On the transition from light to shadow

The importance of the concept of penumbra width in radio propagation studies is stressed in this paper. The propagation over a smooth spherical earth is the mathematical model used for discussing the transition from light to shadow or, in other words, between interference and diffraction zones. The breakpoint between these two zones corresponds to the well-known criterion of a path clearance equal to 0.6 of the first Fresnel zone radius. It is shown here that this criterion can be related to the relative position of the penumbra width associated to each terminal.