We examine the relative importance of coherent and incoherent echo components in near-nadir radar scattering from self-affine surfaces. The coherent component, important only for observations at small incidence angles (nadir-looking sounders and altimeters), is a function of both surface roughness and scattering area. Models for the incoherent component require a minimum scattering area for validity; for smaller regions, the radar echo is overestimated. We propose a simple criterion for the horizontal scale that defines this shift in applicable scattering model and show the range of roughness parameters for which MARSIS, SHARAD, and possibly JIMO radar sounder measurements may be modeled using the two approaches.