Abstract
Recently, metasurfaces (MSs) have continuously drawn significant attentions in the area of enhancing the performances of the conventional antennas. Thereinto, focusing MSs with hyperbolic phase distributions can be used for designing high-gain antennas. In this chapter, we first design a new reflected MS and use a spiral antenna as the feeding source to achieve a wideband high-gain antenna. On this basis, we propose a bi-layer reflected MS to simultaneously enhance the gain and transform the linear polarization to circular polarization of the Vivaldi antenna. Then, we proposed a multilayer transmitted MS and use it to enhance the gain of a patch antenna. This kind of high-gain antenna eliminates the feed-block effect of the reflected ones but suffer from multilayer fabrication. To conquer this problem, we finally propose a single-layer transmitted focusing MS by grouping two different kinds of elements and use it to successfully design a low-profile high-gain antenna.
Keywords
- focusing
- MS
- high-gain antenna
- reflection
- transmission
1. Wideband reflected high-gain antenna based on single-layered focusing metasurface
In last several years, metasurface (MS) has become a research hotspot since it relieves the drawbacks of bulk metamaterials. An MS usually consists of a set of periodic or locally nonperiodic unit cells with subwavelength thickness. The phase gradient metasurface (PGMS) is a special kind of MS which has been proposed by Yu et al. to demonstrate the general Snell’s law [1]. Since the PGMS is able to provide predefined in-plane wave vectors to manipulate the directions of the refracting/reflecting waves, it consequently attracts a lot attention in beam steering. In Ref. [1], the authors designed a PGMS by using nano-V-antennas with different shapes to verify anomalous reflection/refraction effects, which opens the door to the rapid development of MS for beam steering. Over the last 5 years, the MS has ushered in the golden age of theoretical and practical researches. Many applications of MS have emerged in the areas of focusing [2, 3, 4, 5, 6, 7, 8], anomalous refraction/reflection [6, 7], surface-plasmon-polariton coupling [8, 9], radar cross section (RCS) reduction [10, 11, 12], and polarization manipulation [13]. Generally, a wide phase-steering range covering 2π is an essential characteristic for the MS element. The phase is manipulated by changing the structure size or rotating the angle of the particle on the substrate. Then, by fixing proper phase distributions on the MS, we can flexibly manipulate the wavefronts and the polarizations of the EM waves. These characteristics as well as compact size and low loss mean the MS can be a good candidate to improve antenna performance by enhancing the antenna gain [4], reducing the RCS of the antenna [12], converting the antenna polarization [13]. We call this kind of high-performance antenna based on MS as MS antenna. Thereinto, the focusing MSs usually have been used to enhance the gain of the antenna. They can also transform spherical wave emitted by a point source placed at the focal point to plane wave theoretically. In such case, the directivity and gain of the point source can be improved greatly.
1.1. Theory
Figure 1 depicts the schematic used to derive the generalized law of reflection. The introduction of an abrupt phase shift, denoted as phase discontinuity, at the interface between two media allows us to revisit the law of reflection by applying Fermat’s principle. The incident angle of the electromagnetic wave is
where
As shown in Figure 2(a), if the designed
where
1.2. Unit cell design
Figure 3 shows the structure of the MS element, which is utilized to build the reflected MS. The top metallic layer is composed of a cross and a cross-ring (CCR), and the bottom layer is totally metal. The dielectric layer has a substrate with the permittivity of 2.65 and thickness of 3 mm. For characterization, the unit cell is simulated in CST Microwave Studio by using periodic boundary. To illustrate the operating mechanism of the CCR unit cell clearly, Figure 4 shows the current distribution on the upper surface of the unit cell and Figure 5 shows the phase of S11 in a broad frequency band. As shown in Figure 5, the phase of S11 changes fast around 4.5 and 9.5 GHz, which is the two resonances of the CCR unit cell. From Figure 4, it can be find that the lower resonant frequency (4.5 GHz) is brought by the cross-ring structure and the higher one is brought by the cross-structure. Due to the dual-resonance structure, the phase difference of the CCR unit cell has successfully reached about 620° just by changing the parameter of rn as shown in Figure 6.
