Gain of Electromagnetic Horns

Recent experimental evidence indicates that the measured gain of pyramidal electromagnetic horns may be considerably in error if the measurements are carried out at short distances, and the aperture to aperture separation between horns is used in the gain formula G=(4¿R/¿) ¿PR/Pr. Further experimental verification of this effect has been obtained and a theory developed which is in good quantitative agreement with present experimental data and demonstrates the physical reasons why the previous "far field" criterion of 2D2/¿ is invalid. Curves are presented from which the error in gain measured at any distance may be obtained and applied as a correction.