Question

Transmission via sky waves is impossible for the frequencies

• A. less than 40MHz
• B. equal to 40MHz
• C. more than 40MHz
• D. all of these

Answer 1

Answer: (C)

more than 40MHz

Answer 2

Sky wave propagation relies on the reflection of radio waves from the ionosphere. The ionosphere is a layer of the Earth's atmosphere containing free electrons and ions, which can reflect radio waves back to Earth.

The ability of the ionosphere to reflect radio waves depends on their frequency and the electron density of the ionosphere. There is an upper frequency limit, known as the Maximum Usable Frequency (MUF), above which radio waves will penetrate the ionosphere rather than being reflected.

Typically, sky wave propagation is effective for frequencies in the High Frequency (HF) band, which ranges from approximately 3 MHz to 30 MHz. While under certain conditions, frequencies slightly above 30 MHz (e.g., up to 40-50 MHz) might be reflected, frequencies significantly higher than this range (e.g., in the Very High Frequency (VHF) band and above) almost always penetrate the ionosphere.

Therefore, for frequencies more than 40MHz, the radio waves are generally too high in frequency to be reflected by the ionosphere and instead pass through it into space. This makes sky wave transmission impossible for such frequencies.