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

John Ackermann N8UR jra at febo.com
Sun Nov 19 14:16:44 EST 2006


Really interesting analysis, James.  I'll need to do some checking to
determine my distance from W1AW, and where that puts me in the skip zone.

John
----

James Maynard said the following on 11/19/2006 02:08 PM:
> 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.
> 




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