Characterisation of near-field focusing with application to low altitude beam focusing of the Arecibo tri-reflector system

Antennas are normally characterised in terms of their far field radiation patterns, directive gain and aperture efficiency. The paper describes how to characterise antennas that are focused in the near field, in terms of a focusing gain and a focusing efficiency. The paper also presents an algorithm from which the location of the near-field focus can be calculated directly from the aperture field of the antenna. The definitions and the algorithm are applied to an extensive numerical study of focusing of the beam of the Arecibo tri-reflector system at low altitudes. The focusing is obtained by displacing the feed, or the tertiary reflector, or both at the same time. Finally, an approximate formula is presented, from which results can be calculated easily at other frequencies and altitudes than those being computed