# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements
### UAV-Based Far-Field Measurements
#### 2.1.1 Aerial Measurement of Base Station Antennas  
To establish communication, a mobile BASTA [31] is essential and serves as a communication hub for wireless devices. Owing to the exponential increase in the number of devices connected to wireless networks, BASTAs are being deployed at an unprecedented rate to provide connectivity to users. Certain errors may occur when BASTAs are deployed in the field. These errors include undesired antenna twists, antenna tilts, errors in antenna alignments, and the effects of adjacent objects and towers on radiation patterns. In such cases, performing an in-situ measurement allows one to identify faults and repair the system, thereby improving its performance. In traditional airborne measurements, a helicopter is used to measure the radiation pattern. However, these methods are expensive and require heavy equipment. Advancements in UAVs and the miniaturization of RF components, such as portable spectrum analyzers, have enabled the measurement of mobile BASTA systems using UAVs [32].  
UAV-based measurements for BASTAs involve measuring vertical and horizontal radiation patterns. The vertical radiation pattern is determined using the procedure shown in Figure 6. To ensure optimal reception from the AUT, the UAV should be positioned at an appropriate height to maintain LOS with the BASTA. Once the optimal vertical location is determined, the UAV follows a vertical path and reconstructs the radiation pattern. Typically, a BASTA consists of several antenna elements, such as dipoles, arranged in an array. Each of these elements has its own radiation pattern. The resultant radiation pattern  
Figure 5: UAV-based measurement for a parabolic reflector antenna system placed on a ship.  
is formed by combining all the elements in an array. Therefore, the UAV must follow a vertical path to reconstruct its vertical radiation pattern. The electrical down-tilt and null fill can be determined by obtaining the vertical radiation pattern of the BASTA. Electrical down-tilt [33] and null fills are significant parameters that affect the base station coverage area. Ground users experience maximum signal strength when the main lobe is directed towards their area, whereas they do not receive any signal when a null fill is directed towards them. In [31], a UAV programmed with a mask R-CNN was used to automatically determine the base station orientation. R-CNN is an object detection algorithm used to detect specific regions in an image. The proposed method in [31] involves creating a database named UAV-antenna, which consists of 19,715 communication BASTA images. This is achieved by the capturing of BASTA images by UAVs. Secondly, mask R-CNN applies a selective search scheme to identify the pixel coordinates of the BASTA. These pixel coordinates are used to measure the BASTA's tilt angle. Based on the proposed method, after completing the measurements it was found that the actual tilt angle of the antenna system deviated by 1\({}^{\circ}\)-2\({}^{\circ}\) from the intended tilt angle.  
Figure 7 illustrates the procedure followed to obtain the horizontal radiation pattern of a BASTA deployed in the field, which was measured using a UAV with an RX antenna. Based on the optimum height, at which the RX antenna mounted on the UAV receives the maximum power from the AUT, which is determined during the vertical pattern measurements, the UAV follows a trajectory in a circular path around the AUT. Obtaining a horizontal radiation pattern enables one to determine azimuth HPBW, sector power ratio (SPR), and front-to-back ratio (FBR). SPR is the ratio of power outside the desired sector to power inside the desired sector. This helps to improve the antenna design, which requires the SPR to be as low as possible to achieve lower co-channel interference and better call quality. Ideally, a BASTA should have an SPR less of than 3% and an FBR greater than 25 dB [34].  
Figure 6: Vertical radiation pattern of a BASTA using a UAV.