( 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.