On 5/18/22 7:51 AM, Kim Elmore wrote:
I don't mistrust contesters that tell me they're certain of this,
but... I'm wondering how they *know* that the difference is 1-2 dB on
the *receiving* end? If they can increase the smoke by 1-2 dB and all
of a sudden make the QSO, how do they *know* that's what did it? HF
propagation is funny stuff. I do a LOT of statistics in my job as a
research meteorologist/scientist, even though I'm not a formally
trained statistician, and this is always my first question when
presented with statements like this. Show me the data and how it was
analyzed.
I'm pretty sure that there is no such data set in existence and I
don't know of a good way to collect one. However, I suspect that the
innate variability of ionospherically propagated HF signal strength is
far larger than 1-2 dB and that any p-values we'd find at the 1-2 dB
thresholds would be pretty large and so deemed statistically
insignificant. I suspect there are too many degrees of freedom to ever
pin this down.
I have no intention of starting a fight or creating discord. I
*deeply* respect Jim's judgement and experience. Even so, I'd love to
craft an experiment that would allow us to statistically determine the
dB threshold that truly makes a difference on each band. While I'm
spitballing, I might as well include different geomagnetic conditions
as well.
That said, there's also a good argument for not wasting a dB if you
can affordably avoid it.
The variability of an ionospheric path is way more than 1 dB. But as you
say, this is really complex to tease out, because on the Rx end, it's
your signal competing with both near and distant noise, and the other
signals.
If I'm transmitting in the middle of a thunderstorm - the Rx is seeing
both my signal, and my local noise over essentially the same
(scintillating) path.
If I'm trying to break a pileup, then I'm really competing with the
"other guys" who also have varying paths. So what you have is multiple
signals, each with varying power - the distribution isn't gaussian, but
if you think about it, what you might have is a competition between two
gaussian variables with slightly different means, but similar variances.
And getting through depends on the instant where yours happens to be
bigger than the other.
This kind of problem has been studied a lot in two contexts, where some
actual math and models might be available: one is detection of targets
with radar in clutter; the other is jamming and multi signal channels.
Finally, a LOT also depends on the "skill" of the receiver. Some
receivers (the person or machine) are better at isolating a single
signal from the interference.
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