# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 3 UAV-Based Propagation Measurements
### UAV-Based Small-Scale Propagation Measurements  
In [76], to investigate the dependence of small-scale fading on the altitude of a UAV, a hexacopter was equipped with a circularly polarized cloverleaf wire antenna, as shown in Figure 15. The receiving system consisted of a magnetic mount wideband high-frequency (MGRM-WHF) antenna, which is independent of the ground plane and was installed on a mast at the ground station, 1.5 m above the ground. The test environment consisted of multiple buildings and metal containers with the UAV taking a vertical path ranging from 0 to 100 m in height and a horizontal path maintaining 20-60 m from the receiver at ground level. The path loss exponents (PLEs) were estimated by varying the height of the UAVs to determine the relationship between small-scale fading and the UAV altitude. Furthermore, small-scale fading calculations are categorized into LOS and NLOS conditions. The Rician-K factor was utilized in the LOS case to explain the fading behavior. Adding the height parameter of a UAV to the Rician-K factor provided a better understanding of the small-scale fading. At lower altitudes, multipath components [77] from buildings and metallic containers combined vectorially at the receiver, causing fading. The cumulative distribution functions (CDF) [78] estimated small-scale fading in both the LOS and NLOS conditions. In [75], a DJI N3 six-rotor UAV, equipped with a \(\lambda/4\) monopole, was used to determine the fading depth, using UAVs by varying their vertical paths from 0 to 24 m in height, with a receiving station positioned 25 m away from the UAV. A \(\lambda/4\) monopole with a ground plane and gain of 5.2 dBi, connected to a portable signal generator, enabled the continuous transmission of signals. We note from the measurements that the fading depth was independent of the operation frequency, which was more evident for the LOS conditions than for the NLOS conditions. A distribution function, such as the Nakagami, Rayleigh, Weibull, or Gaussian function, can describe the fading amplitude. By maintaining a root mean square error (RMSE) as low as 0.02 dB for both the LOS and NLOS scenarios, the log-logistic function [79] is the best distribution function among the available distribution functions. In [80], to investigate the scattering effect of the buildings, ATG propagation measurements were conducted using a custom-built UAV equipped with a mmWave conical horn antenna (operating in the 26-40 GHz range) configured as the receiver and a planar elliptical dipole ultra-wideband (UWB) antenna (operating in the 3.1-5.3 GHz range) configured as the transmitter. In contrast, the ground station featured an mmWave conical horn antenna as the transmitter and a UWB antenna as the receiver. These ATG propagation measurements were instrumental in understanding the propagation characteristics of urban  
Figure 15: Hexacopter carrying cloverleaf wire antenna. environments. This study presented power angle profiles for ATG propagation, which showed that in urban areas the presence of building rooftops causes a reduction in signal strength due to scattering. Additionally, the power elevation profile results indicated that when the UAV was at a higher altitude (50 m), the dominant propagation mechanism was due to reflections from buildings located behind the ground station. Measurements of outdoor-to-indoor coverage, conducted with the UAV hovering outside a building and the ground station positioned inside the building, revealed significant losses as the signals propagated through the building walls at both mmWave and UWB frequencies. In ATG channels, which consist of a wireless channel between the UAV and the ground system, knife-edge diffraction (KED) is a commonly employed method for estimating the signal strength attenuation caused by diffraction. In KED, the EM wave diffracted by the building corners is determined by considering the obstacles to be thin and perfectly absorbing.