Standing-Wave Current vs Traveling-Wave Current
 

On Dec 31, 11:11*am, Roger wrote:
Cecil Moore wrote:
Richard Clark wrote:
To cut to the chase, Norton and Thevenin sources are appropriate to
network analysis irrespective of your perception.

All my references indicate that those sources are only
appropriate for *steady-state* use. Roger is searching
for a transient state source.

Right!

73, Roger, W7WKB

Odd. Cecil has not named his "references" which is
quite unusual for he truly likes to name-drop: Ramo,
Whinnery, Hecht, IEEE, ...

You would be well served to google "reflection diagram"
or "bounce diagram" where you will find fine examples
of computing re-reflection using the source impedance
of generators modelled using the Thevenin equivalent
circuit. It is not particularly complicated, though
it can be tedious.

...Keith

Standing-Wave Current vs Traveling-Wave Current
 

Keith Dysart wrote:
On Dec 31, 11:11 am, Roger wrote:
Cecil Moore wrote:
Richard Clark wrote:
To cut to the chase, Norton and Thevenin sources are appropriate to
network analysis irrespective of your perception.
All my references indicate that those sources are only
appropriate for *steady-state* use. Roger is searching
for a transient state source.
Right!

73, Roger, W7WKB

Odd. Cecil has not named his "references" which is
quite unusual for he truly likes to name-drop: Ramo,
Whinnery, Hecht, IEEE, ...

You would be well served to google "reflection diagram"
or "bounce diagram" where you will find fine examples
of computing re-reflection using the source impedance
of generators modelled using the Thevenin equivalent
circuit. It is not particularly complicated, though
it can be tedious.

...Keith

One of Cecil's common techniques is to declare any combination of
perfect voltage source and resistance to be a "Thevenin equivalent",
which he then claims relieves him of any obligation to consider the
power supplied by the source or dissipated in the resistor.

Of course, that combination of components has no special properties or
restrictions, and must conform to the same rules as any linear
components. It is, in fact, an excellent choice for many examples and
illustrations because of its bare simplicity. Only when a substitution
is made for some other combination of linear components does the perfect
voltage source and resistor become an "equivalent", and in that case you
can correctly state that the power supplied by the perfect source and
dissipated by the resistor aren't necessarily the same as for the
circuit being replaced. But any analysis which isn't valid when driven
by a perfect voltage source in series with a resistance (or current
source in parallel with a resistance) is fundamentally flawed. Waving
your hands and declaring it a "Thevenin equivalent" and therefore not
subject to the rules all linear circuits must abide by is simply a way
of saying your theory can't handle simple cases.

Moving on, my electrical circuits texts abound with examples in which
some initial condition is assumed, then a source is "turned on" or
connected with an imaginary switch at t = 0, and the transition from the
initial state to steady or final state is studied -- exactly as I did in
my analysis. The source is most often a perfect voltage or current
source and, in the sections dealing with sine wave AC circuits, produces
a sine wave. The assumption of an initial condition (usually, but not
always, that all voltages and currents in the circuit are zero) is
absolutely required when solving the fundamental integro-differential
equations which result from circuits containing inductances and
capacitances.

I'll be glad to give page references from Pearson & Maler and Van
Valkenburg, but it's really not necessary since anyone having any
electric circuits text can find abundant examples. It appears that some
of the posters either never took a basic course in electrical circuits,
or forgot some very fundamental principles which were taught. But used
circuits texts can be purchased for a very modest price, so there's
little excuse for remaining ignorant if a person is truly interested in
learning about the topic. (There's also the Internet, but you have to be
more careful in sorting out the good information from the bad, and this
can be difficult if you're not already pretty well acquainted with the
topic.) I'd really like it, too, since I'd be able to present an
analysis now and then without having the most basic principles of linear
circuit analysis questioned and debated.

I'm afraid that the fuss about the source is primarily a way to avoid
confronting the facts, which are apparently disturbing to some of the
imaginative alternative theories being promoted.

