A probe-compensated near-field-far-field (NF-FF) transformation with planar spiral scan, particularly suitable for flat antennas under test (AUTs), is proposed in this communication. It relies on the nonredundant sampling representations of electromagnetic fields and has been achieved by properly applying the unified theory of spiral scannings for nonvolumetric antennas, when such a kind of AUT is considered as enclosed in a dish with diameter equal to its maximum dimension, thus better shaping its geometry. An efficient two-dimensional optimal sampling interpolation (OSI) algorithm is then developed to recover the NF data required by the standard NF-FF transformation with plane-rectangular scan from those collected along the spiral. Since the number of NF data and spiral turns is related to the area of the modeling surface, the here proposed NF-FF transformation technique allows one to further reduce the measurement time with respect to those based on the modelings for quasi-planar AUTs, which instead involve, in such a case, a residual volumetric redundancy. Some numerical simulations, assessing the accuracy of the OSI algorithm and of the so developed NF -FF transformation, are shown.
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