The propagation of radio waves with frequencies ranging from 2 MHz to 30 MHz is mainly due to the sky wave propagation.

Sky Wave Propagation:
Sky wave propagation occurs when radio waves are reflected by the ionosphere back to the Earth's surface. The ionosphere is a layer of the Earth's upper atmosphere that contains a high concentration of ions and free electrons. When radio waves encounter this ionized layer, they can be refracted or reflected back to the Earth.

Key Points:
- Frequency Range: The frequency range of 2 MHz to 30 MHz corresponds to the High Frequency (HF) band. This range of frequencies is ideal for sky wave propagation.
- Ionospheric Reflection: The radio waves in this frequency range interact with the ionosphere, specifically the F-layer and E-layer, causing them to bend or reflect back towards the Earth's surface.
- Multiple Hops: Sky wave propagation enables radio signals to travel over long distances by undergoing multiple hops between the Earth's surface and the ionosphere. This allows for communication beyond the line of sight.
- Day and Night Variations: The efficiency of sky wave propagation is influenced by various factors, including the time of day. During the day, the D-layer of the ionosphere absorbs the lower frequency radio waves, limiting the effectiveness of sky wave propagation. However, at night, the D-layer disappears, allowing the lower frequency radio waves to penetrate the ionosphere and reach longer distances.
- Angle of Incidence: The angle at which the radio waves hit the ionosphere determines whether they are refracted or reflected. The optimal angle for sky wave propagation is usually between 5 and 30 degrees.

Conclusion:
In conclusion, the propagation of radio waves with frequencies ranging from 2 MHz to 30 MHz is primarily due to sky wave propagation. The interaction of these radio waves with the ionosphere allows them to be refracted or reflected back to the Earth's surface, enabling long-distance communication. Understanding the principles of sky wave propagation is essential for designing and operating HF radio communication systems.