## 3 Airborne Segment
### Airborne Antennas Technologies  
In general, a flying platform is equipped by many electronic components that can be grouped into three main subsystems [27]: flight control, energy management and transceivers. The flight control subsystem, composed of some sensors and actuators, is responsible of platform stabilization and mobility. The energy management subsystem handles the energy and its storage by using solar panels and batteries, overall being responsible for available energy. The transceiver subsystem represents the set of electronic components that allow one to transmit and receive data. According to the mission and the application purposes, different equipment and technologies can be adopted into these onboard subsystems. In this subsection, one of the most important components of the transceiver subsystem is discussed, namely the radiating system. Antennas are certainly among the fundamental components of UAVs, and they are determinant for the performance of the onboard transceiver subsystem. Therefore, high gain, high efficiency and low-profile airborne antennas represent some key requirements to consider during the design phase. For example, an antenna array composed of four printed monopole antennas working at 2.4 GHz embedded in the structural components of a UAV wing is proposed in [139]. An efficient radiator composed of compact and low-profile probes accurately placed on the UAV body has been designed in [140] by exploiting the Characteristic Modes Theory (CMT) [141; 142; 143]. In [144], a broadband slotted blade dipole antenna is described. A conformal phased array antenna for UAVs with wide scanning range is presented in [145]. Some solutions regarding the design of radiating systems for HAPs are illustrated in [146; 147; 148]. Figure 6: Example of Hybrid-Duplex (HBD) wireless communication. Concerning the coverage area, the flying platforms scenario differs from that of ground segment, whose coverage is typically rectangular in the \(uv\) plane, since a circular scan area turns out to be more appropriate [91]. Then, within a predefined circular scenario, there could be several possible cells configurations. Figure 7 illustrates two examples of cells configuration within a circular scanning area with a maximum coverage angle of \(\sin(\theta_{max})\) off broadside direction (\(\theta\) = 0\({}^{\circ}\)), namely with \(n\) = 8 cells (Figure 7a) and \(n\) = 20 cells (Figure 7b). The cell configuration scheme has a strong impact on the UAV antenna parameters such as the Half Power Beam Width (HPBW) on both horizontal and vertical planes as well as lateral lobes. As previously stated, 5G, 6G and future wireless generations open the door to mmWave communications. However, owing to a deeper propagation loss and higher sensitivity to obstacles, they have to cope with a coverage limitation when compared to sub-6 GHz communication systems. Therefore, active electronically beam-scanning antenna arrays represent a pivotal technology for the air segment to provide high gain capable of counteracting high path loss, offering low interference communications as well as concurrent multibeam radiation patterns. However, in the case of mmWave, the radiating system design turns out to be even more important due to the significant losses of phase shifters and a lower Power Amplifiers (PAs) efficiency [149] that lead to a more complicated thermal management [150]. From the energy point of view, passive cooling systems are preferred to active ones by the industry since they do not need electricity. In the framework of antenna array design, the simplest way to help the cooling system to dissipate heat is to increase the distance among antenna elements [20]. However, increasing the inter-element spacing too much could lead to grating lobes or high lateral lobes inside the visible region, with a harmful interference effect in a multisens scenario. The most popular array layouts are organized in square or rectangular lattices.