Re: [AOCC] ARRL Sep VHF K7HP Single Op LP


Mike WB2FKO
 

Yes, offsets in frequency in the waterfall.  To confuse the FT8 decoder would require two signals offset in time by more than 40ms.  If my math is right, this corresponds to a path length difference of 12,000 km.  That's why I think it's multipath coherent interference, which only requires a path length difference on the order of a wavelength.  I suspect greater phase difference (multiple wavelengths) would make it worse and also explains why it would be more problematic as the radio frequency goes up.

If I understand you correctly, multiple static paths would produce multiple ghost traces that should still all decode.  It's the dynamic path difference that causes sufficient signal distortion to prevent decoding.

I should mention that in the ~30 minutes of trying various things with W9RM on 432, I did actually get one FT8 decode.  But we had already agreed to switch modes at that point and start over.

Mike WB2FKO

On 9/17/19 6:14 PM, John Klem wrote:
If I correctly understand, you saw multiple traces offset in frequency, not time?  I think many people have now seen this on 6 m (I'm among them), and it's not hard to believe the effect could be worse on 432.

In order to produce this frequency offset, I believe you must have some sort of dynamics in your path to induce Doppler or something similar.  Multiple static paths shouldn't do it.  I trust your receiver is linear enough that you shouldn't have observable mixing products (unless you have multiple strong signals already on different frequencies and an FT-817 like mine).  As Keith suggested, aircraft would do it, but the shift would vary with time (which I presume you didn't see).  I suppose some sort of atmospheric dynamics could produce this effect as well, and someone invoked that explanation in something I read recently.

I did a quick search on nonreciprocal propagation, and it appears this is generally recognized for ionospheric paths, but not obviously so for tropospheric paths.

Really interesting stuff.  It would be fun to do comparisons on different bands when you observe this, to see if the frequency offset scales with carrier frequency.

John AA5PR


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