It is well-known that the values of power gain of an electromagnetic horn as obtained by measurements made with two identical horns at finite distances are different from the true value which might be obtained if the measurements were carried out at infinite distance. In view of this

E. H. Braun has worked out a method for the calculation of the appropriate corrections which should be applied when the measurements of horns are being made at various finite distances. In his original paper

it is shown that the results of calculation are in fair agreement with experiments. However

in Braun’s method

the phase relationship of incident field on receiving aperture is not considered. This

as a matter of fact

introduces appreciable errors when the distances concerned approach 4D

2

/λ（D——linear dimension of the aperture

λ——wave length）. In this paper

an improved method of approximation is given

the results of calculations are listed in table Ⅰ and Ⅱ

in which it is shown that the accuracy of this new method is superior to that of Braun’s.