# Dual-Probe Near-Field Phaseless Antenna Measurement System on Board a UAV
## 3 Experimental Validation
The acquisition distances were 4.5 (70 \(\lambda\)) for the first measurement grid and 5.3 (82 \(\lambda\)) for the second one (see Figures 5 and 6). Figure 5: Picture of the UAV and the reflector antenna placed on top of a 1.5 m high metallic mast, and the scheme of the distances of the measurement domain. A grid spacing in height (y-axis) of 5 cm (0.77 \(\lambda\)) was chosen as a trade-off between the sampling rate and the number of scans that can be performed taking into account the capacity of the UAV batteries. If the grid spacing was set to 0.5 \(\lambda\) (3.2 cm) to ensure Nyquist sampling rate [13], this would result in 75 scans along \(x\)-axis, which would require replacing the battery before the acquisition can be completed. With a grid spacing of 5 cm, the number of scans is reduced to 48, avoiding UAV battery replacement in the middle of the measurement process. Nevertheless, in the case of planar NF measurements of directive antennas, the relaxation of the sampling rate above Nyquist can be admitted, provided the grating lobes do not fall within the FF angular margin of validity. For validation purposes only, the horizontal (x-axis) component of the field was measured, as it can be deduced from the position of the monopole probe antennas in Figure 1. Nevertheless, the vertical (y-axis) component of the NF could be acquired as well by rotating the two probes 90\({}^{\circ}\) and repeating the flight using the same flight waypoints defined for the measurement of the horizontal component. Once the flight path was defined, the AUT was measured using the UAV-based system described in Section 2. The NF measurements were simultaneously acquired in both the acquisition surfaces after a 12 minutes flight. The overall number of samples collected was 6016, in agreement with the number of samples that would result from a uniform half-a-wavelength (\(\lambda\)/2) sampling of a 3 m \(\times\) 2 m plane, which would be 5766. A video of the UAV-based system in operation can be watched at: [[https://youtu.be/k3](https://youtu.be/k3) CleGTKhxE]([https://youtu.be/k3](https://youtu.be/k3) CleGTKhxE) (see Supplementary Materials). Figure 6: Amplitude (normalized, in dB) of the near field (NF) measured at the geo-referred UAV flight path positions. Notice the offset in the vertical axis (y-axis) of the acquisition surfaces in order to capture the tilted main beam of the reflector antenna. The origin of the coordinates system is at the antenna. Figure 7: Flowchart of the validation methodology followed in the example presented in Section 3. Figure 8: Analysis of the pSRM convergence (cost function error) for different spacing between acquisition surfaces and several distances to the AUT. The simulated geo-referred flight path acquisition points have been considered, evaluating the NF radiated by an electromagnetic model of the AUT at these positions. Regarding validation purposes, on-site NF measurements were compared with the NF estimated from an equivalent currents model of the AUT obtained from complex measurements at a spherical range in the anechoic chamber. The NF radiated by the equivalent currents model was evaluated at the positions shown in Figure 6, as outlined in the flowchart depicted in Figure 7. The comparison of the NF measured and evaluated at the two acquisition surfaces is depicted in Figure 9. Despite the main lobe of the reflector antenna being properly characterized, the positioning and geo-referring uncertainties as well as the limited dynamic range of the dual-channel receiver introduce some distortion in NF measurements acquired with the UAV-based system (Figure 9b). Next, the amplitude-only information of the NF acquired at the two measurement surfaces shown in Figure 6 was introduced in the pSRM to recover an equivalent currents model of the AUT, as outlined in the flowchart of Figure 5.0 The recovered equivalent currents model of the AUT is used to calculate the NF on the AUT aperture plane as well as the radiation pattern.