Nonredundant NF-FF transformation with spherical scan accounting for an offset mounting of a long AUT

An efficient near-field — far-field (NF-FF) transformation technique with spherical scanning for an elongated antenna under test (AUT), which requires a number of NF data minimum and equal in both the cases of its centered and offset mounting, is developed in this work, by applying the nonredundant sampling representation of electromagnetic fields to the voltage acquired by the scanning probe and modelling the AUT by a cylinder with two hemispherical caps. The interest for such a transformation arises from the fact that sometimes, for practical constraints, the AUT cannot be mounted in such a way that its center coincides with the scanning sphere one. In such a case, the number of NF data needed by the classical spherical NF-FF transformation can considerably grow, due to the increase of the radius of the minimum sphere containing the AUT. On the contrary, in the proposed approach, the NF data needed by the classical transformation are accurately reconstructed from the collected nonredundant ones by means of a 2-D optimal sampling interpolation algorithm, thus making possible to obtain a remarkable reduction of the measurement time. Numerical results assessing the accuracy and robustness of the technique are shown.