Flaminio Ferrara

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 Efficient Near-field to Far-field Transformation Using the Planar Wide-mesh Scanning

An effective procedure is here proposed for evaluating the antenna far field from the knowledge of near-field data collected on a planar wide-mesh grid. The choice of the corresponding sampling law on the considered planar surface is based on the theoretical results relevant to the nonredundant sampling representations of the radiated electromagnetic field. Then, the data needed by the standard probe-compensated near-field-far-field transformation with plane-rectangular scanning can be efficiently reconstructed by using an optimal sampling interpolation expansion of central type. The benefit of using this particular and innovative approach is that it allows one to reduce the number of near-field data with respect to the classical plane-rectangular scan, while maintaining the accuracy of the far-field reconstruction. Last but not least, the implementation of the planar wide-mesh scanning does not require to change the plane-rectangular positioning systems, but only the software for controlling them. Numerical tests are reported for assessing the effectiveness of the approach and its stability with respect to random errors affecting the data.

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.

Far-Field Reconstruction from Nonuniform Plane-Polar Data: A SVD-Based Approach

An efficient technique to reconstruct the field radiated by an antenna in the far-field region from the knowledge of the nonuniformly spaced plane-polar data measured by a probe in the near-field region is developed in this work. The singular value decomposition method is applied to evaluate the uniformly distributed samples, whose positions are fixed by a nonredundant sampling representation of the field. Then the plane-rectangular near-field data needed to perform the classical probe-compensated near-field-far-field transformation are efficiently evaluated via the optimal sampling interpolation algorithm. As demonstrated by numerical tests, the far-field reconstruction process is accurate and stable

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.