Quiet-zone evaluation using a spherical synthetic-aperture radar

A compact range, for example, will never produce a perfect plane wave in the target zone. Knowledge of the actual incident field is essential for assessing and/or improving the measurement quality. We use quiet-zone illumination data to produce a synthetic-aperture image of a microwave measurement range, allowing isolation and physical removal of unwanted signal sources. We discuss results concerning resolution, dynamic range, and focusing in such images, and some experimental data are presented. To obtain illumination data, we use a known probe (on a roll-over-azimuth mount) to make measurements on a spherical surface. (Thus, standard positioners can be used and special linear probing devices are not required.) These data can be processed to determine the incident field within the volume of the sphere. The technique is closely related to well-known spherical near-field scanning algorithms for determining antenna radiation patterns. In contrast to common field probing practices, which give qualitative estimates, the present method will produce, in principle, an accurate incident field reconstruction that has been fully compensated for probe pattern effects.