# A Review of Unmanned Aerial Vehicle Based Antenna and Propagation Measurements
## 2 UAV-Based Measurements
### Aerial Measurements of Low-Frequency Antennas  
The SKA is an array of telescopes that operates based on the principle of aperture synthesis and is designed for excellent spatial and angular resolution. A square kilometer array log-periodic antenna (SKALA) is a very large structure, and measuring such a large structure in the far-field region requires a large measurement distance. Performing UAV-based far-field measurements for these antennas is not economical owing to the battery limitations of UAVs. Sometimes, the UAV altitude needs to be more than 120 m, which is not possible owing to UAV flying guidelines. In such cases, the antennas are measured in near-field regions. One such case is the SKALA; it consists of 256 LPDAs with a diameter of 38 m. In the SKALA, each LPDA comprises a bowtie dipole for impedance matching. In [50], a pre-aperture array verification system for SKA was measured with UAVs in the near field. The AUT consisted of 16 active elements. All the elements were designed to feature a dual-polarization operation in the frequency range of 50-350 MHz, a minimum directivity of 8 dBi, and an intrinsic cross-polarization ratio exceeding 15 dB. An inter-element spacing of \(\lambda/2\) was maintained to achieve better control over beam steering. However, maintaining an inter-element spacing of \(\lambda/2\) is subject to mutual coupling [3].  
The pre-aperture array verification system of SKA, which is the AUT, has an overall size of 9.2 m over a 16 m ground plane mesh. The metallic grid ground plane improved the overall directivity of the system and provided protection from humidity, weather, and terrain conditions. The UAV was equipped with a portable signal generator and a dipole resonating at 175 MHz. The main challenge in these measurements is obtaining accurate phase values. In [50], to address this problem, an additional reference antenna with a known phase [51] was used. This reference antenna was placed 12 \(\lambda\) (20 m) from the center of the array, as shown in Figure 13. The UAV was equipped with a dipole and followed a quasi-planar trajectory at an altitude of 24 m. With an average speed of 3 m/s, the UAV took 15 min to complete the trajectory.  
Figure 12: UAV–based measurements for an array of HF wire biconical antennas.  
When the UAV follows the trajectory, the receiver system connected to the AUT at the ground level acquires voltages corresponding to the horizontal and vertical directions. The time stamps of the global navigation satellite systems (GNSSs) on the UAV and the time stamps of the GNSS at the ground level are synchronized at the receiver connected to the AUT. Finally, with the complex voltages and phase values, the embedded element patterns are reconstructed by performing a near-field to far-field transformation. The measured data from the UAV were compared with the simulated data, and the simulations were performed in the CST studio suite. From these comparisons, it was observed that the UAV-based near-field measurements were accurate. A deviation of 1 dB was observed in the amplitude. Thus, UAVs can be used to measure large structures such as the SKA in the near-field region. Thus, UAV-based measurements for the SKALA can help to identify areas where design improvements are required to improve the efficiency of the entire system. Table 2 presents a summary of large antennas, such as reflector antennas, ground plane antennas, ONERA biconicals, and SKALA LPDAs measured by UAV near-field techniques.