Question

(i) Which mode of propagation is used by short wave broadcast service having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically,upto what distance, communication can be achieved, by this mode.

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

Answer 1

(i) Sky wave propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz.
The sky waves reach the receiver after reflection from the ionosphere. The oscillating electric field of an 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, 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.

(ii) There is an upper limit to frequency of waves used in this mode because waves above 30 MHz, penerate the ionosphere and escape whereas the waves less than 30 MHz are reflected back to the earth by the ionosphere.