Near-Field/Far-Field Transformation with Helicoidal Scanning from Irregularly Spaced Data

A fast and accurate technique for the compensation of the probe positioning errors in the near-field/far-field transformation with helicoidal scanning is proposed in this paper. It relies on a nonredundant sampling representation using a spherical modelling of the antenna under test and employs an iterative scheme to evaluate the near-field data at the points fixed by the helicoidal nonredundant representation from the acquired irregularly distributed ones. Once these helicoidal data have been recovered, those required by a classical cylindrical near-field/far-field transformation are efficiently determined by using an optimal sampling interpolation algorithm. Some numerical tests assessing the effectiveness of the proposed approach and its stability with respect to random errors affecting the near-field data are shown

Two efficient procedures to correct the positioning errors in the plane-polar scanning

Two techniques to effectively compensate known positioning errors in a plane-polar near-field–far-field (NF–FF) transformation, using a minimum number of NF data and adopting an oblate ellipsoid to shape the considered antenna, are proposed and validated through experimental proofs. The former makes use of the singular value decomposition method to recover the voltage samples which would be acquired by the probe at the points fixed by the non-redundant sampling representation from the collected positioning error affected ones, whereas the latter employs an iterative scheme. The NF data required by the classical NF–FF transformation with plane-rectangular scanning are then efficiently evaluated via a two-dimensional optimal sampling interpolation formula. The effectiveness of the proposed techniques is assessed by experimental tests performed at the Antenna Characterisation Lab of the University of Salerno

An Innovative Direct NF-FF Transformation Technique with Helicoidal Scanning

A direct near-field-far-field transformation with helicoidal scanning is developed. It is based on the nonredundant sampling representation of electromagnetic fields and uses a spherical antenna modelling to determine the number of helix turns. Moreover, the number of voltage samples on each of them is fixed by the maximum transverse dimension of the antenna, both to simplify the mechanical scanning and to reduce the computational effort. This technique allows the evaluation of the antenna far field directly from a minimum set of near-field data without interpolating them. Although the number of near-field data employed by the developed technique is slightly increased with respect to that required by rigorously applying the nonredundant sampling representation on the helix, it is still remarkably smaller than that needed by the standard near-field-far-field transformation with cylindrical scanning. The effectiveness of the technique is assessed by numerical and experimental results

Near-Field to Far-Field Transformation Techniques with Spiral Scannings: A Comprehensive Review

An overview of the near-field-far-field (NF-FF) transformation techniques with innovative spiral scannings, useful to derive the radiation patterns of the antennas commonly employed in the modern wireless communication systems, is provided in this paper. The theoretical background and the development of a unified theory of the spiral scannings for quasi-spherical and nonspherical antennas are described, and an optimal sampling interpolation expansion to evaluate the probe response on a quite arbitrary rotational surface from a nonredundant number of its samples, collected along a proper spiral wrapping it, is presented. This unified theory can be applied to spirals wrapping the conventional scanning surfaces and makes it possible to accurately reconstruct the NF data required by the NF-FF transformation employing the corresponding classical scanning. A remarkable reduction of the measurement time is so achieved, due to the use of continuous and synchronized movements of the positioning systems and to the reduced number of needed NF measurements. Some numerical and experimental results relevant to the spherical spiral scanning case when dealing with quasi-planar and electrically long antennas are shown

Far-Field Pattern Reconstruction from Near-Field Data Collected via a Nonconventional Plane-Rectangular Scanning: Experimental Testing

This paper deals with the experimental validation of an efficient near-field-far-field (NF-FF) transformation using the planar wide-mesh scanning (PWMS). Such a nonconventional plane-rectangular scanning technique is so named, since the sample grid is characterized by meshes wider and wider when going away from the center, and makes it possible to lower the number of needed measurements, as well as the time required for the data acquisition when dealing with quasi-planar antennas. It relies on the use of the nonredundant sampling representations of electromagnetic fields which employ an oblate ellipsoid or a surface formed by two circular "bowls" with the same aperture diameter but eventually different bending radii to shape a quasi-planar antenna. A two-dimensional optimal sampling interpolation formula allows the reconstruction of the NF data at any point on the measurement plane and, in particular, at those required by the classical NF-FF transformation with the conventional plane-rectangular scanning. The measurements, performed at the planar NF facility of the antenna characterization laboratories of Selex ES, have confirmed the effectiveness of this innovative scanning also from the experimental viewpoint

