[time-nuts] FMT -- 40M strangeness?

[James Maynard]

John Ackermann N8UR wrote:
> Without giving away any actual numbers, did anyone else notice either a
> fuzzy signal, or some interference within about 1 Hz of W1AW on 40M?
> 
> I recorded the entire test run and have been unable to prove to myself
> exactly where W1AW is; almost any sample of data I select shows two
> signals within about 1 Hz -- depending on just what segment of the data
> I analyze, I can sometimes get one peak that is sharper and another that
> is smeared out over about 0.5Hz, but I'm not confident about which one
> is the real thing.  An FFT with enough bins to separate the two signals
> loses the CW keying, so I can't use that to see which one is real.
> 
> Again, nobody post actual frequencies, but if you've looked at the 40M
> signal very closely, I'd appreciate finding out whether this is local to
> me, or something others saw.
> 
> Thanks,
> 
> John
> 
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> .
> 
For me, the W1AW signal was quite faint. It was visible on Specturm 
Lab's waterfall display as a rather broad, fuzzy trace, but I could not 
copy it by ear. (I did, once, hear the call sign, W1AW.) So my 
measurement of its frequency was by averaging the Sprectrum Lab text 
output (File | Text file export... | Export of calculated data) after I 
had imported into a Microsoft Excel file.  I computed a mean of the data 
in one of the spreadsheet columns to get the frequency that I used in my 
FMT submission.  I also computed the standard deviation of that column, 
and saw that it was spread over several hertz.

You were closer to W1AW, and had a stronger signal to work with than I. 
But I surmise that were seeing the same phenonemon: ionospheric doppler 
-- and especially the effect of multipath on the doppler-shifted signal.

Suppose that, at the time of the FMT, the ionosphere was rising. (It 
usually does at and after sunset.)  I assume that you were beyond the 
ground-wave coverage zone of W1AW, but were getting it on sky-wave. 
Let's denote the frequencies of the W1AW signal as transmitted (or 
received on ground wave), and after one-hop, two-hop, etc. skywave 
reflections as follows:

f0 = transmitted frequency = frequency as received on groundwave
f1 = frequency as received after one reflection from the ionoshphere
f2 = frequency after two reflections
f3 = frequency after three reflections
etc,

If the ionosphere is moving, f1 will differ from f0 by some amount that 
depends at the rate at which the ionosphere is moving. For two-hop 
reception, f2 will differ from f1 by a similar amount -- but not exactly 
the same, because of differences in the angles of incidence to the 
reflecting surface.

I surmise that the strongest signal you received was proably W1AW as 
received at frequency f1 (after one hop) and the second, fainter trace 
was W1AW as received at frequency f2 (after two hops). You are probably 
beyond the zone of ground-wave reception, so you did not receive W1AW at 
its actual transmitted frequency, f0.

Which leads to an interesting possibility.  If you assume that the 
difference, f1-f0, is almost the same as the difference, f2-f1, you may 
be able to use this information to infer the true transmitted frequency, f0.

I, on the other hand, had such faint and blurred reception that I was 
unable to discrimate between f1, f2, f3, etc., and so could not try to 
compute f0.

-- 
James Maynard, K7KK
Salem, Oregon, USA