Cecil Moore wrote:

wrote:
Can you expand on why you think a TDR causes me difficulties?
I can't think of any reasons.

There's more than just voltage in those returned pulses.

Yes, there is actually energy in them; easily computed at any point
on the line by using the voltage and current waveform at that point.

As for standing waves, I have no difficulties with forward
and reflected voltage waves. They work perfectly fine.

That takes care of the E-field. But do you think a wave can exist
without an H-field? If not, the wave possesses energy, by definition.
Energy flowing past a point is power.

Your voltage-only waves violate the conservation of energy
principle and the accepted laws of physics for EM waves.

I somewhat sympathize with your dilemma. You have latched on to
these forward and backwards waves for so long that you have started
to believe that they are real and are therefore ascribing to
them all the properties one would expect of a real EM wave such
as current and power.

Some of the authors you quote are not so convinced of their reality.
Consider this quote from sometime back....

"Johnson continues: "We can regard the first term in this
expression as the power associated with the forward-traveling wave,
and the second term as the reflected power (associated with the
rearward-traveling wave)""

As Peter points out in

http://groups.google.ca/groups?q=reg...ing.net&rnum=5

the use of the word "regard" is critical to this passage. Mr Johnson
seems well regarded and it seems likely that if he had intended the
passage to mean "is" he would have used "is" rather than "regard".

Just for a brief moment attempt to relax your hold on forward and
reverse waves and think of the real voltages and currents on the
line. These can be read with real voltmeters and real current meters.
These are the voltages and currents that have to satisfy Es and Hs
and energy flows and power. (And before Richard H pipes up about
directional voltmeters, I take this opportunity to remind him that
all a directional voltmeter actually detects is the real voltage
and current at the point of insertion in the line; all the rest
is computation based on the real voltage and current).

Forward and reflected voltage and current waves are convenient ways
of describing the real voltage and current distributions on the line,
but that does not make them real. And the fact that they correctly
predict the voltages and currents but sometimes fail to predict the
power (when Z0 is not real, for example, but there are many other
examples in simple circuit theory) is a strong indicator that they
are a convenience and not a reality.

Consider a mundane example in the physical world. You have a post
supporting two clotheslines each leaving the post at 90 degrees.
At 135 degrees from the clotheslines is a single guy wire to keep
the post from bending. While for the purposes of analysis you can
pretend that there are two guy wires and this will assist you in
discovering the forces involved, never forget that there is really
only one for otherwise you may be sorely surprised.

Much like this example, the superposition of voltages is a useful
analytical technique, but one must always be aware of its
limitations or one will be lead quite astray.

When one starts believing that the intermediate results represent
reality, trouble begins. It is for this reason that quality authors
use mushy words like "regard" when describing these intermediate
results and not solid words like "is".

....Keith