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Francesco D'Agostino,Flaminio Ferrara, Jeffrey A. Fordham, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, November 2014

The near-field – far-field (NF–FF) transformation with spherical scanning is particularly interesting, since it allows the reconstruction of the complete radiation pattern of the antenna under test (AUT) [1]. In this context, the application of the nonredundant sampling representations of the electromagnetic (EM) fields [2] has allowed the development of efficient spherical NF–FF transformations [3, 4], which usually require a number of NF data remarkably lower than the classical one [1]. In fact, the NF data needed by this last are accurately recovered by interpolating a minimum set of measurements via optimal sampling interpolation (OSI) expansions. A remarkable measurement time saving is so obtained. However, due to an imprecise control of the positioning systems and their finite resolution, it may be impossible to exactly locate the probe at the points fixed by the sampling representation, even though their position can be accurately read by optical devices. As a consequence, it is very important to develop an effective algorithm for an accurate and stable reconstruction of the NF data needed by the NF–FF transformation from the acquired irregularly spaced ones. A viable and convenient strategy [5] is to retrieve the uniform samples from the nonuniform ones and then reconstruct the required NF data via an accurate and stable OSI expansion. In this framework, two different approaches have been proposed. The former is based on an iterative technique, which converges only if there is a biunique correspondence associating at each uniform sampling point the nearest nonuniform one, and has been applied in [5] to the uniform samples reconstruction in the case of cylindrical and spherical surfaces. The latter relies on the singular value decomposition method, does not exhibit the above limitation, but can be conveniently applied only if the uniform samples recovery can be reduced to the solution of two independent one-dimensional problems [6]. Both the approaches have been applied and numerically compared with reference to the positioning errors compensation in the spherical NF–FF transformation for long antennas [7] using a prolate ellipsoidal AUT modelling. The goal of this work is just to validate experimentally the application of these approaches to the NF–FF transformation with spherical scanning for elongated antennas [4], using a cylinder ended in two half-spheres for modelling them. The experimental tests have been performed in the Antenna Characterization Lab of the University of Salerno, provided with a roll over azimuth spherical NF facility supplied by MI Technologies, and have fully assessed the effectiveness of both the approaches. [1] J.E. Hansen, ed., Spherical Near-Field Antenna Measurements , IEE Electromagnetic Waves Series, London, UK, Peter Peregrinus, 1998. [2] O.M. Bucci, C. Gennarelli, C. Savarese, “Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples,” IEEE Trans. Antennas Prop. , vol. 46, pp. 351-359, 1998. [3] O.M. Bucci, F. D’Agostino, C. Gennarelli, G. Riccio, C. Savarese, “Data reduction in the NF–FF transformation technique with spherical scanning,” Jour. Electr. Waves Appl ., vol. 15, pp. 755-775, June 2001. [4] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Effective antenna modellings for NFFF transformations with spherical scanning using the minimum number of data,” Int. Jour. Antennas Prop ., vol. 2011, Article ID 936781, 11 pages, 2011 [5] O.M. Bucci, C. Gennarelli, G. Riccio, C. Savarese, “Electromagnetic fields interpolation from nonuniform samples over spherical and cylindrical surfaces,” IEE Proc. Microw. Antennas Prop ., vol. 141, pp. 77-84, April 1994. [6] F. Ferrara, C. Gennarelli, G. Riccio, C. Savarese, “Far field reconstruction from nonuniform plane-polar data: a SVD based approach,” Electromagnetics, vol. 23, pp. 417-429, July 2003 [7] F. D’Agostino, F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, “Two techniques for compensating the probe positioning errors in the spherical NF–FF transformation for elongated antennas,” The Open Electr. Electron. Eng. Jour. , vol. 5, pp. 29-36, 2011.