Focal-Plane Scanner for Measuring Radome Beam Deflection in a Compact Range
Author: Scott T. McBride
Measurement of radome beam deflection and/or Boresight shift in a compact range generally requires a complicated set of positioner axes. One set of axes usually moves the radome about its system antenna while the system antenna remains aligned close to the range axis. Another set of axes is normally required to scan the system antenna through its main beam (or track the monopulse null) in each plane so the beam pointing angle can be determined. The fidelity required for the beam pointing angle, combined with the limited space inside the radome, usually make this antenna positioner difficult and expensive to build.
With a far-field range, a common approach to the measurement of beam deflection or Boresight shift uses a down-range X-Y scanner under the range antenna. By translating the range antenna, the incident field's angle of arrival is changed slightly. Because the X-Y position errors are approximately divided by the range length to yield errors in angle of arrival, the fidelity required of the X-Y scanner is not nearly as difficult to achieve as that of a gimbal positioner for the system antenna.
This paper discusses a compact-range positioner geometry that approximates the simplicity of the downrange- scanner approach commonly used on far-field radome ranges. The compact-range feed is mounted on a small X-Y scanner so that the feed aperture moves in a plane containing the reflector's focal point. Translation in this 'focal plane' has an effect very similar to the X-Y translation on a far-field range, altering the direction of arrival of the incident plane wave. Measured and modeled data are both presented.