LF signals can propagate by ground wave or by sky wave. Most studies
on LF propagation have
concentrated on the ground wave mode and little data and analysis
exist on sky wave propagation. J.S.
Belrose et al presented data on LF skywave propagation in the
Proceedings of the IEEE in May of 1959.
This paper seems to be the best source of data on the subject.

AMRAD is looking at acquiring data on European LF broadcast stations
to help engineer the system
necessary to achieve a transatlantic QSO on LF.

Some early data has been collected by Sandy, WB5MMB and is shown here.
Note the abrupt drop in
signal level around 0700 GMT which corresponds with sunrise in the
area of the transmitter. We are
curious to know what other phenomena might be exploited to achieve a
transatlantic QSO.

The equipment consisted of an e-field probe on the roof of a 3-story
building, and a military "Cubic 3030" receiver.

You may look up the graph at  http://www.amrad.org/projects/lf/162.jpg

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Andre' N4ICK