Question

Distinguish between ‘sky waves’ and ‘space waves’ modes of propagation in communication system.

(a) Why is sky wave mode propagation restricted to frequencies upto 40 MHz?

(b) Give two examples where space wave mode of propagation is used.

Answer 1

Ground wave − When the radio waves from the transmitting antenna propagate along the surface of the earth so as to reach the receiving antenna. Frequency range is less than a few MHz.

Sky waves − In this wave propagation, the radio waves from the transmitting antenna reach the receiving antenna after reflection from the ionosphere. Frequency ranges from a few MHz up of 30 to 40 MHz.

(a) The e.m. waves of frequencies greater than 40 MHz penetrate the ionosphere and escape so, the sky wave propagation is restricted to the frequencies upto 40 MHz.

(b) In television broadcast and satellite communication, the space wave mode of propagation is used.