NDT: The phenomena of NDT described

Working through the literature, and personal experience, there appear to be two broad types of NDT experienced on the bands:

• at lower VHF frequencies (typically 6 and 4 metres, and on 10 m), best DX is usually up to 500 km. Bigger stations and better conditions can extend this to 700 km, but it very rarely exceeds 1000 km (see Palle OZ1RH's article referenced below). It is usually best in the early afternoon, during the summer months, and work undertaken for the Propagation Studies Committee of the RSGB has demonstrated this very well indeed (watch for a link soon to the report of this study by Ray Cracknell G2AHU, Chris Deacon G4IFX, and Ian Brotherton G2BDV).
• at higher VHF frequencies and above (most classically 2 metres), stations regularly work to 700 km or further even under flat band conditions. Bigger stations and better conditions take this out to over 1000 km, and occasionally out to over 1500 km. [Beyond that, extreme DX usually involves ducting over at least some of the path, or ionospheric modes.] Best conditions usually occur in the early morning or late evening, and in the autumn, but openings are not easily predictable. In the UK, they may be associated with the falling of surface barometric pressure as a high pressure system moves eastwards to central Europe. At SHF and above (70 cm +), NDT is generally harder, but capable of similar results.

Although in good conditions, low power (QRP) operation can achieve good DX, this is not as true as with ducting tropo (where path losses may be very small) or Sporadic E (Es) modes. Fading (QSB) is a problem, even during lifts, as it may be slow and very deep. This may include faster 'flutter' fading, and is particularly noticeable towards the end of a 'lift'.

If you are unsure as to the differences between ducting and non-ducting modes, they are detailed here.

Several papers have measured field or signal strength at various distances from transmitters, beyond line of sight. Among my favourites are:

Kitchen, F A, Richards, E G, & Richmond I J (1958) Some investigations of metre-wave radio propagation in the trans-horizon region. IEE Symposium on Long-Distance Propagation above 30 Mc/s, 28 Jan 1958. Pp. 106-116.

Ames, L A, Newman, P, & Rogers T F (1955) VHF tropospheric overwater measurements far beyond the radio horizon. Proc IRE October 1955, 1369-1373.

There are many excellent descriptions of fading patterns and the like, including:

Rider, G C (1958) Some tropospheric scatter propagation measurements and tests of aerial siting conditions at 858 Mc/s. IEE Symposium on Long-Distance Propagation above 30 Mc/s, 28 Jan 1958. Pp. 143-152.

Bullington K (1955) Characteristics of beyond-the-horizon radio transmission. Proceedings of the IRE Oct 1955, 1175-1180.

An interestingly controversial paper - which proposed that the tropopause could act as a reflective layer - is:

Starkey, B J, Turner, W R, Badcoe, S R, & Kitchen G F (1958) The effects of atmospheric discontinuity layers up to and including the tropopause on beyond-the-horizon propagation phenomena. IEE Symposium on Long-Distance Propagation above 30 Mc/s, 28 Jan 1958. Pp. 97-105

Palle OZ1RH is rather more upbeat about working tropo on 6 m. His excellent (and very encouraging) article from the UKSMG's Six News is available at http://www.uksmg.org/tropo.htm

David Dunham's hypothesis that some far DX can result from refraction in ionised ozone layers, or the D layer, is available in:

Dunham, D (1999) Ozone layer propagation: Pondering the possibility. CQ VHF January 1999, 32-38.

NDT Inner Sanctum

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Last updated 5 Apr 1999
Howard Oakley
Mail howard@quercus.demon.co.uk