The paper presents a detailed theoretical treatment of the beyondhorizon radio scatter propagation problem. The form for the spectral density of refractive-index fluctuation, assumed to be the same as that of velocity fluctuation, is derived from the universal equilibrium theory of turbulence. It is used to predict the spectral densities and magnitudes of brightness fluctuation and angular deflection fluctuation for light from a stellar source, and of field-strength fluctuation for line-of-sight radio propagation. These predictions are compared with experimental data, and support is thereby obtained for the theoretical form of the refractive-index fluctuation spectrum. The height variation of the intensity of the refractive-index fluctuations is derived from the experimental data. There follows a detailed treatment of the beyondhorizon case of radio scatter propagation, including a full analysis of the influence of the geometry of the scattering volume on the received field-strength. It is shown that the form of the scattering crosssection implied in the results is compatible with those obtained in different treatments, based on idealized assumptions.