Page 89 - Communication IFR_Neat

change in phase which occurs at the point of reflection of the ground-reflected
wave. The net result is that, for instance, an aircraft flying towards a ground station

may suffer fading or temporary loss of VHF communications with that station. The

range at which this occurs depends on ground aerial height above the surface,
aircraft altitude, and frequency. For instance, with VHF R/T, except in freak

conditions, the curvature of the earth gives protection from mutual interference

between stations using a common frequency provided they are well- separated
geographically.

9.7 FACTORS AFFECTING VHF PROPAGATION

Attenuation.

The term attenuation means the loss in strength of a radio signal as range from the
transmitter increases. The signal strength received is inversely proportional to the

distance from the transmitter. A wave becomes attenuated as range increases

because:-
• The radio energy available is spread over a greater area.

• Radio energy is lost to the earth, the atmosphere, and sometimes to the

ionized layers above the earth.

One factor on which the operational range of a radio emission depends is the
transmitter power. The range obtainable is proportional to the square root of the

power; in other words if the range is to be doubled, the transmitter power must be

quadrupled.

Refraction.

As a general rule, radio signals travel in straight lines, that is, they follow great circle
paths over the surface of the earth. Under certain circumstances, however, the

path of a signal may change direction. This change of direction is known as

refraction. The amount of refraction varies considerably, depending on conditions
and on frequency. In the VHF range of frequencies there is negligible refraction.

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