Jim Kelley wrote:

Cecil Moore wrote:
And given the following example from "Fields & Waves ..." by
Ramo & Whinnery, what do you think changes the direction of
the Pz2- Poynting Vector such that it vectorally adds to the
Pz1+ Poynting Vector resulting in the Pz2+ Poynting Vector?

100W XMTR---50 ohm line---+---291.4 ohm line---1698.5 ohm load
Pz1+ = 100W-- Pz2+ = 200W--
--Pz1- = 0W --Pz2- = 100W

If you can answer that simple question to the satisfaction of
everyone, the discussion will be over.

Any reflections (and Poynting vector reversals) would be caused by the
presence of impedance discontinuities - not by anything else.

The impedance discontinuity is certainly in the cause and effect *chain*
as is the Poynting vector reversal. But simply asserting that the
impedance discontinuity causes the Poynting vector reversal is
a sophmoric begging of the question and leaves out some important
details. A causes B. B causes C. C causes D. You are certainly logically
correct in saying that A causes D, but your statement leaves out some
important intermediate details, namely B and C. (see below)

For instance, the impedance discontinuity causes nothing unless the
source energy exists and is taken into account as a cause. The
question is what are the necessary and sufficient conditions for an
impedance discontinuity to result in a complete reversal of the
(Pz2-) Poynting vector? Since a source is necessary, the source can
be considered as a cause. (The creator of our species can even be
considered as a necessary cause as can the Big Bang.)

Looking at it from a scientific standpoint, the impedance discontinuity
can only directly reverse half the magnitude of the Poynting Vector because
the physical power reflection coefficient is 0.5. So how does the other half
of the magnitude of the Poynting Vector get reversed? It's all been covered
by the Melles-Griot web page and the Molecular Expressions FSU web page.

In the presence of EM source energy, the impedance discontinuity causes
reflections. One of those reflections reverses half the magnitude of the
(Pz2-) Pointing Vector. That magnitude is (Pz2-)(rho^2).

Two of those reflections engage in wave cancellation. One is (Pz1+)(rho^2).
The other is (Pz2-)(1-rho^2), the other half of the (Pz2-) Poynting Vector.
These two reflections have equal magnitudes and opposite phases and therefore
cancel. Their energy components reverse direction and head back toward the load
as explained on the Melles-Groit and Molecular Expressions web pages.

And that's how the (Pz2-) Poynting Vector gets reversed. It's a two-step
process, each step involving half of Pz2- in the above example.

In the
absence of impedance discontinuities, the transmission line would appear
to be infinitely long and would not produce reflections.

That's true. The impedance discontinuity causes the reflections that engage
in wave cancellation. In steps, it goes like this.

A. Impedance discontinuity driven by a source of EM energy

B. Reflections (implied mismatch)

C. Wave Cancellation (permanent destructive interference)

D. Energy direction and momentum reversal (constructive interference)

A causes B. B causes C. C causes D. This is in any freshman logic book.
You are certainly logically correct in saying that A causes D, but
your statement leaves out some important intermediate details. Those
intermediate details are what this discussion is all about.
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---