# Dual-Probe Near-Field Phaseless Antenna Measurement System on Board a UAV
## 3 Experimental Validation
For validation purposes, the same procedure followed with the on-site NF measurements was conducted with the NF on the measurement surfaces estimated from the NF measurements at a spherical range in the anechoic chamber (flowchart of Figure 7). Concerning the pSRM convergence, it was achieved after 33 iterations when the NF inputs were on-site measurements, and after 25 iterations when the NF was estimated from the anechoic chamber measurements. The calculation time was 55 s in a conventional laptop, that is, \(\sim\)8% of the measurement time using the UAV. Figure 5.1 NF on the measurement surface #1 depicted in Figure 5.2 Comparison between (**a**) the NF estimated from an equivalent currents model of the AUT using anechoic chamber measurements and (**b**) on-site NF measurements. The NF on the AUT aperture plane is depicted in Figure 5.3 Figure 10a,b correspond to the amplitude and phase distribution of the NF when the anechoic chamber measurements are considered, whereas Figure 10c,d are the estimated aperture fields calculated from the on-site NF measurements. In both cases, the amplitude distribution fits the shape and size of the reflector antenna. In the case of the phase distribution, the typical phase-shift of the off-centered reflector antennas can be noticed along the vertical direction (y-axis). For comparison purposes, the aperture fields distribution from the proof-of-concept shown in [12] are depicted in Figure 10e,f. In this case, the AUT diagnostics is not as accurate as in the improved prototype, as some artifacts appear outside the projected aperture of the reflector antenna (upper left side). The differences in the phase distribution are mainly due to the different tilt of the reflector antenna in this validation example and in [12]. Figure 5.4 Amplitude (**a**) and phase (**b**) of the reconstructed aperture fields from complex NF measurements at spherical range in anechoic chamber. Amplitude (**c**) and phase (**d**) of the reconstructed aperture fields from on-site NF measurements, current prototype. Amplitude (**e**) and phase (**f**) of the reconstructed aperture fields from on-site NF measurements, previous prototype [12]. Solid black line represents the projected profile of the reflector antenna. From the aperture fields, the AUT radiation pattern can be calculated by means of NF-FF transformation. To eliminate the noise and artifacts observed in Figure 10 which are outside the area corresponding to the AUT physical aperture size (solid black line in Figure 10), filtering can be applied. The aperture fields outside the AUT physical aperture size are discarded. Thus, the radiation pattern was calculated from the aperture fields of Figure 5.5 The far field pattern is depicted in Figure 5.6 It can be observed that the reflector antenna was slightly tilted when measured at the spherical range in the anechoic chamber. For a better comparison, the main beam vertical cut is shown in Figure 12c. The agreement between the far field calculated from the on-site NF-measurements using the UAV-based antenna measurement system, and the one calculated from the NF measurements at the spherical range in the anechoic chamber can be observed. Figure 5.7 Filtered aperture fields on the AUT aperture plane. Amplitude (**a**) and phase (**b**) of the reconstructed aperture fields from complex NF measurements at spherical range in anechoic chamber. Amplitude (**c**) and phase (**d**) of the reconstructed aperture fields from the on-site NF measurements using amplitude-only information. The solid black line represents the projected profile of the reflector antenna.