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The report by Andre Kesteloot on 15/12/99 re LF
propagation prompted me to read some old IEE journals of the late 1940s when
there was much intrest in VLF.There are an enormous number of refs and I have
access to only a few of them but it seems a great deal of work was done on
LF propagation.Some conclusions are:-
Variations in signal strength mainly due to
interference between steady ground wave and multiple reflections from the
ionosphere.Height of the reflecting layer being 65-82Kms.Very great variations
at sunrise/sunset(indeed for 1-2hours before these events)Other sources of
variation due to SIDs and major mag. storms
The reflection height seems to be higher at
night by some 12-14Km in winter and 15-17Km in summer.Most of this work was done
at frequencies of 16-70Khz using the Decca chain but it was noted that the
effects were more marked at 127Khz.Relating all this to the possibility of
transatlantic QSOs it seems that there will be no ground wave so we are only
concerned with the reflected wave and therefore the height of the ref.layer,but
at only 60-80Kms the number of hops will be very great and thus the distance the
waves have to travel! can anybody do the spherical geometry to work this
out?What about the losses at each reflection? fortunately over sea so not too
great.But it should be possible to put this all together and work out the total
path loss.One more thing it seems to me that the best time for long distance
contacts will be at night with the mid path being at the centre of the
sunset/sunrise periods at the ends of the path.Well theres a lot to think
about here,but why not ,and who knows we might do it! so thanks Andre for
stimulating these thoughts. 73s Laurie.
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