An antenna measurement campaign was conducted in April 2016 at LOFAR station CS302 in collaboration with Italian partners from the Instituto Nazionale di Astrofisica (INAF), the Consiglio Nazionale delle Ricerche (CNR) and the Politecnico di Torino (PT). These measurements support the development of an improved LOFAR beam model and help to develop strategies to support the roll-out, commissioning and characterization of the Low Frequency Aperture Array (LFAA) of the SKA. These measurements were done using an Unmanned Aerial Vehicle (UAV) equipped with a radio frequency (RF) transmitter developed by our Italian partners for antenna characterization below 500 MHz in the context of the SKA.
During this campaign, the team shown in Figure 1 performed measurements on all three antenna arrays of LOFAR station CS302, i.e., the Low Band Antenna (LBA) inner array, the LBA outer array and the High Band Antenna (HBA) array with several different flight strategies. In one of these flight strategies, the socalled spin flight, the UAV is hovering in a fixed position above the array while spinning around its vertical axis. Figure 2 shows the ouput of the central antenna in the array, which was right below the UAV. The plot clearly shows that the probe antenna on the UAV alternatingly aligns with each of the two dipoles that make a LBA. With such measurements, we can very accurately measure the orientation of the antennas and verify that the antenna can discriminate the two polarizations very well.
In the meantime, we have made significant progress on improving the electromagnetic (EM) model for the LBA, whose predictions are now in good agreement with the measured response. Efforts to make a similar improvement to EM model for the HBA will start in 2017. We also successfully demonstrated our ability to derive the antenna positions from the RF measurements. This could lead to significant cost savings in LFAA as this pre-empts the need to physically determine the position of each antenna one-by-one.
Figure 1 The team that conducted the campaign(front row, then rear row): Fabio Paonessa (CNR), Paolo Maschio (PT), Andrea Lingua (PT), Giuseppe Pupillo (INAF), Menno Norden (ASTRON), Stefan Wijnholds (ASTRON), Giuseppe Virone (CNR), Pietro Bolli (INAF) and Irene Aicardi (PT).
Figure 2 Output of the central LBA during a spin flight.
Figure 3 The UAV in action.
Figure 4 Close-up of the UAV