What are the modes of Radio Propagation?

1. Line Of Sight propagation: Line of sight refers to the propagation of HF radio waves in a straight line without being reflected by the ground or other objects. Such signals can travel for relatively short distances, depending on terrain and atmospheric conditions.
    
2. Ground wave propagation: The radio signal travels over the Earth's surface and is absorbed and reflected by the ground in this mode of propagation. Ground wave propagation is most effective at lower frequencies (below 2 MHz) and can be used for short-range communication, typically up to a few hundred kilometers.
    
3. Sky wave propagation: Sky wave propagation involves radio signals reflected by the ionosphere returning to Earth for long-distance communication, particularly effective at the HF range (3–30 MHz). Reflections occur due to multi-refractions.
   Additional modes of skywave propagation:
   * Ionospheric Scatter: Occurs when the radio signal is affected by plasma irregularities in the E and F layers. This phenomenon is known at frequencies ranging from 20 to 50 MHz.
   * Tropospheric scatter: The Troposphere (the lowest portion of the Earth's atmosphere) may scatter radio signals at frequencies ranging from 100 MHz to 2 GHz.

See HF Propagation modes, and an index for HF Radio Propagarion.