Three different modes of propagation of electromagnetic waves are :
1. Ground (Surface) wave propagation.
2. Sky wave propagation.
3. Space wave propagation (LOS communication).
For frequencies about 40 MHz, space wave propagation is being used as the ionosphere will not reflect the signals and ground transmission is not possible. In space wave the transmitter and receiver must be on the line of sight (LOS) together.
Such wave propagation used for television broadcast and satellite communication. In this propagation the uplink and downlink frequencies are kept different to avoid the mixing up of the signals.
