On 06/07/2019 21:06, Rich Zwirko - K1HTV wrote:
Here in the Mid-Atlantic, a number of 6 Meter FT8 DXers have observed
backscatter signals that appear as a 2nd or even a 3rd signal on the
WSJT-X Wide Graph window. The Doppler shifted frequency is usually
lower than the direct signal. In addition to the directly received FT8
signal appearing on the waterfall display, at times both direct signal
as well as one or more of the reflected backscatter Doppler shifted
signals can be decoded. The DF between the main and ghost signals is
usually between 40 and 130 Hz. The average appears to be around
negative 70 Hz.
When these ghost signals are observed, they are usually noted on
multi-KW FT8 signals produced by stations in the Mid-Atlantic region
that are beaming towards highly ionization Es regions. These extra
waterfall signals can last a few minutes then quickly disappear. I
have not observed them when there is no Es opening. They don't appear
to be the result of signals being reflected off aircraft.
73,
Rich - K1HTV
Hi Rich,
I believe you are wrong to discount aircraft reflections from your
observations. Here in the UK at a location where there is a lot of
aircraft traffic from the London airports and also considerable high
altitude traffic heading transatlantic to and from EU, the effect is
particularly noticeable during Es propagation peaks. I believe what you
are seeing is Es backscatter from high ERP stations with narrow beam
widths, combined with aircraft reflections of their signals. The effect
is very prominent because the "main" signal is not direct, i.e. back
scattered, and is weaker relative to aircraft reflection paths. The net
effect is apparently strong Doppler shifting traces either side of a
central trace that is on frequency due to being scattered from static Es
clouds. Note that the true direct signal from these stations is often a
quite weak component due to them beaming away from you, and being over
the horizon.
73
Bill
G4WJS.
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