Q18 of 26 Page 1

(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?

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


(ii)


(iii) The sky waves suffer reflection in the ionosphere and reach the receiver after reflection. The oscillating electric field of electromagnetic wave provides the electrons in the ionosphere with energy and thus, change the velocity of the electrons which in turn affects the effective dielectric constant and hence refractive index. Radio-waves cover a distance of not less than 4000 km after reflection from the ionosphere . 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 the waves which are above the frequency of 40 MHz are refracted through ionosphere and escape.


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