## 4 Antennas for Beam Steering and Beamforming
### Passive Antennas for Beam Steering
These results highlight the antenna's effectiveness in modern applications like satellite communications and 5G networks, combining compactness with advanced polarization and beam control capabilities. However, its limitations include potential challenges in fabricating such intricate structures and maintaining high precision for dual-band beam control. Reference [105] presents a design for a dual-band transmit array at 12.5 GHz and 14.25 GHz capable of generating orthogonally polarized contour beams (360\({}^{\circ}\) phase range), targeting advanced communication systems. The design utilizes independent phase compensation for each band, achieved through the design of high-precision elements,polarization control, and a maximum gain of 25.3 dBi. Its limitations involve the complexity of fabrication and alignment, as well as challenges in maintaining the performance over wide angular ranges. The design holds promise for improving multifunctional antenna systems. Ref. [106] presented a Fabry-Perot-based, all-dielectric Huygens' structure for millimeter-wave beam steering with circular polarization. The design employs dielectric metasurfaces with tailored phase and amplitude responses to achieve compactness and efficient beam steering (360\({}^{\circ}\) phase coverage). It is integrates well with 60 GHz slot antennas, achieving circular polarization with a gain of 34.13 dBic. While the design demonstrates effective beam steering and reduced cross-polarization, its limitations include fabrication complexity and potential performance sensitivity to dielectric material inconsistencies. Reference [107] presents a method for achieving wide-angle beam steering with circular polarization in the Ka-band (29.5-30 GHz) using a plate lens antenna. The approach relies on the in-plane translation of a dielectric plate lens to steer the beam over a wide angular range while maintaining circular polarization (0\({}^{\circ}\) to 50\({}^{\circ}\)). The method demonstrates a high gain of 27.3 dBi and high beam quality. However, the mechanical translation of the lens introduces practical limitations, such as slower response times and potential wear-and-tear issues in dynamic systems compared to fully electronic beam steering. Additionally, the setup can be bulky and less suitable for highly compact or integrated applications. Ref. [108] presented a high-gain aperture antenna using a leaky-wave antenna (LWA) array with substrate-integrated waveguide (SIW) technology designed for CubeSat applications. Operating at 28 GHz, the antenna achieves a gain of 15.2 dBi with a fixed radiation angle of -45\({}^{\circ}\), while its compact design (98.6\(\times\)16\(\times\)1.575 mm\({}^{3}\) per element) ensures integration within a 1U CubeSat. SIW enables a low-profile, efficient structure, making it suitable for space-constrained environments. However, its fixed radiation angle may limit communication flexibility, requiring satellite orientation adjustments, and the frequency-specific design at 28 GHz may necessitate modifications for different operational bands. Despite these limitations, the proposed antenna offers a promising solution for enhancing CubeSat communication with its high gain and compact form factor. Reference [109] proposes a CubeSat microstrip antenna incorporating a metamaterial structure for LoRa communication at 924 MHz. The design utilizes a metamaterial layer to enhance the antenna's performance by improving the gain by up to 2.6 dBi while maintaining a compact form factor suitable for CubeSat applications. The authors demonstrate that the metamaterial structure effectively reduces the antenna's size while preserving its efficiency, making it ideal for low-power, long-range communications such as LoRa. However, while the antenna shows promise for CubeSat-based IoT and remote sensing applications, its suitability for beamforming or beam-steering tasks is limited. The passive nature of the metamaterial structure restricts dynamic beam control, making it less adaptable to varying communication conditions in low Earth orbit (LEO).