Page 102 - GEN-radio 2025 - færdig_Neat

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.

13.5 Freak Propagation

It has been stated that for frequencies above about 30 MHz, transmission is

normally ‘line of sight’ so that propagation is by means of the space wave. Under
certain conditions of freak or ‘anomalous’ propagation, however, ranges much

greater than line of sight ranges can be achieved by means of duct propagation

and scatter propagation.

GEN radio 2025 - Midtjysk Flyveskole 102