Walter Maxwell wrote:
On Tue, 02 Aug 2005 16:04:29 -0500, Cecil Moore
wrote:


Jim Kelley wrote:


Cecil Moore wrote:

You can add Walter Johnson, Simon Ramo, and John Whinnery to
that list. "Power in the reflected wave" is commonly mentioned.

Proving what, exactly? That power is "in" a reflected wave?

Proving that "Power in the reflected wave" is common
usage in RF engineering. You are not going to get the
human race to stop using the word, "sunrise", no
matter what you say or do.


Jim, I'm sure you'll agree that voltage is reflected, and that current
is also reflected. Then isn't the product of voltage and current
power?

Walt, W2DU

I hope I haven't given the impression that I would be unfamiliar with
that relationship. I used it in my work practically every day of my
life for many years.

Here's my best answer to such a question. According to the physics that
I've studied, physicists seem to be laboring under the impression that
it is the electric and magnetic fields which actually propagate and
interact with matter. That interaction can result in, among other
things, reflection. The E&H fields manifest themselves within matter as
voltages and currents, respectively. Although it is the fields which
reflect, it is the resulting voltages and currents that we can most
readily measure. From a practical everyday standpoint this distinction
makes little or no difference. It is only when we start to describe
physical processes such as energy flow that it may become necessary to
consider the distinction.

And yes, if you take a calculator or slide rule and multiply voltage and
current, and make certain assumptions, you can find the rate at which
energy is probably being transferred from one place to some other place.
But some folks have argued descriptions of physical phenomena derived
from an assumption that the number displayed on the calculator
physically propagates through a transmission line, physically reflects
off the ends of the transmission line or other discontinuities, and
physically interferes with itself or other numbers like it. So that
there is no further mistaking my position, I dispute the validity of
these descriptions of the behavior of physical phenomena, and some of
the predictions based upon them. I do not dispute P=V*I, but thanks for
asking.

The wave which propagates along a transmission line can produce energy
at a rate proportional to E x H, but it is not E x H which propagates
along the transmission line. E propagates in one plane, and H
propagates in a plane a right angles to E. When we know the
characteristics of E and H (or V and I) individually, we can predict
what will occur when either encounters a discontinuity. But when we
multiply to two together, the resulting magnitude no longer possesses
the properties of either of the multiplicands, and so no accurate
predictions about interactions with discontinuities can truly be derived
without making assumptions. And as we've seen, given a nebulous laundry
list of assumptions, some inaccurate predictions can be derived.

Your 'Transformer' diagrams come as close as any I've seen to describing
how things behave in an coaxial impedance matching arrangement. I hope
you include them in your 3rd Edition. I have, by the way, found similar
but less detailed drawings of thin film interfaces in two different
physics textbooks. I think the key to understanding them is the
superposition of multiple reflections.

73, Jim AC6XG