Plane-wave spectrum methods for the near-field characterization of very large structures using UAVs: The SKA radio telescope case

The reconstruction of embedded element patterns is analyzed in physically and electrically large aperture arrays from near-field measurements taken with a source carried by a drone. This is a very challenging metrology problem, especially when the arrays are composed of wide-band wide field-of-view antenna elements in a highly coupled environment, as is the case for the SKA-low radio telescope stations. We study the use of both a direct near- to far-field transformation and a model-based approach to overcome the limitations imposed by this type of complex electromagnetic structures. Both methods are explained using a decomposition of the antenna voltages in terms of a spectrum of incoming plane waves. A numerical example with an irregular array of 16 log-periodic elements and a hemispherical path shows their abilities to capture the mutual coupling effects.