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. 73 Conrad PA5Y, JO21VO -----Original Message----- From: Martin Davies G0HDB <marting0...@gmail.com> Sent: 07 July 2019 11:27 To: WSJT software development <wsjt-devel@lists.sourceforge.net> Subject: Re: [wsjt-devel] FT8 ghost signals 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 _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
