Mechanical and Electrical Alignment Techniques for Plane-Polar Near-Field Test Systems
Authors: Michael Carey, Patrick Pelland, Stuart Gregson, Naoki Shinohara
The planar near-field technique is one of the most widely used, methods for measuring electrically large, medium to high gain antennas [1, 2]. Plane rectilinear systems consist of two intersecting, orthogonal linear translation stages which produces data in a convenient regularly spaced rectilinear co-ordinate system. Although the plane rectilinear geometry is by far the most commonly encountered implementation, plane-polar [2, 3] and plane-bipolar  geometries can also be constructed using mechanically convenient, commercially available, positioners. Plane-polar acquisition systems typically comprise the intersection of a rotation stage mounted behind the AUT, with a linear probe translation stage acting as a radial arm. This geometry yields data tabulated in a plane-polar co-ordinate system. Although comparatively rare in industry, plane-polar systems are important because they present certain distinct advantages [3, 5]. They do however pose some unique challenges within their implementation. As with all near-field methodologies, accurate and precise probe positioning is of paramount importance to the success of the technique and comprises an important term within the facility level uncertainty budget [6, 7]. Clearly, this is equally valid for the planar-polar technique with the hybrid angular/linear positioning system presenting unique challenges to the mechanical alignment.
This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure  and can be used on small, and large, plane-polar near-field antenna test systems.