Because the ionospheric plasma drifts, radio sounding signals encounter different irregularities of electron density, even at only slightly different times, causing temporal phase variations; these may be characterized conveniently by the structure function. We have proposed in several recent publications a new irregularity diagnostic method based on this effect, and it is now useful to consider the range of irregularity scales affecting the observed phase fluctuations. We show, in particular, that for the two-dimensional irregularity power spectrum index 2 < ν < 3 (quite typical for the ionosphere) the phase structure function value at the smallest available lag (0.01 s) is sensitive to irregularities in a broad range of scales, between several meters and several kilometers.