Nonredundant NF–FF transformation with helicoidal scanning

A probe compensated near-field-far-field (NF-FF) transformation technique with helicoidal scanning, which makes use of a minimum number of probe output voltage samples, is developed in this paper. This target is achieved by using a nonuniform helix as scanning curve, i.e., a circular helix with a constant angular elevation step, and by developing an efficient sampling representation over it. Then, by choosing the step coincident with the spacing needed to interpolate the probe voltage along a generatrix, it is possible to reconstruct such a voltage at any point over the cylinder. This allows the recovering of the data required by the standard probe compensated NF-FF transformation technique with cylindrical scanning. As compared with the previous technique employing a uniform helix with a constant linear elevation step, it allows one to reduce remarkably the number of needed samples without affecting the numerical efficiency. Numerical examples assess the accuracy and the stability of the FF reconstruction process.