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Abstract-In this paper, the theoretical foundations of near-field-farfield transformations with spiral scannings are revisited and a unified theory is provided. This is accomplished by introducing a sampling representation of the radiated electromagnetic field on a rotational surface from the knowledge of a nonredundant number of its samples on a spiral wrapping the surface. The obtained results are general, since they are valid for spirals wrapping on quite arbitrary rotational surfaces, and can be directly applied to the pattern reconstruction via near-field-far-field transformation techniques. Numerical tests are reported for demonstrating the accuracy of the approach and its stability with respect to random errors affecting the data.

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An experimental validation of a new near-field-far-field transformation technique with helicoidal scanning, tailored for elongated antennas, is provided in this paper. Such a transformation is based on non-redundant sampling representations of the electromagnetic fields. It makes use of a flexible source modeling, which allows one to very well fit many of these kinds of 134 .antennas by properly setting the geometric parameters. By employing such. modeling i~stead of spherical m.odeling,. it is possible to remarkably reduce the error related to the truncation of th~scanning zone, since .measurement cylinders with a diameter smaller than the antenna's height can be used. A comparison of the reconstructions recovered from the non-· redundant measurements on the helix with those obtained from data directly measured on the classical cylindrical grid assesses the validity of this innovative scanning technique.

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