[wsjt-devel] FT8 ghost signals

[Rich Z. - K. <k1...@co...>]

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.

73,
Rich - K1HTV

= = =

From: Jim Brown [mailto:k9...@au...]
Sent: 6. heinäkuuta 2019 9:28
To: wsj...@li...
Subject: Re: [wsjt-devel] 60 Hz + harmonics sidebands on FT8 signals?

There are two common causes of sidebands with a station that is properly
adjusted and uses good equipment.

1) The best gear (Elecraft K3, K3S, or a Flex 6000-series), and a first
class power amp) have sidebands roughly 40 dB down on both sides of their
signal due to IM distortion in the RF stages. These sidebands are generally
symmetrical, and the total -40 dB bandwidth is about 680 Hz.

2) Doppler shift produced by reflections from aircraft. These sidebands are
generally asymmetrical (that is, on one side or the other of the main
signal). Here in the SF Bay Area, which includes Silicon Valley and extends
down along the Monterey Bay, we have glide paths for major airports in San
Jose and San Francisco.

There are roughly a dozen or so locals running 500W with gear like this,
and we all regularly experience what you're seeing.

73, Jim K9YC