Jim and others,
The graphs in Terman show only a modest 3-4 dB variation in
signal level over the sunspot cycle - but what they don't show is
what the signal-to-noise ratio was. The high noise level on LF is
one of the major factors deciding if communications can take place
or not. If a lot of the background noise on the band is from distant
electrical storms, one would expect the noise level to be subject to
the same propagation effects as the signals are. It could go either
way I suppose, so I await the solar minimum with interest.
Cheers, Jim Moritz
As an anecdotal comment, in ZL, when we started on LF in the early 1990s,
the DX propagation seemed to be better than it is now. We were doing quite
well for DX, and first-built transmitters were generally lower power than
are used nowadays.
I've also had a look at ITU-R propagation graphs, but they are confusing to
interpret as they are normalised. As somebody else commented on, the thrust
of some ITU-R work is to determine reliable ground wave coverage for LF and
MF broadcast stations, and the likes of sporadic transcontinental DX is not
in their terms of reference for results.
My opinion is that LF DX is statistically better around a sunspot minimum,
late at night, in winter.