Question

Why sky wave propagation is not possible above 30MHz

Answer 1

Skywave propagation is using the atmosphere to reflect radio signals to places far away from the transmitter on earth.

For frequencies lower than 30MHz, the atmosphere is opaque and will reflect the signal well. If incident angles are low we can transmit far over the horizon and have low losses per “hop”.

But for frequencies between 30MHz and like, 6GHz, the atmosphere is not opaque at all. The power will transmit out to space, which is not what we want. It won't skywave propagate, it'll just head out into space.