• # question_answer 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?

Sky wave propagation is used by short wave broadcast services having frequency range from a few MHz up to 30 MHz. Sky wave can travel very long distances and can even travel round the earth. $\therefore$ Refractive index of ionosphere $(\mu )$ is less than that of free space $({{\mu }_{0}})$ i.e.        $\mu <{{\mu }_{0}}$ As, we go high, the electron density decreases and therefore $\mu$ decreases. The refraction occurs till it reaches critical angle and afterwards it reflects electromagnetic wave back on the earth's surface. The critical frequency is given by             ${{v}_{c}}=9{{({{N}_{\max }})}^{1/2}}$ In general $30\text{ }MHz>{{v}_{c}}>5\text{ }MHz,$ when frequency is greater than ${{v}_{c}},$ then it crosses the ionosphere and never reflects back on the surface of Earth. The ranges of critical frequency up to few MHz sets an upper limit to frequency of waves used in this mode.
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