Some errors in the calculation of the radiation patterns of reflector antennas, using Kirchhoff integration

It is demonstrated that calculation of the antenna patterns of reflector antennas, by means of Kirchhoff integration, leads to unreliable far sidelobes, if theoretical truncated feed patterns are used. The integrated radiation pattern does not yield 4¿, but a greater value; the calculated noise temperature is therefore too high. Integration of calculated radiation patterns over 4¿ steradians is recommended to control the results.