RCS calculations, transformations and comparisons under spherical and plane wave illumination

Abstract

The techniques for determining far-field antenna and RCS patterns at short distances are numerous and well known. One class of techniques involves reflecting the spherical phasefront of a source antenna from one or two curved metal reflectors. These systems, referred to as compact antenna test ranges (CATRs), are used to synthesize planar phasefronts. Using a CATR, far-field patterns can be measured at short distances directly. Another class uses near-field to far-field (NF/FF) transformations to analytically predict far-field radiation patterns from measurements performed over a surface that is near the test antenna. In a usual radar laboratory, a simple approach is used to determine far-field radiation and scattering patterns. For targets that are large relative to the curvature of the spherical wavefront, a two-dimensional analytical algorithm transforms the spherical wave (near-field) measurements into the desired plane wave (far-field) results. Comparisons of measurements performed under spherical and plane wave illumination demonstrate the efficacy of the simple approach and the RCS correction algorithm. A target and its monostatic RCS measured with a dual parabolic-cylinder CATR were available. The target, a 25/spl lambda/ long precision cylinder was measured for the frequencies, orientations, and angular positions which correspond to the CATR measurements. The corrected spherical wave measurements are in very good agreement with the CATR plane wave measurements. In this paper, the algorithm is used to transform RCS patterns measured in the near-field of the spherical wavefront into far-field patterns; however, the algorithm is not a NF/FF transformation in the conventional sense. The algorithm corrects for all deviations of the illuminating field from that of an ideal plane wave. Correction coefficients that are related to the inverse of these deviations, in both amplitude and phase, are calculated for a region of space equal to or greater than the dimensions of the test target. Convolving the correction coefficients with the RCS pattern of the target removes from the measurements errors due to the non-plane wave illumination.