Richard Harrison wrote:

Keith wrote:
"Are there other signals which would produce this result?"

A short-circuit produces a voltage inversion. The plus volts and the
minus volts make a zero in a short.

Imagine a couple of identical positive pulses separated by some distance
and traveling in the same direction along a long transmission line which
has a short approached by the pulses. When the first pulse hits the
short it is inverted and reflected to travel back toward its source as a
minus voltage pulse.

When the first pulse encounters the second pulse they disappear at the
instant of coincidence, but otherwise, travel on their merry ways. What
happens at coincidence is transfer of the energy associated with volts
to the energy wave associated with amps as energy can`t be obliterated
while the volts disappear.

I do like pulses. Thinking about pulses on the line certainly helped
clarify my understanding of how lines worked.

So what happens when two pulses collide? I don't think you will like
this answer either, but here goes.

First, remember that power and energy are not the same. Power can
become zero while the energy remains. It simply means that the
energy is now stored in the capacitance or inductance (or equally,
the E field or H field) but is not moving. So no energy is lost
or destroyed when the power goes to zero.

So what happens to these two pulses? They bounce off of each other
and return whence they came. This must happen since the voltage on
the region of the line where the two pulses collide remains zero
and since p(t) = v(t) * i(t) there is no power in this region
so there must be no energy flowing. You would observe exactly
the same result were you to short the line at the point of
collision just before the collision occurred. Two pulses colliding
is just like a reflection.

As a simple analogue, consider two identical elastic balls rolling
towards each other. They bounce back after the collision.

On a line this is easier to visualize with two negative pulses
colliding. Each pulse consists of a clump of charge. When these
clumps of charge collide, remembering that like charge repels,
they bounce back.

What actually happens during the collision? With balls, the
energy is stored in the deformation of the ball and then
released as the balls separate. With pulses, the power stops
(current is zero), and the energy is stored in the capacitance
of the line in the region of the collision. After all the
energy has stopped flowing (no power, zero current, all energy
in the E field), the energy is released into the reflected
pulses and once again, there is energy moving (power) on the
line.

....Keith