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  4. Assertion: Television signals are received through skywave propagation. Reason: The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

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Q. Assertion : Television signals are received through skywave propagation.
Reason : The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

AIIMSAIIMS 2005Communication Systems

Solution:

In sky wave propagation the radio waves which have frequency between $2\, MHz$ to $30\, MHz$, are reflected back to the ground by the ionosphere. But radio waves having frequency greater than $30\, MHz$ cannot be reflected by the ionosphere because at this frequency they penetrates the ionosphere. It makes the sky wave propagation less reliable for propagation of TV signal having frequency greater than $30\, MHz$. Critical frequency is defined as the highest frequency that is returned to the earth by the ionosphere. Thus, above this frequency a wave whether it is electromagnetic will penetrate the ionosphere and is not reflected by it.