × 3.3 λ0). The accuracy phase manipulation and high transmission indicate a perfect focusing effect. For application, an RCP lens antenna working at the Ku band with a F/D of 0.32 is studied numerically and experimentally. The results indicate that the proposed lens antenna achieves a high gain of 17.6 dB at 13 GHz, a good axial ratio of 2 dB, and also a high aperture efficiency of 41%. Remarkably, the planar lens proposed in this paper empowers significant reduction in thickness of the lens, and achieves perfect focusing behavior, low profile, and high transmission efficiency simultaneously, thereby providing a great practical alternative to conventional lenses.
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