For electrically large antenna systems it is impossible or unpractical to measure directly the radiation patterns on a conventional far-field (FF) range. Therefore, for such antennas, it is useful to exploit near-field (NF) measurements to recover the FF patterns via NF-FF transformation techniques . It is useful to note that NF measurements may be performed in a controlled environment, as an indoor shielded anechoic chamber, which allows to overcome those drawbacks that, due to weather conditions (rain, snow, etc.), electromagnetic (EM) interference, pollution and other, cannot be eliminated in FF outdoor measurements. In addition, NF scanning techniques represent the better choice when complete pattern and polarization measurements are required. Moreover, they provide the necessary information to determine the radiating field at the surface of the antenna (microwave holographic diagnostics). Among the NF-FF transformation techniques, the cylindrical scanning (Fig. 1) has attracted considerable attention [2-5]. At the cost of a moderate increase in the analytical and computational complication with respect to the planar scannings, it allows one to reconstruct, from a single set of NF measurements, the antenna complete radiation pattern save for the zones surrounding the spherical polar angles. By exploiting the analytical properties of EM fields , sampling expansions have been successfully applied to the NF-FF transformation with cylindrical scanning . Unlike the approach in , the number of data on each ring decreases moving from the central measurement rings to the peripheral ones, and the linear spacing between the rings grows when the cylinder radius increases. However, such an approach still exhibits the shortcoming that the overall number of samples becomes unbounded when the height of the scanning cylinder approaches infinity. A new NF-FF transformation from a nonredundant number of data, which stays finite also for an unbounded cylinder, has been developed in  by assuming the antenna under test (AUT) as enclosed in a sphere (spherical modelling). Unfortunately, for elongated antennas, the spherical modelling gives rise to an useless increase in the number of NF data. Moreover, it prevents the possibility of considering measurement cylinders with a diameter smaller than the source height. A NF-FF transformation, which overcomes these drawbacks and reduces significantly the number of required data in the case of elongated antennas, has been proposed in , by applying the recent theoretical results concerning the nonredundant sampling representation of the EM field  and considering the antenna as enclosed in the smallest surface formed by a cylinder ended in two half-spheres. Since in [3-5] the case of an ideal probe has been considered, the effects of the measurement probe have not been taken into account in the evaluation of the antenna FF pattern.
|Titolo:||Direct far-field reconstruction from data measured by a cylindrical scanning facility|
|Autori interni:||D'AGOSTINO, Francesco|
|Data di pubblicazione:||2002|
|Appare nelle tipologie:||4.1.2 Proceedings con ISBN|