Evaluation of near-field compact ranges measurement of tracking antennas

The near-field compact range uses the quasi-plane-wave beam produced by a large dish antenna, thereby greatly reducing the normal
2 D^{2}/\lambda
separation required. This type of range has been experimentally evaluated for both antennas and scatterers; there is partial filling of sidelobe nulls and pattern distortion below some pattern level. No data were found on the use of compact ranges for tracker antenna measurement. This paper analyzes tracker measurement performance. In the near-field, the transverse field has oscillations in both amplitude and phase, and an asymmetric amplitude taper. Geometric diffraction is used to calculate the near-field; this is the only near-field method that is moderately simple. Details of this method are given, along with typical field plots. For the tracker antenna, a simple difference pattern analogous to sin
x/x
is used, with the perturbed pattern obtained from a numerical integration of the aperture distribution modified by the field perturbations. A versatile computer code was written and many test cases were run to determine the effect of varying the several parameters. The difference pattern null depth needed for faithful measurement is derived as a function of tracker accuracy, and it is shown that null depth is primarily controlled by field amplitude taper. Effects of varying the ratio of range to test antenna diameter, range antenna
f/D
, and test area transverse and axial location are described. The effects of an inclined feed, which reduces amplitude asymmetry, are calculated. A clearer understanding of the role of edge diffracted rays is obtained from the calculations; these rays affect primarily pattern shape and sidelobe nulls. The technique allows an effective tradeoff of all parameters to be easily, quickly, and relatively inexpensively made.