How to distinguish ducting from non-ducting tropo

As with most matters of propagation, it is impossible to be certain whether tropo is ducting or non-ducting. Here are some indicators which will help you arrive at the more likely of the two, for a given QSO and path.

Ducting tropo usually:

• skips like sporadic E, that is to say, the distant station will be noticeably stronger than most closer stations which are not accessible by the duct. But note the comments for NDT below.
• results in surprisingly strong signals for the distance. If you work out losses using one of the standard NDT methods, then what you hear will be many dB stronger than you might expect, again not unlike sporadic E.
• is geographically selective. Although you can get ducts and ducting conditions which allow paths connecting quite widely separated stations, normally stations working a duct are quite close together in space, at both ends of the duct. There can of course be multiple ducts, or 'layers' rather than ducts, etc.
• requires low-angle entry into the duct. If your takeoff angle is highly positive, then you are not likely to be able to use it, but your emissions will shoot straight out of the upper side of the duct.
• uses only the lowest part of the troposphere. Ducts are commonest below 1 km, and very rarely accessible above 2 km. If you have a 1500 m mountain in the way, then look for something beyond simple ducting!
• is commonest over water, during high pressure (anticyclonic) conditions, when the air is relatively still.
• is unusual at longer wavelengths, because the ducts have to be larger to be effective. You'll be very lucky to work ducts on 6 or 4 m.

Non-ducting tropo usually:

• shows some signal strength enhancement with increasing distance, but this is normally only relative to calculated losses. This, a station 1500 km away is not likely to be stronger than one 750 km away, but it will be stronger than loss calculations would predict.
• can affect large areas at either end of the path. Indeed, its geographical selectivity may be very poor, with enhancement of propagation over very large areas.
• may still work (with significant attenuation, of course) for stations with positive takeoff angles. It is possible to work distances of over 700 km when your takeoff angle is 10 degrees, although you may then need some help by refraction from the obstacle.
• uses the highest parts of the troposphere. The longer the path, the higher the waves have to travel (although they will not be completely linear because of refraction at lower altitudes). The height of the tropopause may be a significant factor in determining maximum path length, although some NDT may occur through the lower stratosphere.
• is commonest during high pressure (anticyclonic) conditions, but not particularly over water paths. NDT is often best when the high pressure is in decline, but prediction is extremely hard, particularly if you are only studying surface charts. If you want to look at something more meaningful, you will need to get hold of charts showing upper air features.
• occurs at all VHF, UHF and higher frequencies, although longer wavelengths do not usually enjoy as good NDT as shorter ones.

Another important issue when trying to decide the mode of propagation, across the VHF bands, is whether it could have been ionospheric (particularly by ionoscatter or sporadic E). Generally speaking, sporadic E will be much stronger on similar bearings when you listen to lower frequencies. If there is no sign of any enhancement of propagation on lower VHF frequencies, you can usually be quite confident that the mode was tropospheric, although you should not expect to work identical paths (at any given time when the bands are open, the higher the frequency, the longer the predicted path, via sporadic E) at identical times.

NDT Inner Sanctum

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Last updated 5 Oct 1998
Howard Oakley
Mail howard@quercus.demon.co.uk