Why does FM not propagate in sky waves?

Question

I am curious why FM stations don't propagate in sky waves. I'm sure that it might be multiple factors, wattage, directional antennae, etc. Also, it appears that the higher frequencies penetrate the ionosphere rather than reflect. One thing that I am specifically curious about is the reflection. Is it truly reflection or is it refraction? If it is refraction do the different wave lengths refract at different angles which would effectively destroy the fidelity of the signal?

Answer 1

FM does propagate in sky waves. What it does not do is refract and/or reflect off the ionosphere. Frequencies lower than commercial FM do, but by the time you get to 88MHz they don't anymore. Instead, the sky wave penetrates the ionosphere and goes off into space.

Lower frequencies both refract and reflect off the underside of the ionosphere, depending on their angle of incidence. For low (grazing) angles of incidence, the wave front path gets gently bent or refracted back toward the earth, rather than reflecting "hard" off the ionosphere like a light beam off a mirror.