Last Updated on December 4, 2024 by John Berry
Normally, a radio wave launched from an antenna bends (is refracted) towards the Earth. The wave also spreads as it propagates toward the distant receiver and if the path is obstructed, only a small amount of signal energy can be captured beyond the obstruction. Propagation is normally by both refraction and diffraction. Ducting is a special case when radio signals can propagate much further than would be available by refraction and diffraction. Tropospheric ducting is experienced mostly in the VHF and UHF bands.
Duct set-up
Typically, energy spreading at acute angles is lost to space. But sometimes an upper limit is forced on that propagation by the set-up of a layer which reflects energy from its boundary with the higher troposphere. The wave is then trapped between the boundary and the Earth’s surface. The result is a surface-based duct. Sometimes too, a second boundary develops between the upper boundary and the Earth’s surface. The result then is that the wave is trapped in an elevated duct.
Ducts are characterised by a departure in the atmosphere’s refractive index from the normal 50% of time of -40N/km* to a much smaller -157N/km or below. Under normal tropospheric behaviour the refractive index reduces and inverse beam-bending occurs progressively. The normal obstruction of the Earth’s bulge into the radio path falls away, and the stations are effectively left looking at one another.
Inverse beam bending is often mistaken for ducting.
If a duct is formed, the path loss approaches the Free Space Loss (FSL) and occasionally, losses reduce to be less than that. This means that long-distance paths that were previously inoperable (through excessive diffraction loss over free space) become operable.
For the duct to trap and propagate a signal, the angle of entry must be very shallow. The receiver must also have a similar geometry.
Image credit: Sean Oulashin on unspash.com
Ducting frequency of occurrence
Ducts supporting enhanced propagation at VHF and UHF are rare around the British Isles.
Favourable propagation more often comes about in the UK by the more-frequently occurring reduced refractivity to around -100N/km represented by a k of 2.75 exceeded for 5% of time. k is the effective Earth radius representing the refractivity and used to draw straight line path profiles by distorting the Earth’s bulge. This reduces the diffraction loss to something allowing 500-800km paths, rather than the longer 1,000km plus paths of ducting.
Ducting is typically caused by a warm moist air mass when it slides off the land onto a cold(er) sea. The air mass cools rapidly, moisture condenses out (since cold air can hold less moisture) and the refractive index drops. This really only occurs over the North Sea in high summer. There are three centres of frequent ducting in the World – all in the Tropics. One is off the coast of Senegal, West African, where much of the empirical work on ducting was done in the 70s and 80s.
*Refractivity (or rather refractivity gradient) is measured in N units per kilometre (of height). Refractivity is generally negative and hence we’d say, for example, when describing the troposphere, that the refractivity is: negative 40 N units per kilometre, exceeded for 50% of time.