Most everyday amateur communication using VHF (and above) bands relies on non-ducting tropospheric propagation. Although at certain times of the year, and over certain paths, spectacular QSOs are achieved by tropospheric ducting, or via the ionosphere, scatter and refraction below the tropopause (between 8 and 20 km above the earth) remain our mainstay. If you want to know how to distinguish ducting from non-ducting tropo, then visit this page.
Surprisingly, our understanding of non-ducting tropo (NDT) is not as good as it should be, given its commonplace occurrence. The physical basis was set out over 50 years ago, by Booker and Gordon (A theory of radio scattering in the troposphere Proc IRE 38:401-412), and you'll find this described in more detail in the texts listed in the bibliography. There are more popular versions of this, such as that produced by Gannaway in the amateur literature. At the heart of the Booker-Gordon model is the requirement for refractive blobs (an esoteric term!) generated by turbulence in the troposphere. Whilst there is good experimental evidence for these occurring in the lower troposphere, and that their correlation distance is appropriate to the observed propagation, and the variance of the index of refraction fits too, longer paths can get increasingly problematic. This is because of the increasing altitude of the common volume, in which the forward scatter should be taking place.
If you have ever enjoyed a QSO using NDT propagation over a distance of more than 500 km, try visualising where that forward scatter must have been happening, at an altitude of over 8 km above the earth's surface. This is way above the more interesting and active lower regions of the troposphere, up in the zones where most tephigrams get rather boring. The next time you are flying in a passenger aircraft up at that sort of altitude, in good weather, notice how most of the time the air has no gross turbulence, and the only visible peculiarity is the haze trapped just below the tropopause.
The tropopause itself has interesting effects on NDT propagation. If you keep an eye on the BADC MST radar images, you'll notice a strongly reflective band which shows roughly where the tropopause is. Unless NDT extends above the tropopause, for a lot of more distant QSOs, the tropopause must be forming the upper boundary of the common volume of forward scattering. So is the height of the tropopause not a major determinant of NDT propagation at longer distances?
If you'd like to help me try to get a better understanding of what is going on in NDT propagation, then I am very keen to have repeated QSOs with amateur stations who can provide good information on their output, aerials, and precise location, from the M4 northwards. If you can offer 2, 4 and 6 m bands, then so much the better - given the importance of the relationship between the wavelength and correlation distance, this could be very useful. I am also always interested to hear of unusual NDT propagation events, such as enhancements in relation to fronts and weather conditions in general.
If you want to read more reliable accounts, here is a bibliography to get you started:
Geoff Grayer's Chapter 2 in The VHF/UHF DX Book edited by Ian White, RSGB, ISBN 0-9520468-0-6. This is essential reading for anyone with an interest in VHF and above.
Ray Flavell's Chapter 3 in VHF/UHF Handbook edited by Dick Biddulph, RSGB, ISBN 1-872309-42-9. Dwells a lot on tephigrams and the like, but good when read in conjunction with the above.
Beyond Line of Sight. A History of VHF Propagation from the Pages of QST edited by Emil Pocock, ARRL, ISBN 087259-402-5. A wonderful collection of reprinted articles from QST, containing material on other modes, and Gannaway's classic account of tropo scatter.
Propagation of Radiowaves edited by Hall, Barclay and Hewitt, IEE, ISBN 0-85296-819-1. Pricey, and with the main emphasis on commercial radio, but an excellent summary of ITU Recommendations, and with some superb illustrations of ducts.
Doppler Radar and Weather Observations by Doviak and Zrnic, Academic Press, 2nd edn., ISBN 0-12-221422-6. Looking at radio propagation from the other side, perhaps, but a very good insight and well illustrated. Expensive too, I fear.
Wave Propagation and Scattering in Random Media by Ishimaru, reprinted by Oxford/IEEE Press, ISBN 0-19-859226-4. A thorough account of the mathematical physics, expensive.
Fundamentals of Atmospheric Physics by Salby, Academic Press, ISBN 0-12-615160-1. A good account of the troposphere, in the main, which is fairly affordable, although it has little reference to radio wave propagation.
Troposcatter Radio Links by Roda, Artech House, ISBN 0-89006-293-5. Concentrates on commercial applications, but contains a lot of useful information. Out of print, but available direct from Artech House under their In-Print-Forever programme (see http://www.artech-house.com/).
Here are some pages which go into some of the more controversial aspects of NDT in more detail, with speculation on the results of recent research:
The history of VHF propagation beyond line of sight
The troposphere as a medium for propagation
The phenomena of NDT described
Paths of NDT propagation
Refraction as a mode of NDT
Forward scatter as a mode of NDT
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Last updated 21 Dec 1998