# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 3 UAV-Based Propagation Measurements
### UAV-Based Large-Scale Propagation Measurements  
UAVs can be used to measure large-scale propagation effects that occur mainly owing to path loss and shadowing. To estimate the path loss, the commonly used models are the free-space path loss (FSPL), two-ray, basic log-distance, and modified log-distance models. For the path loss calculation, the FSPL requires information about the transmitter antenna gain (\(G_{t}\)), receiver antenna gain (\(G_{r}\)), operating wavelength (\(\lambda\)), and separation (\(d\)) between the UAV and receiver. On the other hand, when the UAV is at a lower altitude and ground reflections are present between the UAV and receiver, two-ray models can be utilized to estimate the path loss. In [56], a DJI Mavic 2 Enterprise UAV equipped with a LoRa sleeve dipole operating at 868 MHz was used for propagation measurements between a UAV and a wireless sensor network (WSN) and between a UAV and an unmanned surface vehicle (USV). The results of the UAV-based propagation measurements for a scenario in which the UAV moved vertically up to 30 m, and another scenario in which the UAV moved horizontally away from the receiver (WSN or USV), were compared with path loss estimations from two-ray models. Based on the comparison, the two-ray model underestimated the path loss, resulting in a mean difference of 6.45 dB between the UAV and USV and 15.5 dB between the UAV and WSN. These findings demonstrate the necessity of improving the two-ray model to increase the accuracy of path loss measurements. A higher level of precision is required for mmWave [57; 58; 59] and for situations in urban areas [60], which are surrounded by multiple buildings and obstacles. Log-distance path loss models are more general and appropriate for calculating path loss [61]. In evaluating the path loss for a channel between a UAV and a receiver, the log-distance method considers the path loss exponent (\(\alpha\)). An improvement in the log-distance model is the modified log-distance model. According to the modified log-distance model [62; 63], we can determine the path loss by  
\[PL(\text{dB})=PL_{0}(\text{dB})+10\alpha log_{10}\bigg{(}\frac{d}{d_{0}}\bigg{)} -10 log_{10}\bigg{(}\frac{\Delta h}{h_{opt}}\bigg{)}+C_{p}+10 log_{10}\bigg{(}1 +\frac{\Delta f}{f_{c}}\bigg{)}, \tag{2}\]  
When estimating the path loss, the modified log-distance model considers an additional parameter known as the height of the UAV from the ground (\(h_{gnd}\)), the minimum height of the UAV (\(h_{opt}\)) that provides the lowest path loss, and a constant loss factor \(C_{p}\), representing the losses due to the antenna orientation on the UAV and carrier frequency \(f_{c}\). \(PL_{0}\) (dB) is the path loss at the reference distance \(d_{o}\), \(\Delta f\) is the Doppler variation in frequency, and \(\Delta h\) is the difference between \(h_{gnd}\) and \(h_{opt}\). To determine the extent to which the various factors discussed above attenuate the signals, we created a scenario with a UAV carrying the transmitting antenna. The propagation measurements [64], as described in Figure 14, consisted of a car and horn antenna that was identical to that mounted on a UAV. The ground-level antenna worked in the receive mode and was mounted on a 2 m mast in front of a 10 m building. The horn antenna was mounted on the UAV hovering 12 m above ground level. Modeling and simulations were performed using commercially available EM software, CST Studio suite 2023 [65] at 5 GHz. Part of the signal was diffracted by the corners of the buildings at a height of 10 m and by metallic components of masts and obstacles, such as cars, before reaching the receiver at ground level. In [66], a UAV equipped with a dipole and scanner capable of measuring RSRP was utilized to calculate the path loss, with the UAV's altitude varying from 1.5 m to 120 m above ground level.