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9.9 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.

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.

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

programmes also suffer from interference due to this effect.
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