Question

The frequency suitable for beyond the horizon communication using sky wave is

• A. \(10\,KHz\)
• B. \(10\,MHz\)
• C. \(1\,GHz\)
• D. \(1000\,GHz\)

Hint:

Sky wave propagation is generally possible for frequencies between \[ 3\,MHz \text{ and } 30\,MHz \] because such waves are reflected by the ionosphere.

Answer 1

The Correct Option is B

Step 1: Recall the principle of sky wave communication.
Sky wave (ionospheric) propagation works because radio waves in a certain frequency range are reflected back to Earth by the ionosphere, allowing communication beyond the horizon without satellites.

Step 2: Understand the structure of the ionosphere.
The ionosphere has layers (D, E, and F layers) of ionised gas at different heights. The F layer, at about 200-400 km altitude, is the most important for sky wave reflection.

Step 3: Identify the frequency range for sky wave propagation.
The ionosphere reflects radio waves only in the High Frequency (HF) range: approximately $3\,\text{MHz}$ to $30\,\text{MHz}$.
Below $3\,\text{MHz}$: waves are partially absorbed by the D layer and the ground.
Above $30\,\text{MHz}$: waves pass through the ionosphere into space and are not reflected.

Step 4: Evaluate each given option.
$10\,\text{kHz} = 0.01\,\text{MHz}$ - too low, absorbed before reaching the upper ionosphere.
$10\,\text{MHz}$ - falls within the HF range (3 to 30 MHz), reflected by the F layer.
$1\,\text{GHz} = 1000\,\text{MHz}$ - far too high, penetrates the ionosphere.
$1000\,\text{GHz}$ - far too high, used in millimetre wave range.

Step 5: Select the correct option.
Only $10\,\text{MHz}$ lies in the suitable HF band for sky wave communication.

Step 6: State the answer.
\[ \boxed{10\,\text{MHz}} \]