Question

In sky wave mode of propagation, why is the frequency range of transmitting signals restricted to less than 30 MHz? 

Answer 1

In sky wave mode of propagation, the frequency range of transmitting signals restricted to less than 30 MHz, because it is the maximum frequency below which the signals can be transmitted from one point to another point via reflection from the ionosphere. Above this frequency, the signals will be transmitted through the ionosphere and are not reflected back.