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Hi Andy, I'll bet against that ;-)
If I recall right, Richard G3CWI used
to have interesting Doppler plots and scattergrams from G3PLX on
his website (eg. http://www.qsl.net/g3cwi/scatterg.htm). It
seems that ionospheric Doppler spectra appear mostly rather diffuse. On the
other hand, in my opinion the sharpness of the slant line in Chris'
capture proves that a point-like object was moving along a regular,
well defined trajectory.
If you look to the left of the spectrogram,
you can see bits of the Doppler shifted line below main line, ie on
the high frequency side from the carrier. Then it crosses over and
reappears LF. This would be typical for an
aircraft approaching, passing by the baseline at a certain minimum
distance from the baseline, and then receding.
Best regards,
Markus (DF6NM)
Sent: Thursday, March 03, 2011 9:54 PM
Subject: Re: LF: Topband Test
Thanks again Andy and Markus,
Yes, my feeling is that the reflective
wave shift is due to doppler as the layer changes height. We need to try it
again over a longer period. It is true there are a lot of aircraft between us,
but would the result have been so smooth?
Has this effect been seen on 137kHz? The info I
have seen on near vertical incidence/reflection suggest it should.
Any thoughts on this appreciated, thanks for taking
the time to comment.
Chris, G4AYT.
----- Original Message -----
Sent: Thursday, March 03, 2011 8:40
PM
Subject: Re: LF: Topband Test
No, it definitely is due to ionospheric shift.
30m/s is no big deal over the period of the day/night
transition. 30 minutes at that speed would be a mean change in
height of 54km. Not unrealistic for an E layer
(??) height of 120km
Try to find a copy of G3PLX's article
'JNT
On 3 March 2011 20:19, Markus Vester <[email protected]>
wrote:
Hi Chris, thanks for sharing this unusual
observation.
From your screenshot, the split was up to about
0.35 Hz, or 0.2 ppm of 1.8 MHz. Interpreted as Doppler, a
reflector would have to move at least 30m/s radially from your
baseline. This seems a bit fast for an
ionospheric day-to-night shift.
Another possible explanation could be airplane
scatter. Large airliners are known to have a large radar cross
section at HF. Seeing that on 160 m may seem extreme,
but considering the wingspan of an A-380 (80 m), half-wave
resonant scattering is a very real possibility. The downward
slope of the Doppler shifted frequency would also be
indicative of an object passing by at constant linear
velocity.
Best 73,
Markus (DF6NM)
Sent: Thursday, March 03, 2011 7:55 PM
Subject: LF: Topband Test
Hi All,
Yesterday evening Pete, M0FMT, transmitted QRSS
on topband for a test. An unexpected effect was noticed with the trace
splitting in two. Nothing new or 'earth shattering' I expect, but new to us
and worthy of further experimentation.
I have never seen this effect on 137, even with
quite strong audible signals, maybe others have.
Vy 73,
Chris, G4AYT, Whitstable,
UK.
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