# Dual-Probe Near-Field Phaseless Antenna Measurement System on Board a UAV
## 1 Introduction  
In the framework of novel wireless communications systems, such as 5G or Internet of Things (IoT), unmanned aerial vehicles (UAVs) are playing an important role for network deployment and monitoring. For example, UAVs are being introduced as network nodes for data gathering in large areas [1; 2] and to extend mobile networks coverage by means of drone base stations [3]. Within this framework, on-site calibration [4] and measurement [5; 6; 7; 8; 9; 10] of antennas for wireless communications have gained interest due to the advances in UAVs technology, which simplify the procedures for on-site measurements. Antenna measurement in anechoic chambers provide an accurate characterization of the antenna parameters (e.g., radiation pattern, gain, return losses). However, it must be taken into account that these parameters can be affected by the environment surrounding the place where the antenna is deployed. For example, buildings and obstacles could result in multipath contributions that could modify the coverage. On-site antenna measurements allow the assessment of the radiation performance of the antenna considering the influence of the surrounding environment. The improvements in geo-referring and positioning subsystems on board the UAVs, capable of providing centimeter-level accuracy, have also enabled on-site antenna measurement at higher frequency bands. The current state-of-the-art on on-site antenna measurement has shown the feasibility of these systems up to C-band [5; 6]. The most common geo-referring and positioning subsystems that can be mounted on UAVs are global navigation satellite systems (GNSS), real time kinematics (RTK), inertial measurement units (IMUs), laser rangefinders, photogrammetry systems and barometers. A combination of these systems can lead to geo-referring accuracies within a few centimeters. Ground-based laser trackers can also be used to increase accuracy by one order of magnitude [6]. Most of the prototypes and systems for on-site antenna measurement using UAVs are intended to operate in the far field (FF) region of the antenna-under-test (AUT) which, in the case of electrically large antennas (e.g., reflector antennas, mobile networks base station antennas) can be hundreds of meters away from the AUT [7; 8]. As explained in [9], there are some restrictions for the direct measurement of the FF pattern using UAVs, namely safety restrictions (forbidden flight areas) and UAV flight autonomy. These issues can be overcome if the measurements are carried out in the near field (NF) region of the AUT, so that the furthest distance from the AUT to the UAV is not greater than 10-20 m. The feasibility of NF measurements using UAV-based systems has been proved in [9; 10; 11; 12]. In order to recover the FF pattern of the AUT from NF measurements, near field-to-far field (NF-FF) transformation techniques must be applied [13]. For this reason, complex (i.e., amplitude and phase) NF measurements are required. However, complex NF measurements require more expensive (and weighty) hardware to be placed on board the UAV. Besides, phase measurements are more sensitive to geo-referring uncertainties. An alternative to the use of complex receivers is provided by phase retrieval techniques, capable of recovering phase information from amplitude-only measurements [14; 15; 16; 17; 18; 19]. Once the phase of the field is retrieved, NF-FF transformation techniques can be applied to calculate the AUT radiation pattern. However, NF measurements can be post-processed for antenna diagnostics (e.g., detection of malfunctioning elements). There are two main groups of phase retrieval methods. On the one hand, holographic techniques have been proved to be very accurate and efficient in terms of measurement time, as just one measurement surface has to be scanned. However, these techniques require an additional hardware component (e.g., a reference antenna or an injected reference signal) [14].