Rich,
I hear what you say about the Doppler effect, and I recall our conversations on
HF about it. I do not challenge the theory and facts behind the Doppler
effect, but I believe there is more to the propagational phenomena we attribute
to the Doppler effect. Being one who often challenges conventional wisdom ...
here is a question I have been pondering. For hams, we consider the Doppler
effect of radio wave bouncing off airplanes, and this causes unique radio
conditions. My question is since the number of airline-only [excluding
worldwide military and private] flights increased from 23.8 [2004] to 39.4
[2019 to-date] MILLION why do we not experience more Doppler effect situations
on ham radio? Another data point in that equation is the number of commercial
[only] flights per day in the US ... which is about 87,000.
I recall us talking about the unique sound of the Doppler effect, and sometime
attribute it to my/our proximity to Dulles International Airport. I am very
close to the take-off and landing approaches to Dulles, and I am even closer to
those of Manassas International which has a lot of private & business traffic.
So my chances of being affected by Doppler should be greater. However, I have
not experienced a proportional increase of Doppler related conditions. What's
going on? Why do we not have more "Doppler propagation instances ... or do we
and just not realize it?"
BTW - sorry for this being somewhat outside the scope of WSJT-X.
Ciao, Danny AH6FX/W4 Nokesville, Virginia
On Sunday, July 7, 2019, 5:31:38 AM EDT, Martin Davies G0HDB
<[email protected]> wrote:
On 6 Jul 2019 at 16:06, 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.
Hi Rich, why do you believe the 'ghost' signals that are being observed aren't
caused by
reflections off aircraft?
I very regularly see, and FT8 will decode, a second and sometimes even a third
signal from
nearby stations that are within a few miles of my QTH; on my waterfall I can
see the direct
signal and also the secondary signal(s) that almost always have a clearly
discernible slant on
them, indicating that the cause of the secondary signal is Doppler shift from a
moving object.
In my experience the secondary signal(s) can be either higher or lower in
frequency, by some
indeterminate amount, than the direct signal so they're not artefacts resulting
from 50/60Hz
power-line modulation of the FT8 audio signal, and the +ve or -ve offset of the
secondary
signal(s) from the direct signal will presumably indicate the direction of
travel of the moving
point of reflection.
IMO there's no almost doubt that such 'ghost' signals are caused by reflections
off aircraft; I
suppose it's theoretically possible that a very fast-moving cloud of Es
ionisation could cause
the same effect but I would guess that such clouds don't move at similar speeds
to aircraft!
---
Martin, G0HDB
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