Re: multipath


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.


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

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