[GreenKeys] HF radio relating to DSP TU Update

[Jim Haynes]

HF radio is all we've got, but it just isn't very nice.  There is flat
fading, meaning the whole bandwidth of the incoming signal fades together.
Then there is multipath or frequency-selective fading, where multiple
reflections from the ionosphere create creative and destructive
interference based on slightly different path lengths.  So a particular
frequency get nulled out, and that frequence keeps changing as the
reflective paths keep moving around up there.  And there are multipath
echoes which are the same thing except the path length differences are
in the range of milliseconds, so it's like receiving several copies of
the signal displaced in time.  The Dovetron demodulators were called
"multipath corrector" but apparently it didn't work very well as the
circuits were redesigned several times.  I think the most successful
attack on the multipath problem was a system developed at MIT called
RAKE, which put the signal through a delay line and then tried to
take signals from several taps on the line and combine them 
constructively.  I'm told the RAKE technology is now used in things
like cell phones because the signals get multipath reflections from
buildings and things in the urban environment.

And then in the amateur radio realm we have to contend with interfering
signals, especially in things like contests and DX chasing.  And the
ever-increasing urban noise floor.

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 		Meredith Willson, The Music Man