Question

(i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode.

(ii) Why is there an upper limit to frequency of waves used in this mode?

Answer 1

(a) Sky wave propagation is used by shortwave broadcast services having frequency range from few MHz to 30 MHz.

The sky waves reach the reciever after reflection from the ionosphere. The oscillating electric field of electromagnetic wave changes the velocity of the electrons in the ionosphere which changes the effective dielectric constant and hence refractive index. In a single reflection from the ionosphere, the radio-waves cover a distance of not less than 4000 km. In this way, a very long distance communication is possible with the help of sky waves.

(b) The ionosphere acts as a reflector for a certain range of frequencies (1710 kHz to 40 MHz). Waves above 40 MHz get refracted through ionosphere and escape. This is why there is an upper limit to the frequency used in this mode.