Reconstruction of the Far-Field Pattern Radiated by an Elongated Antenna Measured over a Perfectly Electric Conducting Ground Plane in a Spherical Spiral Near-Field Facility

Aim of this communication is to devise an effective spherical spiral near-to-far-field (NTFF) transformation for long antennas, which uses a reduced set of spiral near-field (NF) data measured in a hemispherical NF facility. Such a facility is provided of a perfectly electric conducting ground plane to allow the characterisation of heavy and large antennas with the repeatability of the measurements. The proposed technique properly exploits the theoretical foundations of spiral scannings for non-volumetric antennas to develop the non-redundant representation along the spiral wrapping the upper and lower hemispheres and the image theorem to synthesise the voltage NF data which would be collected along the spiral wrapping the lower hemisphere. Once these voltage NF data have been synthesised, then an efficient 2-D optimal sampling interpolation algorithm is conveniently used to recover the NF data required by the classical spherical NTFF transformation. It is here assumed that the considered antenna and its image exhibit a predominant dimension as compared to the other two and, hence, are effectively modeled by a rounded cylinder. Numerical tests show the accuracy of the devised nonredundant NTFF transformation with hemispherical spiral scan.