On 22 Feb 2020 18:29, Bill Lederer <w8lvn.9@...> wrote:

Folks:

Here is something I have seen a couple of times.  A station is decoded on two frequencies at the same time, at vastly different db levels.  Here are two  examples from one station
today:

200222_181830     7.074 Rx FT8     -8  0.1 1356 CQ K5DDE EN90
200222_181830     7.074 Tx FT8      0  0.0  650 CQ W8LVN EN62                       
200222_181830     7.074 Rx FT8     14  0.1 1476 CQ K5DDE EN90


200222_181900     7.074 Rx FT8     -8  0.1 1355 W8LVN K5DDE EN90                      a2
200222_181900     7.074 Rx FT8     17  0.1 1476 W8LVN K5DDE EN90

On the order of 25 db difference, 120 hz difference.

Curious what this might be caused by.

Thanks,
w8lvn

I've noticed this loads of times with both FT8 and PSK31. I assume it's an IMD construct where non-linearity or over-driving of the transmission is generating spurious decodable signals. Or maybe harmonics of the modulating audio?

73 Phil GM3ZZA

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73 Phil GM3ZZA

Nope, it's AC mains-related.  If there's any significant 50Hz or 60Hz ripple coming out of the PSU (especially an SMPSU) on the DC line that's powering the rig, or if there's anything else in the shack that might be injecting a ripple signal somewhere in the audio signal chain, then this will be superimposed on the transmitted audio and will appear as a decodable 'sideband', in the case of the K5DDE decodes the spacing of the spurious signal is 2 x the AC line frequency.

It's not uncommon to see more than two spurii, I think I've seen up to four, all with spacings of exact multiples of the local AC mains frequency, on the signals emanated by some Eastern EU stations...!  I can even see a signal 100Hz away from a nearby amateur's very strong FT8 signal; even though his unintentional sideband is approaching 40dB down on his fundamental signal I would prefer it if it weren't there at all.

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Martin G0HDB

On Sat, Feb 22, 2020 at 07:30 PM, Philip Rose wrote:

Doesn't explain when the difference isn't 120 or 100 Hz.

It's unusual but not impossible to see Doppler shifting aka aircraft scatter of signals on especially the higher HF bands; in such cases the spacing of the secondary signal will almost certainly not be a multiple of the AC line frequency.  The effect is much more prominent, and more easily and more often observed, on 6m and higher.

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Martin G0HDB

On 2/22/2020 11:08 AM, Philip Rose via Groups.Io wrote:
> Here is something I have seen a couple of times.  A station is decoded
> on two frequencies at the same time, at vastly different db levels. Here
> are two  examples from one station

This can be caused by overload in the receive audio path between radio
and computer sound card, and by overdrive in the station being
double-decoded. If the difference is 120 Hz, it might be caused by power
supply currents injected into the audio path, usually via the signal
return (cable shield) when station equipment is not properly bonded.
Double decodes like this can also be caused by Doppler, common for
reflections from aircraft on higher bands (we see a lot on 6M).

73, Jim K9YC

[Edited Message Follows]
[Reason: Added link to the article by GM4FVM]

Nope, it's AC mains-related.  If there's any significant 50Hz or 60Hz ripple coming out of the PSU (especially an SMPSU) on the DC line that's powering the rig, or if there's anything else in the shack that might be injecting a ripple signal somewhere in the audio signal chain, then this will be superimposed on the transmitted audio and will appear as a decodable 'sideband', in the case of the K5DDE decodes the spacing of the spurious signal is 2 x the AC line frequency.

It's not uncommon to see more than two spurii, I think I've seen up to four, all with spacings of exact multiples of the local AC mains frequency, on the signals emanated by some Eastern EU stations...!  I can even see a signal 100Hz away from a nearby amateur's very strong FT8 signal; even though his unintentional sideband is approaching 40dB down on his fundamental signal I would prefer it if it weren't there at all.

I've just found a very informative article on the subject; you''ll find it at:

http://gm4fvm.blogspot.com/2019/11/ft8-and-100hz-ripple-strange-one-for-me.html

--
Martin G0HDB