[TowerTalk] source for propagation data?

[Jim Lux]

There has been mention recently (from Tom, W8JI, among others) that a 
physically very large antenna might not do you as much good as it might 
because the incoming wavefront is distorted by small scale variations in 
the ionosphere, so the assumption of an incident plane wave isn't valid 
when looking at apertures of several wavelengths.  This seems reasonable, 
but I'm looking for some actual numerical data.

If I have two small antennas separated by, say, 100 meters, looking at a 
sky wave signal, how decorrelated are they? Over frequency?

I've been looking into the HF OTH radar literature but haven't found much 
yet.  (probably because it was classified back in the 60's when OTH HF 
radar was a hot topic, and nobody's done much with it since).

Perhaps someone knows of some data from ionosondes that's available?