A ground receiver station is receiving a signal at (a) `5 MHz and 100 MHz` , transmitted from a ground transmitter at a height of `400 m` located at a distance of ` 125 km`. Identify whether it is coming via space wave or sky wave propagation or statellite transponder. Radius of earth
` = 6.4 xx 10^(6) m` , maximum number density of electrons in ionsphere ` = 10^(12) m^(-3)`.

Answer

(a) sky waves (b) satellite communication

Explanation

Maximum distance covered by space wave communication,
` d = sqrt (2 R h) = sqrt( 2 xx 6.4 xx 10^(6) xx 400)`
`= 71.55 xx 10^(3) m = 71.55 km`
Since the distance between transmitter and receiver is `125 km` , hence for the given frequency signals of ` 5 MHz and 100 MHz` , the propagation is not possible via space wave communication .
For sky wave propagation , the critical frequency ,
`v_( c ) = 9 (N _(max))^(1//2) = 9 (10^(12))^(1//2) = 9 xx 10^(6) Hz`
` = 9 MHz`
Since ` 5 MHz lt 9 MHz` , so the propagation of signal of frequency `5 MHz` is possible via sky wave . As `100 MHz gt 9 MHz`, hence the progation of signal of frequency `100 MHz` is not possible via sky waves . For that satellite communication is used .