## 4 Antennas for Beam Steering and Beamforming
### Passive Antennas for Beam Steering  
Beam steering is a critical technology for modern communication systems, enabling high directional gain and interference mitigation by focusing transmitted or received energy in specific directions. Passive antennas, which do not rely on active electronic components such as amplifiers, are increasingly being explored for beam-steering applications in small satellite platforms, including CubeSats. These systems offer compact, lightweight, and energy-efficient alternatives to active beamforming systems, making them particularly attractive for space missions constrained by mass, volume, and power budgets. In CubeSat communications, achieving high data rates and reliable links over long distances are challenging due to their small form factor and limited power availability. Passive antennas designed for beam-steering can address these issues by enhancing signal directivity without adding complexity and power consumption to active elements. Despite their advantages, passive antennas for beam-steering face several challenges. Passive beamforming systems require meticulous design to achieve precise phase control and high efficiency. This often involves intricate geometries, tight tolerances, and advanced materials. In the harsh environment of space, passive antennas must withstand extreme temperatures, radiation, and mechanical stresses, which can degrade performance over time. Manufacturing compact and lightweight passive antennas with high precision is also challenging, especially when integrating them into the constrained volume of a CubeSat. Moreover, passive antennas can achieve beam steering; the range and accuracy of the steering are typically less flexible than active systems. To address these challenges, several innovative techniques have been developed, such as passive phased arrays that utilize time-delay elements or phase shifters to control the direction of the radiated beam. These systems achieve high gain and directivity, although their operational bandwidths often limit them. In the reflect arrays method, antennas combine the features of reflectors and arrays by using a flat panel of passive elements to reflect and steer the beam. Their planar structure is ideal for CubeSat integration. In addition, the metasurface technique, composed of subwavelength elements, enables beamforming through phase manipulation across the surface. These antennas offered ultrathin profiles and enhanced design flexibility. To overcome volume constraints, deployable passive antennas inspired by origami techniques have been developed, allowing for compact stowage during launch and deployment in orbit. Last but not least, using lightweight, radiation-hardened, and thermally stable materials can also improve the durability and performance of passive antennas in the space environment. Recent antennas utilized for CubeSat communications, as well as explanations of their limitations, techniques, and outcomes, have been presented. It would be informative to consider several recent passive antennas for beam steering to investigate their challenges and methods, as well as explain how they can be used for CubeSat communications. Reference [94] explores phased array antennas operating in the D-band frequency range (110-170 GHz), focusing on high-frequency performance. Microstrip-based techniques were used for compactness and integration. The antenna demonstrated the design feasibility of D-band phased array antennas, with notable gains of 26.75 dBi and beam-steering capabilities of \(-\)13.5-13.7\({}^{\circ}\). It also addresses the need for miniaturization, which is crucial for high-frequency applications. Operating in D-band frequencies requires high precision of fabrication because of the small dimensions and tight tolerances, which are not always feasible in mass production. Microstrip antennas suffer significant losses at high frequencies, impacting the overall efficiency.