Page 90 - Communication IFR_Neat

9.8 EFFECTIVE RANGE OF VHF

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

aircraft could be restored by either raising the height of the ground aerial or

increasing the aircraft’s altitude.

A formula used for calculating the maximum range of direct wave reception is:

Range (nm) = 1.25 ( √h1 + √h2)
Where h1 = height of ground aerial (feet AMSL)

h2 = aircraft altitude (feet AMSL)

This formula allows for a small amount of refraction in the lower layers of the

atmosphere, which gives a slightly better range than would be obtained if the

direct wave followed a perfectly straight path without any downward refraction.
Under normal conditions, the space wave provides the only propagation path for

frequencies above about 30 MHz.

Therefore, except on fairly rare occasions, communications in the VHF and higher

bands suffer from line of sight transmission with range limited by the curvature of

the earth and any high ground which interrupts the line of sight. Note that the
range limitation imposed by Line of sight transmission is useful when there is a

shortage of available frequencies.

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