# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 3 UAV-Based Propagation Measurements
### UAV-Based Small-Scale Propagation Measurements
The magnitude of the diffraction losses is calculated using mathematical formulas that consider Fresnel diffraction parameters. According to the UTD, diffraction losses are estimated using wedge geometry, which involves the wedge angle and reflection coefficient of obstacles and empirical models such as the linear regression model and the creeping wave linear model [81]. In [82], to understand the accuracy of KED and the empirical models, diffraction loss measurements were performed over a roof top in urban environments at 28 GHz. The measurement setup consisted of a transmitter antenna with a beamwidth of 10\({}^{\circ}\) and a receiver antenna with a beamwidth of 30\({}^{\circ}\). The measurements were conducted at two sites to understand the influence of the TX distance from the LOS boundary and the RX distance from the LOS/NLOS boundary. The study found that the diffraction losses increased when the distance from the diffraction edge increased and decreased when the distance between the TX and the building decreased. The loss was shown to be proportional to the diffraction angle. Using UAV-based propagation measurements, we can estimate the attenuation of the signal when the transmitter follows vertical and horizontal paths. In contrast, conventional methods fail to evaluate diffraction losses and multipath components from the corners and edges of buildings, which are typically between 10 m and 25 m in height. In [83], horn antennas with a gain of 20 dBi and an HPBW of 17\({}^{\circ}\) were used indoors and outdoors for propagation measurements. The indoor measurements were analyzed using three types of wall construction: plastic boards, wooden walls, and dry walls. During the measurements, the receiving and transmitting horns were placed at a 1.4 m height above the corner of the wall. The measurements were performed at 10 GHz, 20 GHz, and 26 GHz. The measured data were compared with the theoretical estimates using the KED model. The practical measurements were in good agreement with the theoretical calculations for a dry wall. However, the KED overestimated losses by 2-4 dB in the case of wooden walls and plastic boards. For outdoor measurements, it was found that KED accurately calculated the diffraction losses for sharp edges, whereas linear models using a minimum mean square error (MMSE) linear fit derived from actual measured data were more accurate for rounded edges. To investigate the scattering effects of buildings, we created an ATG propagation scenario. In this setup, two buildings with heights of 10 m and 20 m were modeled using CST Studio Suite. Building 1 was modeled with a height of 20 m while Building 2 was modeled with a height of 10 m. A horn antenna configured as a receiver was mounted on a mast placed on top of Building 2 to identify potential scattering regions. In the simulation environment, we modeled a UAV equipped with a horn antenna flying at a height of 17 m above ground level. The complete simulation setup, including the scattered rays, is shown in Figure 16. The two buildings were positioned 30 m apart in this scenario. The structure was analyzed using an asymptotic solver based on the shooting and bouncing ray (SBR) technique, which allowed us to observe how signals were diffracted at the corners of the buildings. The SBR technique provides an initial estimation of ATG propagation. However, to accurately understand scattering effects in real-world scenarios, practical UAV-based propagation measurements are necessary. In such measurements, a UAV equipped with a transmitter antenna and a portable signal generator would be used. On the ground, a receiver setup consisting of a horn antenna mounted on a mast and a spectrum analyzer connected to the antenna would be used to calculate the power levels of the received signal. Several propagation measurements were carried out in [84] using a hexacopter equipped with a narrowband antenna resonating at 440 MHz and a wideband antenna operating between 1 GHz and 6 GHz. The measurements were performed in a suburban area at 440 MHz and 1 GHz.