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


Mike WB2FKO
 

Duffey and I discussed this on the long ride home today.  Thinking about it some more, the idea of multipath is that two or more (coherent) signals from the same source travel by slightly different paths to the receiver.  This path difference only has to be a fraction of a wavelength. This causes interference and mixing in the detection circuit, producing ghosts of the digital signal in the audio waterfall.  This is exactly what I saw when we were running FT8 and especially JT65 (the synch signal is very obvious in the traces).  Seems this would be become a greater issue as the frequency goes up into UHF. All that said, FT8 should have been able to successfully decode the baseline signal and any harmonics.  Or maybe the frequency separation of the ghost signals was not sufficient and confused the decoder.

I am convinced the equipment was not at fault, as we had been working very successfully with various digital modes throughout the weekend.

The question of reciprocity is a good one.  W9RM's signal was very up and down, but his digital signals refused to decode independent of the widely varying receive level.  My hunch is that the reflection(s) occurred closer -- much closer -- to our QTH than his.  Beam divergence and diffraction would substantially weaken the reflected/scattered signal by the point they reached him, allowing decodes there.  Also the lower gain, wider aperture of the rover antenna may make us more susceptible.  But we were at an ideal location with almost nothing on the near horizon that suggested a reflection source. We experimented over the course of about 30 minutes with different digital modes and especially beam headings to no avail.  I would have wagered that moving the beam around would have fixed it.  And lost.

I have never had this problem (that I can remember anyway) on 6 or 2.  My experience with FT8 on UHF has been entirely this past weekend, or about 3-4 QSOs total.

Mike WB2FKO


On 9/16/19 9:40 PM, John Klem wrote:
I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR

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