Phase-center effects on wide-band horn pattern measurements in small anechoic chambers

The use of large feed horns is well known, with reference to satellite ground station reflector antennas. These antennas essentially produce high gain pencil beams. Due to the use of shaped reflectors for satellite contour beam applications there is renewed interest in large primary feeds such as corrugated horns, trifurcated horns and pyramidal horns. Due to the sensitivity of the large contour beams with reference to the incident feed illumination, it is necessary to characterize these primary feeds very accurately. The theoretical radiation patterns are used for synthesizing the shape of the reflector. Once the shaped reflector is fabricated, very limited mechanisms are available in a shaped reflector system to compensate for the imperfections in the measured feed radiation pattern. Thus, it is necessary to verify the feed radiation patterns in amplitude and phase by measurements before integrating the feed with the shaped reflector. This paper discusses a few precautions to be undertaken while carrying out radiation pattern measurements on these large feeds. In many C-band shaped reflector antennas the feeds are required to operate over the transmit (3.6-4.2 GHz) and receive (5.85-6.425 GHz) bands. These feeds are about about 10 to 15" in diameter and are characterized by large variations in phase centers over the frequency band. This variation leads to an interesting phenomena while carrying out measurements in small anechoic chambers which are commonly used for feed horn measurements. The variation in phase centers requires a correction to the measured radiation pattern which is described. Failure to do so will lead to erroneous radiation patterns as illustrated.