(i) Ground wave or surface wave propagation .
(ii) Sky wave propagation or ionospheric propagation.
(iii) Space wave propagation / Line of sight propagation.
When radio waves (frequency range 3 MHz to 30 MHz), emitted from the transmitting antenna, reach the receiving antenna after reflection from the ionosphere which acts as a reflector for radio waves the corresponding mode of propagation is known as sky wave propagation.