An effective approach to remarkably reduce the input data in the NF/FF transformation with spherical spiral scan for offset mounted elongated auts

This communication concerns the experimental validation of an effective near-field/far-field (NF/FF) transformation technique for offset mounted long antennas under test (AUTs), wherein the NF data are gathered by means of a spherical spiral scanning. Its main feature is to require a minimum amount of NF measurements, that is exactly the same in both the cases of onset and offset mounting of the AUT. An optimal sampling interpolation formula, attained by modelling the AUT with a cylinder terminated by hemispherical caps and suitably exploiting the non-redundant sampling representations and the unified theory of spiral scans for non-spherical antennas, allows one to accurately determine, from the collected non-redundant samples, the massive NF data required in such a case by the traditional spherical NF/FF transformation. In fact, if an onset mounting of the AUT is not allowed owing to practical constraints, the number of the input NF data for the traditional spherical NF/FF transformation and the corresponding acquisition time can remarkably increase due to the minimum sphere rule. Some results of experimental tests, which fully prove the effectiveness of the technique also from the practical point of view, are shown.