Topics

multipath

John Klem
 

Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR

Bruce Draper
 

What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR

Keith Morehouse
 

There are, most certainly, two different phenomenon with regard to "multi-path" (I'll put that in quotation marks).

Bruce describes one, which is heard frequently.  Rapid fading due to out of phase paths mixing at the receive end.  There is another, which manifests itself as multiple coherent signals.  I see both, although the latter is much more obvious on CW or FT8.

-W9RM

Keith Morehouse
via MotoG


On Wed, Sep 18, 2019, 6:36 AM Bruce Draper <bruceaa5b@...> wrote:
What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR

Scott K5TA
 

Just got around to reading these messages —

A couple of questions come to mind:

Is there any noticeable difference in the appearance of the waterfall traces between the fundamental signal and the ghosts?  Signal strength?  Freq stability?

Was the only available spectrum display at audio?  It would be neat to look at an RF panadapter display alongside of the WSJT audio waterfall to compare.  A local RX issue could be in play, which would also provide a possible explanation for the non-reciprocal observations.

-TA


On Sep 18, 2019, at 8:34 AM, Keith Morehouse <w9rm@...> wrote:

There are, most certainly, two different phenomenon with regard to "multi-path" (I'll put that in quotation marks).

Bruce describes one, which is heard frequently.  Rapid fading due to out of phase paths mixing at the receive end.  There is another, which manifests itself as multiple coherent signals.  I see both, although the latter is much more obvious on CW or FT8.

-W9RM

Keith Morehouse
via MotoG

On Wed, Sep 18, 2019, 6:36 AM Bruce Draper <bruceaa5b@...> wrote:
What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR

Mike WB2FKO
 

John's explanation has cleared up the physical picture I have of what is happening.  Two or more coherent signals traveling over different static paths and interfering at the receiver can't produce new frequencies.  All they can do is enhance or reduce signal strength, but there won't be any new harmonic content.  The path length has to be changing/moving to induce a Doppler shift.

If I assume a steady 10mph wind, pushing an air mass that can somehow reflect/scatter RF, I estimate a Doppler shift of about 7 Hz at 432 MHz. Stronger winds and higher carrier frequencies induce bigger frequency shifts. I am unsure if atmospherics can do this, but such a shift is consistent with the fuzz I remember seeing on the FT8 waterfall.  If the wind is steady, the signals should still decode. If the air mass is not moving steady, then the Doppler shift will fluctuate and and decodes may not happen. FT8 is not tolerant of frequency instability beyond ± 1 Hz. 

This problem should have been reciprocal, however, and Jay seemed to be decoding us just fine. This makes me suspect there was a problem in our setup.  I'm quite sure the audio level coming into WSJT was set below the point of clipping, although it might have been getting close. The indicator on the GUI turns red to warn the op of this condition, but I don't recall seeing that.

Didn't save any traces, so just going by memory.  I think the JT65 ghost signals (yes, they were weaker) were spaced at periodic intervals in frequency, suggesting a nonlinearity like clipping somewhere in the demodulation or decoding.

Mike

On 9/17/19 11:02 PM, John Klem wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR