# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements
### UAV-Based Far-Field Measurements  
To perform far-field measurements, it is essential to maintain a minimum distance of \(2D^{2}/\lambda\), as discussed previously. A UAV equipped with high-precision controller boards and RF measurement equipment, such as an SDR [21] with directional antennas, can be used for far-field measurements. For instance, in the inspection of reflector systems [7] and structurally large antennas, UAV-based measurements are cost-effective and reliable. The measurement techniques proposed here have several advantages over conventional methods, such as the use of helicopters equipped with RF payloads [22], in terms of cost and maneuverability. A compact and lightweight design allows UAVs to easily reach any location for measurements. They can also hover at a specific location, which enhances their ability to conduct RF measurements, with improved results. Various techniques, such as fast Fourier transform (FFT) [23], angular deconvolution [24], spatial mode filtering [25], frequency impulse response, and Hilbert transform, can be used to filter noise when measurements are conducted outdoors in a noisy environment. Considering that the above methods are not generic and cannot be applied to all environmental conditions, [26] proposed a filtering technique referred to as locally weighted regression and dispersion smoothing, which can be used to filter out high-frequency noise. To validate this methodology, measurements were conducted in an anechoic chamber as well as an outdoor environment using the proposed filtering technique on a Yagi-Uda antenna operating at 2.4 GHz and a horn antenna at 5.3 GHz. An analysis of the HPBW measured in an anechoic chamber and an outdoor environment showed that they were in good agreement, with a difference of only \(1^{\circ}\). In the far-field measurements, the far-field patterns on a sphere of a constant radius were estimated. The elevation and azimuth angles, denoted by \(\theta\) and \(\psi\), were the variables used to identify the location on the sphere. The phase information of the AUT was obtained using a vector network analyzer (VNA), and the two-dimensional amplitude information may be calculated using the total electric field:  
\[|E|=\sqrt{\left(E_{\theta}\right)^{2}+\left(E_{\psi}\right)^{2}}, \tag{1}\]  
where \(E_{\theta}\), \(E_{\psi}\) are the electric field in the elevation and azimuth planes, respectively. In UAV-based far-field measurements, the AUT is placed on a tripod and the UAV follows a vertical and horizontal path around the AUT. The UAV carries an RX antenna while moving and collecting data points. The AUT is stationary in this scenario, unlike the conventional outdoor ranges, which can be either elevated or elevated slant ranges, and requires the AUT to rotate by means of positioning commands operated by a computer.An example of a conventional elevated slant range is shown in Figure 3, in which a TX antenna, such as a quad-ridged horn, is mounted on a tall structure, and an AUT, such as an offset-fed parabolic reflector system, is mounted on an azimuth positioner. To eliminate errors caused by ground reflection, absorbers are placed between the source antenna and the AUT. Generally, a TX antenna is designed to have a low SLL, and the height at which the source antenna is mounted should be selected such that the main beam illuminates the AUT. Traditional measurements are performed using a stationary source antenna, and the radiation pattern can be obtained in both the elevation and azimuth cuts by rotating the AUT placed on the positioner. However, in UAV-based measurements, the UAV carrying the antenna follows a trajectory and the AUT is stationary. The advantage of this setup is that it does not require heavy and expensive positioners to obtain AUT radiation patterns. As shown in Figure 4, a UAV with a quad-ridged dual-polarized horn operating at 6-24 GHz was used as a far-field transmitting antenna [27] to measure the microwave vision group (MVG) SR40 parabolic reflector system.