# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 3 UAV-Based Propagation Measurements
### Selecting the UAV Antenna  
Choosing an appropriate antenna for UAV-based measurements is essential before conducting measurements. To ensure that the UAV-based in situ measurements are accurate, it is important to calibrate the antenna before mounting it on the UAV. Several factors are considered when selecting a UAV antenna: it should be compact, lightweight, mechanically stable, unaffected by wind, and electromagnetically insensitive to the structure of the UAV. Because of the several metallic components on the body of the UAV, directional antennas are likely to experience EM coupling with the UAV body, which can degrade its performance.  
To understand the EM behavior of the antenna [85] mounted on the UAV, we simulated a complete UAV structure using the CST Studio Suite. Figure 17 depicts a DJI F450 UAV equipped with a pyramidal horn antenna simulated at 8 GHz using the SBR technique. Apart from the main beam from the horn, there is a portion of signals scattered from the UAV body, which can create errors in antenna measurements. This explains the necessity for care to be taken before selecting an antenna and understanding its behavior after mounting [86] it on the UAV [87]. There are several ways to mitigate the effects of scattering, including changing the antenna design [87; 88], optimizing the antenna location [89], and using RF absorbers in areas where the UAV body exhibits potential reflections. Whenever we choose antennas for UAVs, there is always a tradeoff between an antenna with a narrow beamwidth and an antenna with wider beamwidth. Antennas with a wider beamwidth  
Figure 16: Scattering effect in an urban area simulated in CST Studio Suite.  
cover wide angles, which means that small deviations in the alignment of the UAV relative to the AUT have a less pronounced effect. The signal remains closer to the intended polarization, minimizing the introduction of unwanted cross-polarized components. However, with an antenna with a wider beamwidth, there will be high scattering from UAVs, affecting the co-polarization and cross-polarization patterns.  
On the other hand, antennas with narrow beam widths have an advantage in terms of low scattering from the UAV body. However, they have some limitations as well. The major challenge for these antennas is alignment between the antenna mounted on the UAV and the AUT. In the case of an antenna with a narrow beamwidth, vibrations from the UAV body can create a misalignment between the antenna mounted on the UAV and the boresight of the AUT. To minimize misalignment errors, additional efforts must be made to maintain the antenna's beam peak at the AUT's boresight throughout its trajectory.  
Table 3 presents the different antennas used in the literature. An omnidirectional or directional antenna was used depending on the area of application. These antennas are specially designed for UAV applications, considering beamwidth and radiation pattern constraints. In cases such as dipole [90] and helix [91] antennas, the ground plane is included as part of the antenna. The commonly used antennas with directional or omnidirectional patterns have limitations. Directional antennas are prone to misalignment errors; hence, additional precautionary steps are required to overcome them. Omnidirectional antennas have the limitations of low gain. On the other hand, in [92], an array of half-bowtie antennas was designed to cover all hemispherical regions. This design has better coverage with an HPBW of 240\({}^{\circ}\) in the azimuth and 98.6\({}^{\circ}\) in elevation, similar to an omnidirectional antenna; with a gain of around 5.9 dBi.  
Figure 17: Fields scattered by the UAV body.