# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements
### UAV-Based Far-Field Measurements
#### 2.1.2 Aerial Measurement of Broadcasting Antennas  
In [35], a SixArms custom-built hexacopter with a log-periodic antenna was used to perform broadcasting antenna measurements in the far-field region. It was used to measure the effective radiated power (ERP) and horizontal radiation pattern (HRP), as well as the vertical radiation pattern (VRP). As shown in Figure 8, high-power broadcast antennas can experience certain deviations in their performance, leading to a degradation in the overall coverage area. The feeding mechanism of a broadcast antenna [36] plays a significant role, and these systems are vulnerable to changes. Upon performing the in-situ measurements and comparing the measured VRP with the design specifications, it was observed that there was a 1\({}^{\circ}\) deviation in the tilt in the test case of [32]. Similarly, a change of 0.5\({}^{\circ}\) in the electrical tilt was observed owing to the change in the mechanical lean of the broadcast antennas. By measuring the HRP, other common errors, such as incorrect panel orientation and inverted panels, could be identified and eliminated, thereby enhancing the overall performance of the broadcast antenna [37]. By comparing the HRP of the broadcasting antenna measured with the UAV with the design specifications, it became apparent that a 10\({}^{\circ}\) deviation arose in this test case from an incorrect panel orientation after the panel was installed on the tower. Similarly, measuring the HRP of the broadcast antenna, when there was a taller tower at just 650 ft, showed a 3 dB notch in the plots, which implies that the adjacent tower impacts the measured system. Figure 8: Common errors in broadcasting systems. Figure 7: Horizontal radiation pattern of a BASTA using a UAV. Ideally, when amplitude-only measurements are performed, UAVs carrying transmitters/receivers are in the far-field region (the Fraunhofer region) to measure the radiation pattern [20]. To validate the theoretical concept of far-field regions, the SixArms Airborne Radio Measurement Systems (ARMS-RFX) UAV was used to measure a DTV station at 720 m and 2025 m from the AUT. DTV broadcasting antennas are composed of antenna arrays formed by similar elements. The total height of a DTV broadcasting antenna with all the elements in an array is typically 20 m, with a maximum antenna dimension of 20 m and a frequency of operation at 515 MHz (UHF channel 21 in the USA). Far-field measurements with UAVs can be performed by maintaining at least 1450 m from the AUT. As shown in Figure 9, ARMS-RFX UAVs equipped with an ARMS receiver comprising a real-time spectrum analyzer and an embedded PC [38] were used to measure the DTV transmitter station at 720 m and 2025 m from the AUT. Log-periodic antennas (LPDA) mounted on top of the UAV received signals from the AUT. The radiation pattern of the AUT was calculated instantly for every 0.1\({}^{\circ}\), as the UAV took a vertical path. Using a telemetry link, the measured radiation patterns were transmitted to the ground user for quick verification. The elevation patterns taken at 720 m, which should have been 1450 m according to the theoretical far-field distance calculations, and at a far-field distance of 2025 m from the AUT, are depicted in Figure 10. In Figure 10a, the red dashed lines represent the elevation pattern of the AUT according to the manufacturer's data sheet, and the solid black line represents the measurements performed with the UAV at 720 m. Similarly, Figure 10b represents the measurements performed at 2025 m from the AUT. From the elevation patterns measured at 720 and 2025 m, it was observed that the measurements do not always have to be in the far-field region. UAV-based measurements can be performed closer to the AUT and are still valid. From the two cases depicted in Figure 10, it is evident that when measurements are performed at 720 m from the AUT, the null fill and null depths vary slightly compared to measurements performed at 2025 m from the AUT.