An antenna pattern measurement technique for eliminating the fields scattered from a finite ground plane

When making ground plane based antenna pattern measurements in small anechoic chambers or compact ranges one is often restricted to a ground plane which is only a few wavelengths in extent. The fields scattered from the edges of this finite ground plane interfere with the fields radiated by the antenna, and are usually apparent in the radiation pattern as scalloping on the frontside of the pattern, and as spurious lobes on the backside. These features are not present in a theoretical analysis which assumes an infinite ground plane, and therefore make it difficult to compare theoretical field patterns with those measured experimentally. Previous investigators have made attempts to reduce the edge effects, with only moderate success [1,2]. Time gating (the technique of placing a temporal window filter around the desired signal) is effective at eliminating edge diffraction for relatively broadband antennas, near the zenith, where adequate temporal resolution is obtained. However, away from zenith, and for narrow band antennas, the diffracted and desired signals overlap, thus making time gating ineffective [3,4].