Laboratory measurements of super-resolving Toraldo pupils for radio astronomical applications

The concept of super-resolution refers to various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. Although several techniques to narrow the central lobe of the illumination Point Spread Function have been developed in optical microscopy, most of these methods cannot be implemented on astronomical telescopes. A possible exception is represented by the variable transmittance filters, also known as "Toraldo Pupils" (TPs) since they were introduced for the first time by G. Toraldo di Francia in 1952 (Toraldo di Francia, Il Nuovo Cimento (Suppl.) 9, 426, 1952). In the microwave range, the first successful laboratory test of TPs was performed in 2003 (Mugnai et al. Phys. Lett. A 311, 77-81, 2003). These first results suggested that TPs could represent a viable approach to achieve super-resolution in Radio Astronomy. We have therefore started a project devoted to a more exhaustive analysis of TPs, in order to assess their potential usefulness to achieve super-resolution on a radio telescope, as well as to determine their drawbacks. In the present work we report on the results of extensive microwave measurements, using TPs with different geometrical shapes, which confirm the correctness of the first experiments in 2003. We have also extended the original investigation to carry out full-wave electromagnetic numerical simulations and also to perform planar scanning of the near-field and transform the results into the far-field

Experimental Proofs on an Effective Near-Field to Far-Field Transformation Technique with Spherical Spiral Scan Suitable for Long Antennas

This communication provides the experimental assessment of an efficient near-field – far-field (NF–FF) transformation with spherical spiral scanning for elongated antennas, which allows a remarkable measurement time saving with respect to the classical spherical NF–FF transformation. Such a saving is due both to the use of continuous and synchronized movements of the positioning systems and to the reduced number of NF data to be acquired. This technique relies on a nonredundant sampling representation of the probe output voltage, obtained by using the unified theory of spiral scans for nonspherical antennas and adopting a prolate ellipsoidal modelling of the radiating source. An optimal sampling interpolation scheme is then used to efficiently recover the NF data needed by a probe compensated NF–FF transformation with spherical scanning. The experimental tests have been carried out at the UNISA Antenna Characterization Lab and both the reported NF and FF reconstructions assess the effectiveness and reliability of the technique

Laboratory tests on a near-field - far-field transformation with spherical scan using a very flexible antenna modelling

An effective near-field – far-field (NF–FF) transformation with spherical scanning for quasi-planar antennas is experimentally validated in this work. It is based on a nonredundant sampling representations of the electromagnetic fields using a flexible source modelling formed by two circular “bowls” with the same aperture but different lateral bends. Such a modelling, suitable to fit very well a lot of quasi-planar antennas, allows a remarkable reduction of the number of data to be acquired and, as a consequence, of the measurement time. An optimal sampling interpolation algorithm is employed to efficiently recover the NF data needed by a probe compensated NF–FF transformation with spherical scanning. The effectiveness of the approach is experimentally assessed at the UNISA Antenna Characterization Lab by comparing the FF patterns reconstructed from the nonredundant NF measurements with those obtained from the NF data directly measured on the classical spherical grid

Probe conpensated near-field to far-field transformation with helicoidal scanning for elongated antennas

An innovative near-field - far-field transformation technique with helicoidal scanning is proposed in this work. It makes use of an ellipsoidal modelling of the source, instead of the previously adopted spherical one. Such a modelling, tailored for elongated antennas, allows one to consider measurement cylinders with a diameter smaller than the source height, thus reducing the error related to the truncation of the scanning surface. Moreover, it contains the spherical modelling as particular case and lowers significantly the number of required data when dealing with elongated antennas. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported

Spherical near-field to far-field transformation for quasi-planar antennas: an experimental validation

This work concerns the experimental validation of a near-field – far-field transformation with spherical scanning for quasi-planar antennas. Such a technique is based on the nonredundant sampling representations of the electromagnetic fields and on the optimal sampling interpolation expansions, and uses an oblate ellipsoid to model the antenna. It is so possible to remarkably lower the number of data to be acquired, thus reducing in a significant way the required measurement time. The effectiveness of such a technique is experimentally assessed at the UNISA Antenna Characterization Lab by comparing the far-field patterns reconstructed from nonredundant measurements on the sphere with those obtained from the near-field data directly measured on the classical spherical grid