Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 10 MHz
In sky wave propagation radio waves are reflected back to the earth by the ionosphere. The ionosphere reflects only a limited band, about 3 MHz to 30 MHz, so a frequency from this band is needed.
10 MHz lies inside this band. 10 kHz is far below it, while 1 GHz and 1000 GHz are above 30 MHz and escape through the ionosphere.
Note: The same question was also asked in CUET (UG) 2023 on 21-05-2023 (Slot 3).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Sky Waves – “In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.”
Sky Waves – “… The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.”