What is the primary frequency range for terrestrial communications using skywave propagation?

The primary frequency range for terrestrial communications using skywave propagation is indeed between 3 and 30 MHz. This range is particularly significant because it includes the HF (High Frequency) band, which is capable of utilizing the ionosphere for long-distance communication.

Skywave propagation occurs when radio waves are reflected off the ionosphere, allowing signals to travel beyond the horizon. Frequencies in this range have the ability to achieve effective transmission over considerable distances by bouncing off the ionospheric layers, especially during specific conditions such as during the night or at certain times of the year when the ionosphere behaves more favorably for this type of propagation.

The other options provided fall outside the effective range for skywave propagation in a terrestrial context. Frequencies below 3 MHz often suit long-range maritime and aviation communications better but do not exhibit the same effectiveness for terrestrial communications. Frequencies above 30 MHz (like the 30-300 MHz range) are generally more suited for line-of-sight communications due to their shorter propagation distances, while those in the 10-30 MHz range can indeed support some functions, they do not encapsulate the full range of effective skywave propagation as thoroughly as the 3-30 MHz range does. Thus, the choice of