A theory of spread F based on a scattering-screen model

To shed some light on the phenomenon of spread F, a thin scattering screen is postulated above the E region. The virtual height which is associated with a pulse radiated from the sounder, forward scattered by the screen and then reflected back to the sounder via the F region, is calculated. For frequencies appreciably larger than the penetration frequency, the minimum virtual height versus the operating frequency, on a linear scale, is a straight line, the slope of which depends on the height of the screen. As the height of the screen increases, the slope decreases. When the scattering screen is assumed at the level of reflection, the slope of the line coincides with the tangent from the origin to the regular vertical-incidence trace. Experimental ionograms are presented that fit with the suggested mechanism.