Question

Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode. Why is there an upper limit to frequency of waves used in this mode? 

Answer 1

Sky wave propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz.

Sky waves are used for long distance radio communication. The successive reflection of these radiowaves at the earth’s surface and the ionosphere make it possible to transmit these waves from one part to another part of the earth. 

These waves are reflected by ionosphere by means of total internal reflection, which arises due to change in refractive indices of different layers of ionosphere. Critical frequency of reflection for a particular layer is given by f_c = 9√N_max where N_max = maximum electron density in the given layer.For any frequency greater than f c ionosphere is not able to reflect the radiowave. Hence there exists an upper limits to frequency of waves used in this mode.