On Fri, 25 Jan 2008 04:13:56 GMT, Gene Fuller
wrote:

The irradiance equations
work fine for detailing the external effects, but they don't give any
hint of what happens inside the interface. Think Thevenin.

Hi Gene,

Cecil isn't going to rise far enough to catch a breath of air on this
one. You may as well drop the shoe for lurkers (and me).

I don't see the connection to Thevenin (specifically); but, for me,
inside the interface we can draw the correlation of TIR failure (the
chapter that Cecil hasn't drug across the Xerox yet) to evanescent
waves to near fields to how antennas work.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
inside the interface we can draw the correlation of TIR failure (the
chapter that Cecil hasn't drug across the Xerox yet) to evanescent
waves ...

In the Ramo & Whinnery discussion of the distributed
network wave reflection model, I don't recall them
mentioning evanescent waves. Perhaps you could point
me to the correct page.
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote:
On Fri, 25 Jan 2008 04:13:56 GMT, Gene Fuller
wrote:

The irradiance equations
work fine for detailing the external effects, but they don't give any
hint of what happens inside the interface. Think Thevenin.

Hi Gene,

Cecil isn't going to rise far enough to catch a breath of air on this
one. You may as well drop the shoe for lurkers (and me).

I don't see the connection to Thevenin (specifically); but, for me,
inside the interface we can draw the correlation of TIR failure (the
chapter that Cecil hasn't drug across the Xerox yet) to evanescent
waves to near fields to how antennas work.

73's
Richard Clark, KB7QHC

Richard,

Perhaps I have misread the message traffic for the past 5 years or so,
but it appears that most of the heat over wave reflections is about what
happens during the reflections, including detailed concern about energy
and momentum. The issue is never (or at least rarely) about what one
would find in external measurements on the transmission line (or free
space, as the case may be). This goes back at least to the battles
between Steve Best and Walt Maxwell. (I pointed out that both models
were correct, although they arrived at the proper conclusions in quite
different manners.)

More recently, we have great battles over interferometers and cute Java
demonstrations. Absolutely nobody would question the overall wave
superposition principles shown in the FSU Magnet Lab Java applet. Most
people know how to add sine waves, or at least they can look up the
technique. The entire debate usually comes down to arguments about how
that addition actually takes place physically. This is what I am calling
the Thevenin equivalent. In particular, the external observations are
unambiguous and non-controversial. The standard models and equations
give all the correct answers for the observables. At the same time those
models and equations say nothing about how the two FSU input waves
suddenly jump together to superpose and interfere. Do the waves
"cancel"? Is there a requirement for auxiliary waves that are created
and immediately destroyed? How does one account for missing momentum?

That attempted under-the-hood analysis with tools suitable only for
external description drives my suggestion of the parallel to the
Thevenin model.


73,
Gene
W4SZ

"Gene Fuller" wrote in message
...

That attempted under-the-hood analysis with tools suitable only for
external description drives my suggestion of the parallel to the Thevenin
model.

the Thevenin (or Norton) equivalent circuits i believe are rarely used in
analysing transmission lines and antennas, they are more commonly used for
breaking down lumped circuits and networks. In fact Jackson doesn't even
have them in the index of Classical Electrodynamics 2nd ed... It is
mentioned in Ramo,Whinnery, and VanDuzer Fields and Waves in Communications
Electronics in the index, but in scanning the 2 chapters it is listed for i
don't see an actual reference to Thevenin... one of those chapters deals
with microwave networks and components and does break them down into lumped
equivalents so that is probably where the reference belongs, the other
chapter wouldn't seem to be related so may be a typo.

The problem with the use of the Thevenin or Norton equivalents is that you
have to exactly respect the limitations in order to use them properly...
that is the part of the circuit being replaced with the 'black box'
equivelent must be linear and time invariant, and the analysis is only valid
for sinusoidal steady state. This last one is what gets everyone, it
eliminates all the transients and makes it impossible to use to figure out
what happens when that first reflection physically happens... you have to
ignore all that stuff and only consider the steady state solution.

We have seen that some posters on here don't want to accept those
limitations as they try to figure out 'what is in' the black box
equivalent... this is of course a non-sequitar as the whole purpose of
replacing a part of a circuit with the Thevenin equivalent is to simplify
the problem so you don't have to know what is inside and can focus on the
rest of the problem. So any attempts to measure the length of the line in
the black box, or figure out if it is a lumped circuit break the rules for
using it in the first place.

Dave wrote:
So any attempts to measure the length of the line in
the black box, or figure out if it is a lumped circuit break the rules for
using it in the first place.

