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[TowerTalk] An Interesting Experiment

To: <towertalk@contesting.com>
Subject: [TowerTalk] An Interesting Experiment
From: hwardsil@WOLFENET.com (Ward Silver)
Date: Wed, 22 Jul 1998 11:49:40 -0700 (PDT)

> 
> Off into the world of make believe:
> 
> Consider a LOSSLESS transmission line connected to a load with an
> impedance different than that of the line (i.e, the VSWR is NOT 1:1).
> Then, for the sake of argument, let us say that we can insert a
> single pulse at the "generator" end of the line, and, moreover we
> can disconnect the generator before the pulse reaches the load.
> Where does ALL of the energy go?
> 
> Doug W6JD

It's a darn good thing that Lewis Carroll never learned about transmission
lines...

Actually, this is an easy one - ALL of the power is eventually dissipated
by the load, but in an infinite amount of time.  Envision the little pulse
banging back and forth, losing some fraction of its remaining energy each
time it meets the load.  

And...

Because the fraction lost is not 1.0, the pulse energy will never, ever go
to zero.

This is actually an easy process to observe.  Look at a fast, unterminated
signal path on a PC board with a good scope.  You will observe a
stair-step rising (and falling) edge for each signal transition as the
energy slams back and forth between output pin and input pin.  In fact,
the stairstep can be so extreme in cases of bad mismatches that the gate
can be turned on and off several times before steady-state is reached.
Needless to say, that could be bad...

Digital designers at one time thought that they had left the analog world
behind only to find it in front of them :-)

73, Ward N0AX



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