A Precise Measurement of the Gain of a Large Horn-Reflector Antenna

The gains of horn-reflector antennas have been measured many times in the past at Bell Telephone Laboratories, usually with the result that the effective area is about 1.5 db below full area. In other words, the measured aperture efficiencies run between seventy and seventyfive per cent.* Traditionally, such gain measurements employ a source located in the Fraunhoffer region of the antenna to be measured. The field radiated by the source is then sampled at the antenna by taking "height runs" with a standard (or reference) horn. A thorough job involves several height runs at various lateral positions to examine the field over the entire area occupied by the aperture of the antenna. Inevitably, due to the presence of the ground and other environment, variations exist in the field that illuminate the antenna under test. If the measurement is made over the flat ground of an antenna test range, it is possible to apply corrections to the measured gain. However, when large, highgain antennas are involved, it is difficult to find a sufficiently long range over flat ground. Antenna sites are often surrounded by terrain covered with vegetation, and the radiation from the source located on a tower * A precise measurement of a horn-reflector antenna recently was made elsewhere.1 1019