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?
(i) Sky wave propagation is used by short wave
broadcast services having frequency range from a few MHz upto 30 MHz.
(ii) Sky wave can travel very long distances and can even travel round the earth.
Refractive index of ionosphere X is less than of free space
As we go high, the electron density decreases and therefore X decreases. The refraction occurs till it reaches critical angle and afterwards, it reflects electromagnetic wave back on the earth’s surface. The critical frequency In general, 30 MHz > { V }_{ c } > 5 MHz, when frequency is greater than
{ V }_{ c } then it crosses the ionosphere and never reflects back on .the surface of the earth.
The range of critical frequency upto few MHz sets an upper limit to frequency of waves used in this mode.
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