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Topband: Bi-directional prop

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Subject: Topband: Bi-directional prop
From: (Tom Rauch)
Date: Fri, 27 Sep 2002 07:38:58 -0400
> Not agreeing or disagreeing with whether bi-directional prop exists, I
> need to state what that this quite simply is not proof!
> Proof has a *very* strong meaning to me (like the directionary meaning
> perhaps). For example, I almost always TX on a somewhat
> omnidirectional antenna, and listen on beverages. At sunrise, I'm
> usually deaf to the Carribean.

If the effective radiated power, background noise, and operator's 
ability to hear signals are different at each end of the path... 
there is no possible way we can expect equal copy at both ends.

Background noise or QRM, which is the primary limitation in most 
cases, can vary significantly from hour to hour. That is why the only 
possible way to tell if propagation is non-reciprocal (non-linear 
would imply all of our transmitters would generate IM products and 
harmonics in the ionosphere, and that a 3dB transmitter power 
increase would not result in a similar change at the receiver) is by 
having calibrated level meters at both ends of the path, similar 
antennas, and known effective radiated power levels.        

Signal-to-noise ratio, or hearing a signal vs not hearing a signal, 
does not allow us to evaluate propagation because it is a mixture of 
several effects that change quite a large amount. Without sorting 
those effects out, we know nothing.73, Tom W8JI 

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