Page 101 - GEN-radio 2025 - færdig_Neat

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.

13.4.2 Factors Affecting VHF Propagation

13.4.2.1 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.

13.4.2.2 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.

13.4.3 Effectiv Range VHF

13.4.3.1 Line of Sight Range

The curvature of the earth limits the use of the direct wave. It can be seen that
the aircraft ‘below the horizon’ cannot use the direct wave for communications.

The lowest direct wave is just tangential to the surface and is known as the

‘horizon ray’. It will be appreciated that direct wave communications for the

GEN radio 2025 - Midtjysk Flyveskole 101