Question

Which of the following frequencies will be suitable for beyond the horizon communication using sky waves?
A) 10 kHz
B) 10 MHz
C) 1 GHz
D) 1000 GHz

Answer 1

Hint : Sky-wave propagation works due to the reflection of radio waves by the ionosphere. The ionosphere can only reflect waves up to a certain critical frequency value and is transparent to wave with a frequency higher than that of critical frequencies.

Complete step by step answer
Sky-wave propagation also known as the skip is a type of radio wave propagation. It works on the principle that the ionosphere can reflect radio waves so the signals sent by the transmitter from one point on the surface of the Earth will bounce from the ionosphere and then will be reflected back towards the surface of the Earth and the waves will keep on bouncing and can be received at very far distances.
Since the angle at which the waves hit the ionosphere and the surface of the Earth is very small, there is very little scattering and hence little loss of intensity of the signal. This way we can send signals beyond the horizon which is just a way of saying that we can send signals where the transmitter is not in the line of sight of the receiver.
However, this method can work only in the range of radio frequencies that lies between 3 MHz and 30 MHz.
Hence the only frequency in the options between 3 MHz and 30 MHz is (B).

Note
Waves with a frequency lower than the range will be scattered in the atmosphere and won’t be able to travel long distances. Waves with frequencies higher than this range will not interact with the ionosphere and hence, they won’t be reflected in the ionosphere. So, the signal sent by the transmitter will directly escape the Earth without being reflected so it will not be reaching the receiver hence sky-wave propagation won’t be possible.