Question
State various modes of propagation of electromagnetic waves. Explain briefly using a proper diagram, the mode of propagation used is the frequency range from a few MHz up to 40 MHz.
Solution
Electromagnetic waves are propagated through the following modes:
(i) ground wave propagation
(ii) sky wave propagation, and
(iii) space wave propagation.
For transmission of signal having frequency range from a few MHz up to 40 MHz, sky wave propagation mode is used.
Sky wave propagation:
The mode of wave propagation in which the radiowaves emitted from the transmitter antenna reach the receiving antenna after reflection by the ionosphere. 