In addition, the curves in Figure 7 have good linearity especially during 10–12 GHz, which makes the CCR unit cell very suitable for broadband design. To completely evaluate the reflected phase character of the unit cell, a parameter scan has been made with the step of 0.01 mm for
1.3. Focusing metasurface antenna design
After successfully designing the required broadband unit cell, we use it to design the focusing MS. The reflected phase difference distribution in the plane which is perpendicular to the direction of the incident plane-wave should satisfy the profile mentioned in Eq. (3). The period of the CCR unit cell is
Then, the 3D radiation patterns at 10, 11, and 12 GHz are shown in Figure 11, it can be concluded from the figures that the gain of the feed source is remarkably enhanced and pencil-shaped radiation patterns are achieved. To clearly depict the antenna gain-enhancement via reflected MS, the 2D radiation patterns of the antenna with/without the MS are shown in Figure 12. As shown, the antenna beam width has been decreased greatly and the peak gain has been enhanced greatly compared with the planar spiral antenna. It is also necessary to notice that the source is right circular polarization, while the MS antenna is left circular polarization, and the cross-polarization component of the source antenna is also enhanced by the MS while it is still much lower than the copolarization at the main radiation direction.
At last, the proposed MS antenna has been fabricated and assembled, and the photographs are shown in Figure 13. The farfield results of the novel MS antenna are measured in an anechoic chamber. Figure 14 shows the simulated and measured peak realized gain in the band of 8–13 GHz. As shown, the −1 dB gain bandwidth of the proposed antenna is 10–12.3 GHz (with the fractional bandwidth is 20.6%), and in this band, the peak gain has an enhancement of 13.5 dB comparing with the spiral antenna in average. The peak gains at the frequencies of 10, 11 and 12 GHz are 19.2, 20.1 and 19.4 dB, respectively. Then, the aperture efficiency (AE) can be calculated by Eq. (4), where
2. Wideband multifunctional metasurface for polarization conversion and gain enhancement
The polarization state is one of the most important characteristics of the EM waves. We can classify the polarization conversion MS (PCMS) [14, 15, 16, 17, 18, 19] into two categories according to the format of the MS—transmitting type [14, 15, 16, 17] and reflecting type [18, 19]. Also, the PCMS also can be classified into cross-polarization conversion one [14, 15, 16, 18] or linear-to-circular/circular-to-linear (LTC/CTL) one [17, 19] according to specific functionalities. However, the mentioned PCMSs are all illuminated by plane waves and the radiation performances will be more or less deteriorated when they are directly feed by a spherical feed source like Vivaldi antenna. Taking the overall performances into consideration, a technique should be adopted for a PCMS design to control the direction of the scattering wave for spherical wave excitation. The focusing MS mentioned above can transfer the incident plane wave to its focal point, and vice versa. So, it can be predicted that the combination of the PCMS with focusing MS will improve the radiation performance of the system.
2.1. Linear-to-circular metasurface design
Anisotropic MSs have the character of manipulating electromagnetic waves with different polarizations, respectively. We still adopt the CCR unit cell shown in Figure 16 to design an anisotropic MS. Compared with the unit cell shown Figure 1, we set
where
We suppose that the LTC-PCMS is illuminated by a plane wave propagating along −z direction, then the formulation of the incident wave can be described as Eqs. (7) and (8).
where
For the case of
Then, the reflected E fields can be calculated as:
As described in Eq. (11), it can be concluded that the polarization of the reflected wave is LHCP. In the second case of
In this situation, a RHCP reflected wave has been obtained. Figure 18 depicts a practical realized scheme of our system, where the LTC-PCMS fed by a Vivaldi antenna is built by 13 × 13 single-layered CCR unit cells. The voltage standing wave ratio (VSWR) with its geometrical parameters has been shown in Figure 19. We can conclude that the Vivaldi antenna has a VSWR less than 2 dB within the frequency band of 9–15 GHz. Three-dimensional radiation patterns and AR results of the LTC-PCMS are shown in Figure 20. As shown, the far-field patterns achieve CP radiation but the directivity of the Vivaldi antenna has been broken by the MS. To conquer this problem, an additional focusing profile needed to be brought in to correct the wave-front of the outing wave.
2.2. Multifunctional metasurface design
To ease the design, we want the unit cell can manipulate the
For focusing MS, the phase difference distribution on the MS has to satisfy Eq. (13).
where
While for
where, in the focusing LTC-PCMS design, following equations have to be satisfied: Arg(
2.3. Circular-polarized antenna design
Then, the proposed MS model is built in CST based on the matrixes of
In addition, we simulate the models of
3. High-gain lens antenna based on multilayer metasurface
Compared with the reflected MS, the feed of the transmitted MS does not block the radiated wave, making it more suitable for a high-gain antenna design. However, the design of the transmitting MS is more difficult since the transmitting efficiency must be taken into consideration. In this section, we proposed a four-layered transmitting MS with a parabolic phase profile at 10 GHz. The MS elements are cautiously designed and optimized, aiming at affording high transmitting efficiencies for all the building elements. In that case, the MS can focus the incident plane wave with high efficiency. In current design, a patch antenna operating at 10 GHz is placed at the focal point of the MS to feed the PGMS and the F/D is designed as 0.19. The quasi-spherical wave emitted by the source will be transformed to near-plane wave by the MS, and thus a high gain lens antenna with pencil-shaped beam will be achieved.
