A new and efficient NF-FF transformation with spherical spiral scanning

Antenna near-field (NF) facilities are employed to overcome those drawbacks which, for electrically large antennas, make it impractical to measure antenna patterns on a conventional far-field (FF) range. Among the NF-FF transformation techniques, employing spherical scanning is particularly interesting since it gives the full antenna pattern coverage. The aim of this work is to determine the antenna FF pattern from NF measurements made over a spiral wrapping a sphere from pole to pole with a constant step in /spl theta/. By considering an ideal probe, a convenient way to do this is to develop an efficient interpolation algorithm for the electromagnetic (EM) field reconstruction over the sphere from knowledge of the nonredundant number of samples over the considered spiral. This allows one to determine the NF data required by the NF-FF transformation technique with spherical scanning. The starting point has been represented by the development of a non-redundant sampling representation of the field over the spiral, based on theoretical results concerning the analytical properties of the fields radiated by finite size sources enclosed in a convex domain with rotational symmetry and observed over a surface having the same symmetry. Then, the step of the spiral has been chosen coincident with the sample spacing needed by the approach of Bucci (1991) to interpolate the field along a meridian, thus allowing one to develop the required two-dimensional interpolation formula. The accuracy and stability of such a formula and the effectiveness of the proposed innovative NF-FF transformation technique have been assessed by many numerical simulations.