In sky wave propagation, the radio waves emitted from the transmitting antenna reach the receiving antenna after reflection in the ionosphere. It is also called ionosphere propagation.
The frequency range of transmitting signals is restricted to less than 30 MHz because for every layer of ionosphere, there is a certain maximum frequency of electromagnetic wave above which the wave is not reflected back but gets refracted. Thus, the electromagnetic waves of frequency more than 30 MHz are not reflected back from ionosphere but gets refracted.