Ramo and Whinnery warn against even trying to calculate
the power dissipation inside the black box - something
that is regularly attempted on this newsgroup. The
calculated power dissipation inside a Thevenin equivalent
and a Norton equivalent can result in an infinite difference
even though they are both "equivalent".
--
73, Cecil http://www.w5dxp.com

Dave wrote:
. . .
The problem with the use of the Thevenin or Norton equivalents is that you
have to exactly respect the limitations in order to use them properly...
that is the part of the circuit being replaced with the 'black box'
equivelent must be linear and time invariant, and the analysis is only valid
for sinusoidal steady state. This last one is what gets everyone, it
eliminates all the transients and makes it impossible to use to figure out
what happens when that first reflection physically happens... you have to
ignore all that stuff and only consider the steady state solution.
. . .

That's not true. A Thevenin or Norton equivalent generator can produce a
voltage or current, respectively, which is any function of time. This of
course includes single pulses and pulsed sinusoids, as well as an
infinite number of others.

Roy Lewallen, W7EL

One more note about Thevenin and Norton equivalent circuits:

In spite of the frequency with which this topic appears on this
newsgroup, I don't recall seeing anyone actually use a Thevenin or
Norton equivalent circuit in illustrating a point on this newsgroup. I
don't believe I ever have.

Roy Lewallen, W7EL

Roy Lewallen wrote:
I don't recall seeing anyone actually use a Thevenin or
Norton equivalent circuit in illustrating a point on this newsgroup. I
don't believe I ever have.

Some of your example sources walk and quack like
a Thevenin equivalent circuit. :-)
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Roy Lewallen wrote:
I don't recall seeing anyone actually use a Thevenin or Norton
equivalent circuit in illustrating a point on this newsgroup. I don't
believe I ever have.

Some of your example sources walk and quack like
a Thevenin equivalent circuit. :-)

While some of yours just walk and quack.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
Cecil Moore wrote:
Roy Lewallen wrote:
I don't recall seeing anyone actually use a Thevenin or Norton
equivalent circuit in illustrating a point on this newsgroup. I don't
believe I ever have.

Some of your example sources walk and quack like
a Thevenin equivalent circuit. :-)

While some of yours just walk and quack.

I need to address this, because Cecil has frequently declared all
combinations of an ideal voltage source and resistor as a "Thevenin
equivalent" as I see he's implied here once again. He has claimed this
relieves him of the nagging problem of accounting for such things as
power dissipation in a source resistance. I'll explain how this
characterization and claim are false.

I often use an ideal voltage source in series with a resistance for
illustration of transmission line phenomena. This very simple circuit
allows complete analysis without the unnecessary clutter of more
complicated circuits. You'll find this technique very widely used in
elementary electrical circuits texts for the same reason, and well
before the introduction of Thevenin or Norton equivalent circuits.

But these aren't Thevenin equivalent circuits. Let's review what a
Thevenin equivalent is. I'll quote here from Pearson and Maler,
_Introductory Circuit Analysis_, but you can find an equivalent
definition in any elementary circuit analysis text.

"A theorem named after Leon Thevenin is often useful in reducing a
complex circuit to a simpler one. This theorem, which is proved in
Appendix E, may be stated as follows.

"Any one terminal pair (one port) network which is linear and which may
have any number of independent and dependent transform sources (as long
as the dependent sources are not functions of quantities outside the
network) may be replaced by a transform voltage source in series with a
transform impedance. The transform voltage source is the voltage across
the terminal pair when these are open circuited and the transform
impedance is the ratio of this transform voltage to the transform
current which flows between these terminals when short circuited."

Pay particular attention to the first sentence of the quote. A Thevenin
equivalent circuit is a reduction of a circuit to a simpler one. If you
have a complex circuit containing multiple components and reduce it to a
Thevenin equivalent, the theory says that the equivalent circuit looks
just the same to the outside world as the original. Because the Thevenin
equivalent could represent any number of very different original
circuits, you can't determine anything at all about the internal
workings of the original, such as power dissipation, by looking at the
Thevenin equivalent. That's a completely valid statement which has
frequently been misapplied.

A circuit consisting of a perfect voltage source and an impedance isn't
a Thevenin equivalent circuit unless it's used to replace a more complex
circuit. If it's used simply to represent those two circuit elements and
none others, then all the conclusions we draw from the circuit,
including dissipation in the source and impedance, are and must be
valid. For that matter, the circuit analysis for a Thevenin equivalent
must obey all rules and laws, including source and impedance voltage,
current, and power. We only have to realize that any quantities within
an equivalent circuit aren't necessarily the same as those of the
circuit being replaced by the equivalent.

Declaring all circuits consisting of an ideal voltage or current source
and impedance to be a Thevenin or Norton equivalent is wrong. Declaring
that Thevenin or Norton equivalent circuits don't have to obey
fundamental rules of circuit analysis is also wrong. Continuing to do so
after more than ample evidence has been presented to the contrary is
dishonest.

Roy Lewallen, W7EL