(1) 10 kHz
(2) 10 MHz
(3) 1 GHz
(4) 1000 GHz
Answer – 10 MHz
In case of beyond – the – horizon communication, the signal needs to cover large distances. For a signal of frequency 10kHz, the signal can not be radiated efficiently as the size of the antennae becomes large enough as –
\[\left( \lambda \alpha \frac{1}{\nu } \right)\]
The 1 GHz – 1000 GHz signal waves are highly energetic and escape by penetrating the ionosphere.
However, signal waves of 10 frequencies MHz easily get reflected from the ionosphere and hence for beyond – the – horizon communication signal waves such frequencies are suitable. Therefore, correct answer will be 10MHz.