This paper is concerned with the evaluation of errors on a wideaperture type of direction-finder. The errors considered are those due to the wave interference between two rays incident on the aerial system, such as may be produced in ionospheric propagation. Expressions are derived for the mean-square error taken over all phase relations of the down-coming rays, and also over a distribution in bearing of the smaller of the two rays. In all cases a comparison is made with the corresponding figures for an Adcock direction-finder.It is suggested that, for the d.f. system discussed, an aperture of four wavelengths would be sufficiently large to give a useful reduction in the component of error variance due to wave interference, over that obtained with an Adcock direction-finder.