# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements  
Characteristics such as the matching impedance, polarization, radiation efficiency, directivity, gain, and radiation patterns are used to ascertain the antenna performance. For radar systems with stringent specifications, such as polarimetric weather radar, calibration of the systems after deployment in the field is crucial. Figure 2 describes the methodology adopted for UAV-based measurements, in which the UAV is equipped with an antenna at a height \(h_{uav}\) that serves as a signal source, while the AUT is at a height \(h_{aut}\) from the ground plane. An antenna mounted on a UAV can act as either a transmitting (TX) or a receiving (RX) antenna. As shown in Figure 2, the AUT and the TX antenna mounted on the UAV were separated by distance \(R\). Here, \(\alpha\) denotes the half-power beam width (HPBW) of the TX antenna mounted on the UAV, and \(\alpha_{h}\) denotes the plane angle subtended at the antenna mounted on the UAV by the AUT height [18].  
In the process of measuring the radiation pattern of an AUT using a UAV, several factors, such as the phase curvature of the incident wave, ground reflections, and amplitude taper of the source antenna, affect the measurement accuracy. To avoid measurement inaccuracies, caution should be exercised regarding the variations in the phase and amplitude of the incident field and the interference from ground reflections. For UAV-based measurements, as a rule of thumb, the TX antenna is selected such that it has a wider HPBW than the AUT to prevent measurement errors. Similarly, the phase curvature of the incident field on the AUT affects the accuracy of the measured SLL. To overcome these errors, the phase deviation over the planar test aperture is maintained below \(22.5^{\circ}\). To achieve this phase deviation, it is necessary to have a separation distance greater than \(2D^{2}/\lambda\), where \(D\) is the maximum dimension of the antenna and \(\lambda\) is the operating wavelength [19]. The other factors that affect the accuracy of the measurements are ground reflections.  
Ground reflections can often cause signals to be added constructively/destructively, resulting in apparent gain values that are higher or lower than expected. Also, maintaining a sufficient distance between the TX and RX antennas prevents the distortion of the patterns caused by ground reflections. Different approaches have been used to reduce ground reflections, including diffraction screening and absorbers between the TX and RX antennas. The use of a TX antenna with a low SLL can also prevent ground reflections [20]. At the same time, aligning the beam peak of the TX antenna with the AUT is essential. This can be achieved in a couple of iterations, which involve measuring the power received at the AUT when a UAV is carrying the TX antenna that flies at different altitudes. At a particular height, the UAV can further vary its altitude slightly to detect the position corresponding  
Figure 2: Measurement configuration of the UAV system.  
to the maximum received power of the AUT. At this height, the beam peak from the TX is appropriately aligned with the AUT. With the TX aligned with the AUT and by maintaining a low SLL, ground reflections can be avoided to a certain extent.