A theory is given of the diversity effects produced in spaced aerials by incident waves having a distribution in direction of arrival. The general case of a continuous coplanar distribution of rays with random phase is considered, and when this distribution is narrow and symmetrical it is shown how its extent can be estimated from observations of either the amplitudes or the phase difference of the signals in the spaced aerials. The case of a strong steady signal superimposed on the centre of the continuous distribution is also analysed and the results are obtained in terms of a parameter which may be called the signal/noise ratio. It is shown how, by making measurements of the diversity effect in both amplitude and phase, both this ratio and the angular spread of the continuous distribution can be obtained.Some daytime measurements at nearly vertical incidence have been made on first-order ionospheric reflections at frequencies between 4 and 7 Mc/s, and it is found that the diversity effects observed are most satisfactorily explained in terms of a steady “signal” (or specularly reflected component) superimposed on a “noise” background (composed of the continuous angular distribution due to diffractive reflection). Values of signal/noise ratio of about two or three have been obtained, with a noise background which has an angular spread with standard deviation of about 1°.