Laboratory Tests Assessing the Effectiveness of the Nf-Ff Transformation with Helicoidal Scanning for Electrically Long Antennas

This paper deals with the experimental validation of an efiective near-fleld-far-fleld transformation technique with helicoidal scanning particularly suitable for electrically long antennas, whose validity has been numerically assessed in a previous authors' paper. Such a technique relies on the results relevant to the nonredundant sampling representations of the electromagnetic flelds and makes use of an optimal sampling interpolation algorithm, which allows the reconstruction of the near-fleld data needed by the near-fleld-far-fleld transformation with cylindrical scan. The use of a prolate ellipsoid instead of a sphere to model an elongated antenna allows one to consider measurement cylinders with a diameter smaller than the antenna height, thus reducing the error related to the truncation of the scanning zone. Moreover, a signiflcant reduction of the needed near- fleld data is also obtained. The comparison of the far-fleld patterns reconstructed from the acquired helicoidal measurements with those obtained from the data directly measured on the classical cylindrical grid assesses the efiectiveness of the near-fleld-far-fleld transformation using this innovative scanning technique. At last, its validity is further conflrmed by the very good agreement with the direct far-fleld measurements.