Near-Field-Far-Field Transformation Technique With Helicoidal Scanning for Elongated Antennas

Abstract

Abstract-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 nearfield 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.

Laboratory Tests Assessing the Effectiveness of the Nf-Ff Transformation With Helicoidal Scanning for Electrically Long Antennas

“…The main results [13] concerning the reconstruction of the voltage from a nonredundant number of its samples collected by the measurement probe along a helix are reported in the following.…”

“…When considering a long AUT, it is no longer convenient to adopt the smallest sphere [7] as surface Σ enclosing it, but, as suggested in [13,14], the smallest prolate ellipsoid having major and minor semiaxes equal to a and b (Fig. 1).…”

“…As explicitly pointed out in [13,14], the use of the spherical AUT modelling is not very effective when considering electrically long antennas, i.e., those having a predominant dimension, such as the radiating systems for radio base stations. As a matter of fact, it does not permit to consider measurement cylinders with a radius smaller than one half the antenna maximum size, thus reflecting in an increase of the error related to the truncation of the scanning surface.…”

“…The goal of this paper is to experimentally assess the effectiveness of the NF-FF transformation technique with helicoidal scanning for electrically long antennas [13]. The experimental validation has been carried out both at the laboratory of antenna characterization of the University of Salerno, where an advanced cylindrical NF measurement facility supplied by MI Technologies is available, and at the anechoic chamber of the University of Naples "Parthenope", provided with a FF measurement range.…”

“…In order to overcome the above drawbacks, an efficient NF-FF transformation technique with helicoidal scanning tailored for elongated antennas has been recently proposed in [13], by assuming the AUT as enclosed in a prolate spheroid, a shape particularly suitable to deal with such a kind of antennas, but which remains quite general and contains the spherical modelling as particular case. The use of an oblate ellipsoid has been proposed in [15] to get a NF-FF transformation with planar spiral scanning suitable for quasiplanar antennas.…”