On the Edge !

Probably the main cause of radio telemetry systems becoming unreliable is signal levels being "on the edge" with no fade margin available on the radio path. Fading is as a result of particles obstructing the radio signals e.g. mist, fog, rain, and heat waves. Remembering that radio waves are the same as light waves, and what affects light will also affect radio, only the intensity of effect varies. Fading is not as a result of obstructions such as buildings; that is path loss. Fade is the extra loss through the effects of elements on a pre-determined path loss. Do not confuse the two.

During a really nice day at about 25°C signals should have a fade margin in excess of approximately

dBfm = 20log(D/10)

dBfm is minimum fade margin required, and,
D is distance in km.

It will be noticed the above formula is logarithmic and not the flat 20dB often stated. This is sometimes sneered at as it is believed that all radio paths should have a 20dB fade margin. As stated earlier, radio is just another form of electro-magnetic energy and will react the same as light. If there is mist, the mist does not obscure near objects from view but will blot out further objects. The same will happen with radio. This formula is used to determine if fading is likely to be the fault and not intended for planning purposes. Should a signal on a path be many dB in excess of the result calculated above, then the fault is not one of riding on the edge.

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