In article ,
Jim Kelley wrote:

Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?

But you're missing, or trying to
circumvent, the most interesting aspect
of the problem. It's the one which
highlights the very core of our
disagreement. The energy stored in the
line, remains stored in the line as long
as steady state is maintained without a
single Joule of additional energy moving
into or out of the line. To me, this
illustrates clearly how the fields at
the impedance interfaces of a matching
transformer can be maintained without
requiring multiple rereflections of
energy. I'm hoping some day you'll see
it too.

Jim,

How would your model view the case in which the source transmitted
into the T-line for 1.5 seconds (delivering 150 joules into the line)
and was then disconnected, leaving both ends of the T-line
open-circuited.

In this case, you'd continue to have 150 joules of total energy stored
in the line (modulo the amount of energy which does manage to radiate
out sideways). However, there would be periods (of 500 milliseconds,
one per two seconds) when the voltage near the source end of the
T-line, and the amount of current flowing through the T-line in this
area, were both zero. For the intervening 1.5 seconds out of each 2
seconds, there would be strong current flow through this portion of
the line (having a standing-wave characteristic for all but a very
short transition time on either end).

This state of affairs can, I don't doubt, be modeled purely as a
matter of interaction and interference between fields. The model
would appear to me to have to become extremely complex, in order to
produce the correct results at all points over the two-second
long-term periodicy of this system.

It can also be modeled as the effect of interference between forward
and reflected waves... and this is a somewhat simpler model to use to
describe systems such as this which do not exhibit a purely
steady-state behavior.

As far as I can see, *neither* of these models (fields, or reflected
waves) is fundamentally superior to the other. They are both equally
capable of producing an accurate description of the output of the
system at any point in time, given a set of inputs to the system.
Hence, by the usual standards of the validity of a scientific theory,
both models are equally valid.

Under certain circumstances, one model may be more "practically
useful" than the other. My impression is that your model of fields
may be more useful in looking at relatively local behavior (e.g.
within a wavelength or two) within a system that's at, or close to a
steady state. Cecil's preferred model of reflected power may be more
practical to use (i.e. simpler computations to produce a valid result)
when dealing with systems far from steady state.

In between those two extremes, it looks to me as if which model one
prefers is simply that - a personal preference.

--
Dave Platt AE6EO
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