Mystery Recurring Signal in the 40M Band

[padre]

Good Evening,

Over the last week, there has been a signal I can't ID occuring in the lower portion of the 40M amatuer band. I first noticed it on the JS8CALL QRG where it appears as a 300hz wide 8FSK signal. It transmits for 15 seconds starting at 00:15 and 00:45 past each minute. At first glance, it seems to be in time with the adjacent FT8 traffic but this isn't FT8 traffic. On further investigation with an SDR, I've seen the transmissions across a swatch as wide as 15khz. On the first occassion I noticed it, it transmitted continuously in 15 second intervals for over 36 hours. Since then, it has occurred sporadically. I have examined all the waveforms in the signalwiki database and can't find anything like it. FLDIGI won't decode it. I have IQ data and audio recordings but it is slightly above my technical level to do a full analysis.

https://imgur.com/a/JO8i4Aq

The signal, or a portion of it, has been appearing regularly at 7079khz. The above image shows the signal, or signals, across a larger bandwidth and was taken at 0200z on 16 APR.  At first glance I say signals, plural, because they are different bandwidths but at the time I observed this they all came on and off at the same time.

If there is a good place to post an audio recording, I will share that as well.

73,

Padre

[padre]

Using the TDOA feature on KiwiSDR it seems probably the signal is originating somewhere in the triangle formed by Springfield, MO, St. Louis, MO, and Memphis, TN. I added a close up of the 8 tone portion of the signal and another shot of the whole activity on imgur.

https://imgur.com/a/JO8i4Aq

This activity was roughly 0405z. There are several narrowband signals but there are also tones in between them at roughly 100hz spacing across the roughly 15khz area of interest.

73,

Padre

[Token]

Yes, I think this is FT8.  Just not the way it is intended or as you are used to seeing it.

Similar issue being discussed here:  https://www.reddit.com/r/signalidentification/comments/1k5axrd/signal_from_1406714070_mhz/

Or look at my annotated picture in response to that posting:  https://a4.pbase.com/o12/50/78250/1/175389876.zV3H8DxH.FT8_spurs_20meters_22042025_1721z.jpg

Basically, you are seeing FT8, but you are seeing spurs, audio harmonics, an overdriven signal, poorly suppressed sideband, or something along those lines.  I see it quite often.

T!

[padre]

Token,

Wow! I've seen some splatter but nothing that bad before. Now that you've explained it, it makes sense. A friend of mine said something similar but I didn't understand him until seeing your diagram of the inverted signals. You learn something everyday!

Padre