Extrapolation of data external to the measurement region: In the bi-polar scanning

Abstract

Among the near field-far field (NF-FF) transformation techniques, that employing the bi-polar scanning, proposed by Rahmat-Samii et al. in [1], is particularly attractive due to its peculiar characteristics. In such a scanning, the antenna under test (AUT) rotates axially, whereas the probe is attached to the end of an arm which rotates around an axis parallel to the AUT one. This allows one to collect the NF data on a grid of concentric rings and radial arcs (Fig. 1). The bi-polar scanning maintains all the advantages of the plane-polar one while providing a simple and cost-effective measurement system. In fact, since the arm is fixed at one point and the probe is attached at its end, the bending is constant and this allows to hold the planarity. Moreover, rotational movements are preferable to the linear ones, since rotating tables are more accurate than linear positioners. An efficient probe compensated NF-FF transformation using a bi-polar scanning has been recently developed by the authors in [2] by considering the AUT as enclosed in an oblate ellipsoid, a source modelling particularly suitable to deal with quasi-planar antennas. Such a technique is based on the theoretical results [3] concerning the nonredundant sampling representations of the electromagnetic (EM) fields for antennas enclosed in arbitrary convex domains with rotational symmetry and observed on surfaces having the same symmetry. It allows one to lower the number of needed NF data in a significant way with respect to the classical bi-polar scanning without losing the efficiency. An optimal sampling interpolation (OSI) algorithm has been applied to recover the plane-rectangular data from the bi-polar ones, thus enabling the use of the FFT in the NF-FF transformation.