Page 103 - GEN-radio 2025 - færdig_Neat

13.5.1 Duct Propagation

This effect, also called ‘super-refraction, is associated with a temperature
inversion and a rapid decrease in humidity with height. Such meteorological

conditions are most often found at the surface over land in high pressure

conditions at night and in the early morning. A warm air mass over a cold sea can
also produce the effect. It can also occur at higher levels.

The way in which radio signals can be ‘trapped’ in a duct of cold air is shown

above. This process sometimes permits reception of signals at the surface

hundreds of miles beyond the horizon. The effect is most common in the SHF
and UHF bands, but is also encountered in the VHF band if the duct is sufficiently

deep (say, 500 ft). Duct propagation can cause annoying interference between

control towers using the same R/T frequency, and false range indications on
ground radar screens.

13.5.2 Scatter Propagation
The E-layer sometimes contains areas of very high ionisation density which can

produce weak sky waves (known as ‘Sporadic-E’ reflections) in the VHF band. The

effect is unpredictable and the sky waves are scattered at random in the forward
direction from the transmitter.

With specially designed aerials, scatter propagation can sometimes be used to

provide intermittent extended range VHF R/T but it is not a reliable means of
communication. Scatter propagation can cause mutual interference between

VHF radio aids sharing a frequency and normally protected from interference by

line of sight transmission. Television programs also suffer from interference due
to this effect.

GEN radio 2025 - Midtjysk Flyveskole 103