Rocco Guerriero

Effective antenna modellings for NF – FF transformations with spherical scanning using the minimum number of data

Two efficient probe-compensated near-field-far-field transformations with spherical scanning for antennas having two dimensions very different from the third one are here developed. They rely on the nonredundant sampling representations of the electromagnetic fields and on the optimal sampling interpolation expansions, and use effective antenna modellings. In particular, an antenna with a predominant dimension is no longer considered as enclosed in a sphere but in a cylinder ended in two half spheres, whereas a surface formed by two circular “bowls” with the same aperture diameter but different lateral bends is adopted to shape an antenna with two predominant dimensions. These modellings are able to fit very well a lot of antennas by properly setting their geometric parameters. It is so possible to remarkably lower the number of data to be acquired, thus significantly reducing the measurement time. Numerical tests assessing the accuracy and the robustness of the techniques are reported.

THE UNIFIED THEORY OF NEAR-FIELD-FAR-FIELD TRANSFORMATIONS WITH SPIRAL SCANNINGS FOR NONSPHERICAL ANTENNAS

The unified theory of near-field–far-field transformations with spiral scannings for quasi-spherical antennas is extended in this paper to the case of nonspherical ones, i.e., antennas with two dimensions very different from the third one. To this end, these antennas are no longer considered as enclosed in a sphere, but in a proper convex domain bounded by a rotational surface. The extension, heuristically derived by paralleling the rigorous procedure valid for the spherical source modelling, allows one to overcome its main and serious drawbacks. In fact, the corresponding near-field–farfield transformations use a reduced number of near-field measurements and, above all, allow one to consider measurement surfaces at a distance smaller than one half the antenna maximum size, thus remarkably reducing the error related to the truncation of the scanning zone. These are very important features, which make the spiral scannings more and more appealing from the practical viewpoint. Some examples of the application of this theory to spirals wrapping the conventional scanning surfaces employed in the near-field–far-field transformations are reported, and the accuracy and robustness of the far-field reconstructions are assessed.

An Effective NF-FF Transformation Technique With Planar Spiral Scanning Tailored For Quasi-Planar Antennas

An effective probe compensated near-field-far-field transformation technique with planar spiral scanning is developed in this paper. It makes use of an ellipsoidal modelling of the source, instead of the previously adopted spherical one. Such a modelling, tailored for quasi-planar antennas, allows one to consider measurement planes at a distance smaller than one half the antenna maximum size, thus reducing the error related to the truncation of the scanning surface. Moreover, it is quite general, containing the spherical modelling as particular case, and reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.

NEAR-FIELD — FAR-FIELD TRANSFORMATION TECHNIQUE WITH HELICOIDAL SCANNING FOR ELONGATED ANTENNAS

A fast and accurate near-field — far-field transformation technique with helicoidal scanning is proposed in this paper. It is tailored for elongated antennas, since a prolate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling allows one to consider measurement cylinders with a diameter smaller than the antenna height, thus reducing the error related to the truncation of the scanning surface. Moreover, it is quite general, containing the spherical modelling as particular case, and allows a significant reduction of the number of the required near- field data when dealing with elongated antennas. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.

Experimental Testing of Nonredundant Near-Field to Far-Field Transformations with Spherical Scanning Using Flexible Modellings for Nonvolumetric Antennas

This paper deals with the experimental testing of effective probe compensated near-field-far-field (NF-FF) transformations with spherical scanning requiring a minimum number of NF data. They rely on nonredundant sampling representations of the voltage measured by the probe, based on very flexible source modellings suitable for nonvolumetric antennas characterized by two dimensions very different from the other one. In particular, a cylinder ended in two half-spheres is adopted for modelling long antennas, whereas the quasi-planar ones are considered as enclosed in a rotational surface formed by two circular “bowls” having the same aperture diameter, but eventually different bending radii. The NF data needed to perform the classical spherical NF-FF transformation are then accurately and efficiently retrieved from the acquired nonredundant ones via optimal sampling interpolation formulas. A remarkable reduction of the number of the required NF data and, as a consequence, a significant measurement time saving can be so obtained. The experimental tests have been carried out at Antenna Characterization Lab of the University of Salerno and both the NF and FF reconstructions are resulted to be very good, thus confirming the accuracy and reliability of these NF-FF transformations from the experimental viewpoint too.

