## 4 Antennas for Beam Steering and Beamforming
### Beam-Steering/Beamforming Techniques and Their Possible Features for CubeSat Communications
A secondary metasurface antenna was also positioned 10 mm from the primary radiating antenna to further enhance the radiation and directivity. The design utilized a partial ground and line feeding with a conventional coplanar waveguide (CPW) technique to optimize the overall performance. A rectifier circuit was also integrated to convert the harvested electromagnetic energy into direct current (DC) output power. The proposed model achieved a gain of 8.1 dB, a backward radiation reduction to \(-\)13.1 dB, and a DC output power of 2.82 V. While the antenna demonstrated effective gain enhancement and beam-steering capabilities for RF energy harvesting, its suitability for dynamic beamforming or beam-steering applications is limited. The design focuses on static beam steering achieved through the fixed configuration of metasurfaces and metal slots. This static approach restricts the antenna's ability to adaptively steer beams in response to changing signal environments or target movements, a key requirement in advanced communication systems. Moreover, integrating multiple metasurface layers and the specific placement of components introduces fabrication complexities. It increases the overall size of the antenna system, potentially limiting its applicability in compact or dynamically reconfigurable platforms. Therefore, while effective for enhancing the gain in RF energy harvesting, this design is not suitable for applications requiring agile and real-time beamforming or beam-steering capabilities. Reference [81] introduced a metasurface antenna capable of electronic beam steering without mechanical movement. The design features an array of subwavelength resonators, each integrated with tunable elements, allowing for dynamic control over the phase and amplitude of emitted microwaves. This configuration enables rapid adjustments to the beam direction, enhancing imaging, radar, and wireless communications applications. The antenna's planar structure offers a compact, cost-effective alternative to traditional phased array systems. However, challenges such as the fabrication complexity, power consumption of tunable components, and potential limitations in bandwidth and efficiency affect the performance. Further research is necessary to optimize these aspects for practical deployment. Reference [82] focuses on the application of [PERSON] metasurfaces--ultra-thin structures capable of controlling electromagnetic wavefronts--to achieve precise antenna beamforming and beam steering. By engineering these metasurfaces, the authors demonstrated the ability to manipulate the direction and shape of antenna radiation patterns without relying on traditional bulky feeding networks. This approach offers a low-profile solution for directing electromagnetic beams, which is advantageous in various applications such as wireless communications and radar systems. The study highlights the potential of [PERSON] metasurfaces to provide efficient and compact alternatives to conventional beamforming techniques. Reference [83] introduces a metasurface-driven antenna system designed for three-dimensional beam steering in satellite communications. The system employed a pair of phase gradient metallic metasurfaces (PGMMs) operating in the Ku-band, engineered using a near-electric field phase transformation method. By independently rotating these PGMMs above a fixed-beam base antenna, the design achieves wide-angle beam steering in both the azimuth and elevation planes. The experimental results demonstrated effective beam scanning, with the elevation angles reaching \(\pm 38^{\circ}\) and full \(360^{\circ}\) coverage in the azimuth plane. This approach offers a low-profile, passive solution for dynamic beam steering, eliminating the need for complex feeding networks or mechanical movement, which are common in traditional systems. One notable development involved the use of metasurfaces for beam steering. A study published in Ref. [84] demonstrates beam steering by employing two vertically oriented metasurfaces.