Working through the literature, and personal experience, there appear to be two broad types of NDT experienced on the bands:
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
Last updated 5 Apr 1999