On the Compensation of Probe Positioning Errors When Using a Nonredundant Cylindrical Nf-Ff Transformation

Two different approaches for compensating the probe positioning errors in a near-field-far-field transformation with cylindrical scanning using a nonredundant number of measurements are presented and experimentally validated in this paper. In order to evaluate the uniformly distributed samples from the irregularly spaced ones, the former makes use of the singular value decomposition method, whereas the latter employs an iterative technique. In both the cases, the near-field data needed by a standard near-field-far- field transformation are efficiently evaluated via an optimal sampling interpolation algorithm.

AN EFFECTIVE TECHNIQUE FOR REDUCING THE TRUNCATION ERROR IN THE NEAR-FIELD-FAR-FIELD TRANSFORMATION WITH PLANE-POLAR SCANNING

An effective approach is proposed in this paper for estimating the near-field data external to the measurement region in the plane-polar scanning. It relies on the nonredundant sampling representations of the electromagnetic field and makes use of the singular value decomposition method for the extrapolation of the outside samples. It is so possible to reduce in a significant way the error due to the truncation of the measurement zone thus increasing the far- field angular region of good reconstruction. The comparison of such an approach, based on the optimal sampling interpolation expansions, with an existing procedure using the cardinal series has highlighted that the proposed technique works better. Some numerical tests are reported for demonstrating its effectiveness.

Experimental Assessment of an Effective Near-Field–Far-Field Transformation With Spherical Spiral Scanning for Quasi-Planar Antennas

Experimental tests on a near-field–far-field (NF-FF) transformation technique with spherical spiral scanning suitable for quasi-planar antennas are presented in this letter. Such a technique is based on the nonredundant sampling representations of the electromagnetic fields and employs an oblate ellipsoid to model the antenna under test. A two-dimensional optimal sampling interpolation formula is then employed to recover the NF data at any point on the scanning sphere and, in particular, at those needed by the classical spherical NF-FF transformation. It is so possible to lower the number of needed measurements, as well as the time required to acquire the data. Both the reported NF and FF reconstructions assess the effectiveness of the technique.

An Effective Near-Field-Far-Field Transformation Technique for Elongated Antennas Using a Fast Helicoidal Scan [Measurements Corner]

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 antennas by properly setting the geometric parameters. By employing such modeling instead of spherical modeling, it is possible to remarkably reduce the error related to the truncation of the scanning zone, since measurement cylinders with a diameter smaller than the antennas 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.

Non-Redundant Spherical NF - FF Transformations Using Ellipsoidal Antenna Modeling: Experimental Assessments [Measurements Corner]

Two non-redundant near-field-to-far-field (NF-FF) transformation techniques with spherical scanning, for antennas having a dimension very different from the other ones, are experiment ally tested in this paper. These techniques are based on the non-redundant sampling representations of the electromagnetic fields. They use a rotational ellipsoid to shape non-spherical antennas: a prolate ellipsoid for elongated antennas, and an oblate ellipsoid for the quasi-planar antennas. A two-dimensional optimal-sampling interpolation formula is employed to accurately recover the near-field data required by the classical spherical near-field-to-far-field transformation from the non-redundant near-field data. It is so possible to obtain a remarkable reduction in the number of the near-field samples to be acquired and, as a consequence, a remarkable reduction in the measurement time. Some experimental results, carried out at the Antenna Characterization Lab of the University of Salerno and assessing the effectiveness of the proposed techniques, are shown.