## 4 Antennas for Beam Steering and Beamforming
### Passive Antennas for Beam Steering
The arrays adopt a linear arrangement to enable efficient beam-steering capabilities. The design also incorporates techniques for efficient energy usage, supporting a high-frequency performance while maintaining a small footprint. The antenna arrays demonstrate an enhanced gain of up to 9.19 dBi and directional beam steering. The results indicate their potential for deployment in 5G applications and beyond. However, the design does not support broader frequency ranges, limiting the flexibility. Reference [100] presents a compact and lightweight multi-beam antenna system tailored for CubeSat applications operating in the Ka-band (26.5-40 GHz). The design incorporates a 3D printed dielectric lens to enhance beam directivity and a planar antenna array to generate multiple beams with a coverage of 14-18\({}^{\circ}\) for efficient connectivity. This approach enables passive beam steering, reducing the complexity and power consumption while maintaining high performance, such as a maximum gain of 16 dBi. Three-dimensional printing also lowers the manufacturing costs and facilitates scalability for CubeSat systems. However, the design faces challenges, such as sensitivity to manufacturing tolerances, limited adaptability to other frequency bands, and environmental factors like signal attenuation in adverse weather. Reference [101] introduces a dual-band antenna design leveraging phase-gradient metascreens (PGMs) and near-field meta-steering capabilities. The design employs innovative dual-band, phase-gradient metascreen structures to achieve effective beam steering of \(\pm\)46\({}^{\circ}\) and \(\pm\)51\({}^{\circ}\) for near-field applications. These metascreens are designed to operate efficiently at two distinct frequency bands (11.57-11.97 GHz and 13.95-15.0 GHz), with independent beam-steering capabilities for each band. The PGMs were engineered to precisely control the electromagnetic wave propagation through phase adjustments across the metascreen surface. However, the approach relies on intricate multi-layer structures, which increase the manufacturing complexity and cost. Metal layers in the design introduce conductivity losses, impacting the overall system efficiency. Reference [102] explores the design of a dual-band, shared-aperture antenna system at 3.2-3.45 GHz and 25.5-29.5 GHz, tailored for 5G technologies. This system integrates multiple antenna elements within a compact shared aperture, supporting dual-band operation and beam-steering capabilities. Beam steering of up to 112\({}^{\circ}\) in the sub-6 GHz band and up to 56\({}^{\circ}\) in the mm-wave band are achieved using an innovative phase-shifting network that facilitates precise control of the antenna beam across both frequency bands. The design prioritizes compactness and achieves a maximum efficiency of 78% and gain of 19.31 dBi, making it suitable for integration into modern 5G infrastructure. Reference [103] proposes a hybrid reflectarray antenna combining passive and active unit cells for efficient two-directional beam steering in the coverage of \(-\)18\({}^{\circ}\) and 18\({}^{\circ}\) around 10.1 GHz. The active cells enable dynamic control of phase gradients, while passive cells ensure a lightweight and cost-effective design. Integrating active and passive unit cells for phase manipulation and phase-gradient control to direct beams toward two target directions offers a high efficiency of up to 69.5%. However, the design's complexity increases because of the integration of active and passive cells, and the scalability for multi-directional beam steering is limited. Reference [104] introduces a low-profile, dual-band, and dual-circularly polarized folded transmit array antenna for independent beam control in the K- and Ka-bands (12 and 15 GHz). The design utilizes a unique folded transmit array mechanism and polarization control to steer beams independently at each band (up to 45\({}^{\circ}\)). The system provides high gains of 24.7 dB and 26.1 dB for the respective bands, with efficiencies exceeding 55%.