( i ) Ground Wave or Surface Wave Propagation It is suitable for low and medium frequency up to 2 MHz . It is used for local broad casting .
( ii ) Sky Wave Propagation It is suitable for radio waves of frequency between 2 MHz to 30 MHz . It is used for long distance radio communication .
Critical Frequency The highest frequency of radio wave that can be reflected back by the ionosphere is called critical frequency .
Critical frequency , v c = 9 ( N max ) 1 / 2 Where , N max = number density of electrons / metre 3 .
Skip Distance The minimum distance from the transmitter at which a sky wave of a frequency but not more than critical frequency , is sent back to the earth .
Skip distance ( D skip ) = 2h ( V max / V c ) 2 – 1 where h is height of reflecting layer of atmosphere ,
V max is maximum frequency of electromagnetic waves and V c is critical frequency .
Fading The variation in the strength of a signal at receiver due to interference of waves , is called fading .
( iii ) Space Wave Propagation It is suitable for 30 MHz to 300 MHz . It is used in television communication and radar communication . It is also called line of sight communication .