Hi Conrad,
On 7/8/2019 18:29, Conrad PA5Y wrote:
In fact Martin, sometimes they clearly AP reflections are but often they are
not. They have no discernible doppler slant at all and appear to be a fixed
offset and have me completely puzzled. I regularly see 2 or 3 'ghost' signals
between 6 and 20dB lower in amplitude than the main signal on 2m from stations
with 100W to a 9ele or more. Stations can be several hundred km away from me.
The offsets vary between 70-150 Hz and can be positive or negative. In order
to have a doppler shift something must be moving and with an offset of 100Hz or
more quite quickly. You would think therefore that the frequency offset would
change and it often does not but remains static for several periods and then
just disappears. How on earth can this happen? I have no reasonable explanation
for it but I assure you it is real.
Aircraft scatter on 6m occurs of course but not so often, however during
intense Es events sure enough I also see these ghosts on 6m and again no shift.
It really is very puzzling indeed.
Evidently what's puzzling you is Doppler shifts that remain nearly
constant for a minute or so. Aircraft at 10,000 meters can be
line-of-sight visible at distances up to several hundred km. With
typical aircraft speeds, in such cases the geometry for bi-static
reflections will not change much over a minute. So the Doppler shift
can remain nearly constant. Does this not make sense?
-- Joe, K1JT
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