# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements
### UAV-Based Near-Field Measurements  
To meet the far-field criteria, low-frequency antennas require a considerable distance between the AUT and UAV carrying the antenna. Such long distances may result in excessive free-space path loss, which can reduce overall system accuracy. In such cases, near-field measurements can be useful for EM wave measurements in the radiative near-field region. It is easy and quick to conduct near-field measurements with a UAV and does not require heavy and complicated equipment. In [39], a biconical antenna operating at 110 MHz was measured using a UAV under indoor conditions. Time domain gating techniques were applied to avoid ground reflections and UAV motion. Similarly, in [40], navigation systems were measured in the near-field region. The measurement results from [39; 40] prove that UAV-based near-field measurements can be performed accurately with low-cost equipment. As shown in Figure 11, UAV-based measurements were performed at radiating near-field regions beginning at 2.5 m for a grid reflector with a maximum diameter of 1 m that operates at 4.65 GHz [41]. Because the accuracy of UAV-based near-field measurements depends on UAV coordinates, [42] used dual-band real-time kinematics (RTK), which made it possible to obtain UAV coordinates within 10 arcseconds. The UAV was equipped with two monopoles operating in the 4-7 GHz frequency band that were separated from \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Frequency of Operation** & **Far–Field Distance** & **AUT** \\ \hline
[27] & 14.5 GHz & 350 m & Parabolic reflector. \\
[28] & 9.5 GHz & 60 m & Reflector mounted on ship. \\
[35] & 515 MHz & 2025 m & Broadcasting antenna. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Far–field measurements. Figure 10: (a) UAV–based measurements at 720 m and (b) UAV–based measurements at 2025 m.  
each other by a space of 80 cm. This setup measured a reflector system operating at 4.65 GHz fed by a circularly polarized helix antenna. The monopoles were mounted on a UAV using three-dimensional (3D) printed components, which are highly resistant to mechanical vibrations and transparent to EM waves. Near-field measurements were performed by obtaining the equivalent current distribution over the surface of the AUT. A phase-less retrieval technique was utilized to plot the radiation pattern based on the integral equation method [43], in which the simulated AUT was modeled with equivalent electric and magnetic fields on a closed surface. After obtaining the radiation patterns for the reflector system deployed in the field, measurements were conducted in anechoic chambers. Comparisons between the results obtained from the UAV-based near-field measurements and the measurements in anechoic chambers indicate that when the reflector system is deployed in the field, there is an offset in the beam position and the main beam is widened owing to misalignment errors and ground reflections. In Figure 12, a UAV carrying a monopole is used to characterize an antenna operating at 3-30 MHz. The AUT is a Notradamus ONERA system [44] consisting of a set of 288 biconical antennas arranged along a branch separated by 120\({}^{\circ}\). Biconicals are omnidirectional, and each has a height of 7 m and a width of 6 m. To validate the accuracy and functionality of UAV-based near-field measurements for characterizing the high-frequency (HF) antenna by ONERA, ref. [6] used a Da-Jiang Innovations (DJI) Matrice 600 Pro carrying an antenna to measure a monopole that was 6 m high when placed on a ground plane. The measured data were compared with simulated data, and good agreement was achieved, thus proving that UAV-based near-field measurements are a cost-effective solution for characterizing HF antenna systems. When selecting the appropriate material for mounting the antenna on the UAV, it is important to ensure that the UAV body does not degrade the performance of the antenna.