# Dual-Probe Near-Field Phaseless Antenna Measurement System on Board a UAV
## 3 Experimental Validation  
The aforementioned improvements in the proposed UAV-based system for on-site antenna measurements have been assessed experimentally. For this purpose, an offset reflector antenna working at 4.65 GHz, and fed with a circularly-polarized helix antenna, was selected as AUT (Figure 4). This AUT has a directivity of \(\sim\)30 dB, being the main beam (which has a \(-\)10 dB beamwidth of BW\({}_{-10\text{dB}}\) -12\({}^{\circ}\)) tilted \(\theta\) = 20\({}^{\circ}\) with respect to the vertical axis for this scenario. For this AUT, the FF distance is approximately 8 m (124 \(\lambda\)) at the frequency of 4.65 GHz. One of the reasons that led to the choice of this antenna was to prove the capabilities of the system for testing high directive antennas, which have been widely used in applications, such as radioastronomy or satellite communications. For reference purposes, the AUT was also measured at the spherical range in anechoic chamber of the University of Oviedo (Figure 4a). As this AUT was chosen for the on-site measurements conducted with the previous version of the prototype presented in [12], the results shown in this contribution are compared with the results in [12] to highlight how the proposed improvements have an impact in the measurement results. Figure 4: Pictures of the reflector antenna chosen as AUT. (**a**) Measurement at spherical range in anechoic chamber of the University of Oviedo; (**b**) Placed outdoors for on-site measurement. As shown in Figures 4b and 5, the AUT was mounted on top of a 1.5 m height metallic mast, and then placed in the airfield of the University of Oviedo. The transmitted signal was a tone at 4.65 GHz generated using an oscillator and later amplified up to 15 dBm. First, a preliminary flight along a vertical axis 4.5 m (70 \(\lambda\)) away from the AUT was conducted to determine the placement of the maximum of the AUT main beam. Once its position was found, the flight path and, consequently, the acquisition surfaces, were defined as shown in Figure 5.  
The implemented system allows defining the different kind of measurement domains, e.g., planar, cylindrical, spherical [29]. In the case of high directive antennas, such as the AUT selected for validation purposes, a planar measurement domain is sufficient for the proper characterization of the AUT, since most of the power radiated is expected to be within the acquisition domain. An acquisition domain of \((x,y)=(4,2.4)\) m size was defined, later truncated to \((x,y)=(3,2)\) m to reduce the amount of NF data, as shown in Figure 6.  
The convergence of the iterative phase retrieval methods, such as the pSRM, is influenced by the distance between the acquisition surfaces and the AUT, as well as the spacing between the acquisition surfaces. A quantitative assessment of the influence of these parameters was provided for this AUT, taking advantage of the fact that this antenna was characterized at the spherical range in the anechoic chamber. In particular, an analysis based on simulated flight path positions using the NF calculated on these positions from an electromagnetic model of the AUT was conducted. This model was obtained from the NF measurements at a spherical range in the anechoic chamber (Figure 7) following the methodology presented in [18]. The results depicted in Figure 8 show that the best cases correspond to a spacing between acquisition surfaces within 60 cm-120 cm (9.3 \(\lambda\)-18.6 \(\lambda\)) and an AUT-acquisition surface distance not greater than 5 m (77.5 \(\lambda\)). However, the minimum and maximum UAV flight distances from the AUT (depicted in Figure 8) must be taken into account: too close increases the risk of accidental collision (e.g., due to a sudden wind gust) and too far increases the size of the acquisition grid. Based on the results of Figure 8, it was decided to set the AUT-UAV distance at approximately 4.5 m away from the AUT (1.5 m further than in [12]).