Abstract
A near-field to far-field transformation technique with helicoidal scanning for elongated antennas, which allows the evaluation of the antenna far-field pattern in any cut plane directly from a nonredundant number of near-field data without interpolating them, is developed in this paper. It is based on the nonredundant sampling representations of electromagnetic fields and employs a flexible source modelling suitable for long antennas to determine the number of helix turns. The number of near-field measurements on each turn is on the contrary dictated by the minimum cylinder rule, as in the classical cylindrical scanning, in order to reduce the computational burden and to simplify the scanning from the mechanical viewpoint. Some numerical and experimental results assessing the effectiveness of the proposed technique are reported.
Highlights
Near-field – far-field (NF–FF) transformation techniques with helicoidal scanning, exploiting as suggested in [1] continuous and synchronized movements of the positioning systems of the probe and antenna under test (AUT), have been developed in [2,3,4,5,6,7,8,9] to significantly reduce the measurement time, which is currently very much greater than that needed to carry out the transformation
Apart from that in [2], they rely on the nonredundant sampling representations of electromagnetic (EM) fields [10, 11] and employ optimal sampling interpolation (OSI) formulas to efficiently recover the NF data required by the classical NF– FF transformation with cylindrical scanning [12, 13] from the acquired nonredundant helicoidal ones
The goal of this paper is to develop and validate both numerically and experimentally an even more effective direct NF–FF transformation with helicoidal scanning suitable for electrically long antennas and requiring a nonredundant number of NF measurements
Summary
Near-field – far-field (NF–FF) transformation techniques with helicoidal scanning, exploiting as suggested in [1] continuous and synchronized movements of the positioning systems of the probe and antenna under test (AUT), have been developed in [2,3,4,5,6,7,8,9] to significantly reduce the measurement time, which is currently very much greater than that needed to carry out the transformation. Besides the use of continuous movements, the drastic measurement time saving that characterizes the nonredundant NF–FF transformations with helicoidal scanning [3,4,5,6,7,8,9] is due to the significantly reduced number of needed NF data, which can be further lowered if the surface chosen to enclose the AUT fits better its actual shape This occurs, e.g., when considering elongated antennas which can be properly modelled as enclosed in a prolate ellipsoid [6,7,8] or in a cylinder ended in two half spheres (rounded cylinder) [8, 9], instead of the smallest sphere able to contain them [3,4,5]. As a matter of fact, such a flexible modelling allows one to fit better the shape of a lot of actual antennas by properly setting its geometric parameters
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