Hi Uwe, yes it is a bit confusing as at first sight one would expect the
signals to be present or absent on both grabbers.
At night I think the main mode is a hop length of about 2000 km. See
http://www.btinternet.com/~alan.melia/geometry.htm
This is about the distance from Brian to DCF39. The night-time signals Brian
recorded over several years were fairly consistent but did sometimes show
severe dips in strength which I think are due to destructive interference
(optical term) between the main mode and other paths (maybe a 2-hop path).
The path to the States is roughly 6000kms which is 3 hops, but under
conditions of low absorption it may be that a two hop signal (or a 4 hop
signal) arrives, sometimes at a very similar strength. The result of this
can be deep fades, but the timing does depend on the path length. It is
interesting to watch Joe, Warren, John and Dex's signals. In particular
although John and Warren are only about 80km apart there is a distinct
difference in the times they fade and return....it can be as much as 15 to
20 minutes.
The positive side to this effect is that if the phase difference is right
you get a 6dB enhancement a short time later in the night. These are the
"lifts" that lead to some of the first TA successes on 3 sec dot QRSS (all
that was available at the time using Robert Horne's Spectrogram)
Interesting stuff this LF propagation !!
Cheers de Alan G3NYK
