The frequency bands allocated to the ham radio make allow the use of every types of propagation:
Normally a radio wave is propagated like the light, in straight line, and is stopped by the obstacles, mountains or buildings; that means that the signal cannot normally reach a correspondent behind a mountain, or beyond the horizon.
That is fortunately no so on the decametric frequencies because solar flow when it reaches the high layers of the atmosphere ionizes the rarefied gases which are there, and makes them conductors of electricity, which gives them the property to reflect the radio waves.
The signal emitted by a French ham, for example, will be reflected on these ionized layers several hundred kilometers away from the transmitter and returned towards the ground he will reach well beyond the horizon. Moreover, it can then be reflected again on the ground and jump again toward the sky for a new reflexion if it meets a new correctly ionized layer.
With multiple jumps it is not rare to have contact with correspondents on other continents, even with the antipodes
The propagation on these frequencies primarily depends on the solar activity: when the sun passes by its maxima of activity (every eleven years) the propagation is excellent, it is not the case in calm period of sun. The alternation of illumination day/night or summer/winter has of coarse also its influence.
On these frequencies, the reflexion on the ionized layers does not function any more, except very rare exception, but the wave may go away the horizon thanks to another phenomenon, the refraction: the atmosphere is not homogeneous in temperature and water content, and the propagation velocity of the radio wave depends on these two factors, under certain conditions of inhomogeneousness, the wave is slightly curved to the bottom, which enables it to follow temporarily the curve of the earth and to fall down beyond the horizon, sometimes 1000 km away.
It is exactly the same phenomenon as with the mirages in the desert where the overheated ground creates this inhomogeneousness.
Centimetric and millimetric bands
The ionospheric phenomenon of reflexion does not exist on these frequencies, only the preceding refraction makes it possible to exceed the horizon; moreover, any obstacle stops the wave, even the foliage of tree.
For those which do not know the emission of amateur, see:
Frequency bands types of modulations types of traffic organizations
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