Sometimes, the laying of signal transmitting terrestrial channel is too difficult, and in some instances (at the uplands or at sparsely neighboured places, for example) it is economically unviable.
In these cases, radio relay equipment is the best solving. The requirement of it is an optical visibility of a receiving antenna and a transmitting antenna. The transmission range depends on its frequency: more higher frequency – the less of a range.
A radio relay equipment package usually includes a frequency-division multiplex module, which allows a transmission of a few signals in one communication line.
If the transmission range is small enough, it is possible to use atmospheric optical lines. A TV-signal, transformed by transmission equipment, transforms in a powerful luminous flux, which comes on the receiving equipment through the atmosphere.
The concept is looks like a FOCL concept. But a high-precise optics is used here to form a straight optic beam. It allows to make the receiving leaks lower. The transmission range of this system is not so big – 1500 m, and the operational reliability absolutely depends on atmospheric phenomenons.
That is why the signal may disappear if there is a thick fog.