[Elecraft] Re: Band condx Questions from a Liberal Arts Major

[John Magliacane]

--- On Thu, 11/13/08, Jim Brown <jim at audiosystemsgroup.com> wrote:

> Yes, but not necessarily what you think. Most of the "in and out"
> fading we hear on HF is the result of multipath. That is, the signal
> travels over two paths that differ a bit in time, which puts them
> out of phase. When they out of phase by some odd multiple
> of 180 degrees, they cancel, and by an even multiple of 180
> degrees, they add.

Absolutely correct!

Being a on-air frequency measurement "nut" (originally started with my K2),
I recently started looking into the issue of fading, and the effects it has
on making accurate frequency measurements.

Here's a short video clip illustrating WWV's carrier phase plotted against a local
frequency standard as a Lissajous pattern as received over a 1622 mile path.

Everything you described can be seen through careful observation of this clip:

http://www.youtube.com/watch?v=nf9KWfS9QBs
 
> The DEPTH of fading (that is, the weak part of the fade) depends
> on perfect cancellation of the signal, which happens when the
> two arrivals are almost precisely equal in strength.

Sometimes the second path "wins out" after the fade, and the resultant signal
emerges 180 degrees out of phase with what it was before the fade.

Aside from multipath, much of the slow and shallow fading seen on low HF frequencies
during the day can be the result of variations in D-layer absorption.


73, de John, KD2BD

--
Visit John on the Web at:

	http://kd2bd.ham.org/
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