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 --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
