## 3 Antennas with General Aspects Utilized for CubeSat
### Hybrid Beamforming Techniques  
In Ref. [62], the hybrid beamforming combines analog and digital processing to mitigate the high cost and power consumption associated with fully digital beamforming, which requires a dedicated radio-frequency (RF) chain for each antenna element. This technique leverages analog beamforming to manage a large number of antennas with fewer RF chains. In contrast, digital beamforming provides fine-tuned signal processing control, thereby balancing performance and hardware complexity. The proposed method in the paper addresses challenges such as hardware impairments and inter-user interference by optimizing the hybrid beamforming architecture, resulting in improved spectral efficiency and reduced computational complexity. However, the limitations include potential performance degradation due to analog hardware constraints and the need for precise calibration to maintain beamforming accuracy. In the context of CubeSat communications, hybrid beamforming offers significant advantages. CubeSats are constrained by size, weight, and power (SWaP) limitations, making deploying extensive RF chains impractical. By employing hybrid beamforming, CubeSats can utilize large antenna arrays to achieve the high-gain directional beams necessary for long-distance space communications while keeping the number of RF chains minimal to conserve power and space. This approach enhances data transmission rates and link reliability without imposing substantial hardware burdens, making it a viable solution for advanced CubeSat communication systems.  
In Ref. [63], the authors suggest a two-band single monopole antenna to minimize the number of antennas needed for CubeSat communications. This antenna functions at VHF and UHF, simultaneously enabling data transmission and reception. The main idea is the use of a diplexer to manage uplink and downlink communications through a single antenna. The design incorporates a strip that attaches the antenna to the CubeSat's surface. The proposed deployable monopole antenna has several significant drawbacks. The primary issue is the potential for mechanical failure in the deployment system, which can lead to a loss of communication with the ground station and possibly result in mission failure. Additionally, the relatively low gains of 2.06 dBi (uplink) and 3.35 dBi (downlink) restricts the antenna to short-range communications and lower data rates, which might not meet the requirements of specific CubeSat missions.  
In Ref. [64], retractable dipole and monopole antennas works at dual frequencies within the VHF and UHF bands, optimizing transmission and reception while reducing the number of required antennas. By utilizing a single dual-frequency antenna instead of separate ones, the design effectively conserves space on a CubeSat, making it more efficient. The antennas are integrated with an LC circuit, leveraging the CubeSat's outer structure as a ground plane, particularly for the monopole configuration. When fully deployed, the dipole antenna spans 980 mm in length and exhibits 2.59 and 3.91 dBi directional gains. Additionally, it measures 313.5 mm in total length and demonstrates an exceptionally low reflection coefficient. The monopole antenna provides a superior gain, lower reflection coefficient, and more compact form factor than its dipole counterpart.  
One paper aimed to design a practical shape for a microstrip patch antenna for the X-band (8 to 12 GHz) and Ku-band (12 GHz to 18 GHz). An artificial neural network was used to optimize the microstrip antenna's dimensions. The network inputs the different microstrip antenna parameters and delivers their dimensions in X-/Ku-band satellite communications. An error and validity analysis of the neural network results was conducted in MATLAB 2015. Finally, HFSS simulation software was used for the prototype of the microstrip antenna, which had the best antenna parameters compared with the actual value [65].