The influence of re-radiation on measurements of the power gain of an aerial

While measuring the power gain of a sectoral horn by a method devised by E.M. Purcell, it was found that re-radiation caused serious errors unless precautions were taken to allow for it. The theory of the effect is developed, and experiments are quoted which tend to confirm it. The mechanism of re-radiation is discussed for particular examples. Finally, a modification of Purcell´s method is suggested, and it is shown that the direct method of measuring power gain is not seriously affected by re-radiation.