3.1. Multilayer metasurface unit cell design
The structure of the unit cell, as shown in Figure 29, consists of a four metal layers and a three dielectric layers. The metal layer consists of a circular patch and a square outer frame. The transmission phase changes with the radius of
3.2. High-efficiency transmitted focusing metasurface design
For designing the transmitted focusing MS, the phase distribution should also obey Eq. (3). To not lose generality, we arbitrarily select
3.3. Lens antenna assembling and measurement
The multilayer MS is fabricated and then assembled with a patch antenna. The sample photographs are shown in Figure 37. We test the MS antenna in the microwave anechoic chamber. Figure 38 shows the simulated and measured patterns of the
4. Single-layer metasurface for ultrathin planar lens antenna application
As we all known, it is hard to cover 360° phase shift range with satisfying efficiencies by single-layered (bi-layered metal) structures, though they are easier of fabrication. Multilayer stack adopted in above section is a valid technique to expand the phase shift range of MS. However, it is not the only method to achieve this goal. In Ref [20], three kinds of single-layer unit cells are used together to provide adequate phase range by skillfully connecting each phase shift section of them. In this section, an element group consist of two similar single-layered transmitting unit cells was designed. The phase steering ranges of the two elements have been well connected to achieve a phase shift range of 415°. And we use this element group to successfully design an ultrathin planar lens antenna.
4.1. Single-layer element design
The structures of the element 1 and the element 2 are shown in Figure 39, in which the metal layer of the element 1 is a cross and a double cross-ring structure (cross and double cross-ring, CDCR), and the metal layer of the element 2 is a cross and cross-ring structure (cross and cross-ring, CCR). The dielectric layers of the two elements are all 3 mm thick with a relative dielectric constant of 4.3. The dimensions shown in Figure 39 are, p = 8 mm, w = 0.9 mm, g = t = 0.15 mm,
In order to achieve the phase tuning range of 360°, we remove the outermost cross-frame structure on the basis of CDCR element and get the second element namely the CCR element. To ensure that the simulated conditions are exactly the same, the size of the CCR element is slightly adjusted. When
4.2. Single-layer metasurface design
From the previous analysis, it can be seen that the combination of CDCR and CCR unit cells can completely control the phase of transmitted wave under the precondition of the transmittance higher than 0.7, which meet the requirements of the transmitted focusing MS. The same as the Section 2, we use 13 × 13 elements to build the transmitted MS and improve the gain of the patch antenna. According to Eq. (13), we fix
According to the distribution of Figure 45(b) and (c), a single-layer transmitted focusing MS is constructed, and the patch antenna shown in Figure 34 is used as the feed source. Figure 46 shows the electric-field distribution at 10 GHz on the
4.3. Lens antenna assembling and measurement
As is shown in Figure 48, the single-layer transmitted MS is fabricated and then assembled with a patch antenna. The lens antenna is measured in the microwave anechoic chamber. In Figure 49, the simulation and test patterns of the
5. Conclusions
In this chapter, we have reviewed our recent efforts in utilizing metasurface to enhance the gain of the conventional antenna. For reflected MS, we propose a novel single-layer unit cell to greatly widen the phase-steering range and use it to design an MS antenna which achieves a wide working band of 10–12.3 GHz. On this basis, we propose a bi-layer reflected MS to simultaneously enhance the gain and transform the linear polarization to circular polarization of the Vivaldi antenna. The new MS enhances the gain and decrease beam width of the antenna in a 3 dB axial ratio band of 9.12–10.2 GHz. While for transmitted MS, we not only try to widen the phase-tuning range but also struggle to maintain high transmissions. Two methods have been proposed in this chapter to design transmitted MS. One is based on multilayer stack and the other is using an element-group. Compared with the patch antenna, the gain enhancements at 10 GHz are 11.6 and 10 dB for the methods of multilayer stack and group-element, respectively. In addition, both of the aperture efficiencies have reached 30%. These above MS antennas not only open up a new route for the applications of focusing MSs in microwave band, but also afford an alternative for high-gain antennas.
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