The SPICE simulation of the circuit I analyzed was, of course, a
transient analysis. The source was a perfect voltage source which
produced a sine wave beginning at t = 0 and continuously after that,
just as in my analysis. For anyone having SPICE, here's the netlist:

* 360 degree transmission line with open end and voltage source,
* transient response showing runup

..TRAN .05 30

v0 1 0 sin(0 1 1hz)
tl1 1 0 2 0 td=1 z0=50
tl4 2 0 3 0 td=4 z0=50
rl 3 0 1meg

..PROBE
..END

TL_1_sec.gif is a plot of v(2), which is the junction of tl1 and tl4.
TL_5_sec.gif is a plot of v(3), which is the open far end of the line. A
plot of v(1) would of course show the source voltage, a constant 1 volt
peak sine wave beginning at t = 0. As I mentioned, the 1 megohm
terminating resistor rl is necessary to keep the version of SPICE I have
from blowing up; it can be any value that's large enough to not have an
appreciable effect on the result.

Roy Lewallen, W7EL

Standing-Wave Current vs Traveling-Wave Current
 

On Dec 31 2007, 11:37*pm, Keith Dysart wrote:
On Dec 31, 11:11*am, Roger wrote:

Cecil Moore wrote:
Richard Clark wrote:
To cut to the chase, Norton and Thevenin sources are appropriate to
network analysis irrespective of your perception.

All my references indicate that those sources are only
appropriate for *steady-state* use. Roger is searching
for a transient state source.

Right!

73, Roger, W7WKB

Odd. Cecil has not named his "references" which is
quite unusual for he truly likes to name-drop: Ramo,
Whinnery, Hecht, IEEE, ...

You would be well served to google "reflection diagram"
or "bounce diagram" where you will find fine examples
of computing re-reflection using the source impedance
of generators modelled using the Thevenin equivalent
circuit. It is not particularly complicated, though
it can be tedious.

...Keith

Also, googling '"lattice diagram" reflection' will yield
a different set of interesting examples.

...Keith

Standing-Wave Current vs Traveling-Wave Current
 

On Dec 30 2007, 6:18 pm, Roy Lewallen wrote:
Keith Dysart wrote:

I predict that the pulse arriving at the left end will
have the same voltage, current and energy profile as
the pulse launched at the right end and the pulse
arriving at the right end will be similar to the
one launched at the left.

They will appear exactly AS IF they had passed
through each other.

The difficulty with saying THE pulses passed
through each other arises with the energy. The
energy profile of the pulse arriving at the left
will look exactly like that of the one launched
from the right so it will seem that the energy
travelled all the way down the line for delivery
at the far end. And yet, from the experiment above,
when the pulses arriving from each end have the
same shape, no energy crosses the middle of the
line.

So it would seem that the energy that actually
crosses the middle during the collision is
exacly the amount of energy that is needed to
reconstruct the pulses on each side after the
collision.

If all the energy that is launched at one end
does not travel to the other end, then I am
not comfortable saying that THE pulse travelled
from one end to the other.

But I have no problem saying that the system
behaves AS IF the pulses travelled from one
end to the other.

You said that:

What will happen? Recall one of the basics about
charge: like charge repel. So it is no surprise
that these two pulses of charge bounce off each
and head back from where they came.

Yet it sounds like you are saying that despite this charge repulsion and
bouncing of waves off each other, each wave appears to be completely
unaltered by the other? It seems to me that surely we would detect some
trace of this profound effect.

. . .

Is there any test you can conceive of which would produce different
measurable results if the pulses were repelling and bouncing off each
other or just passing by without noticing the other?

There are equations describing system behavior on the assumption of no
wave interaction, and these equations accurately predict all measurable
aspects of line behavior without exception. Have you developed equations
based on this charge interaction which predict line behavior with
equal accuracy and universal applicability?

No equations. I expect that such equations would be more complex
than those describing the behaviour using superposition. Since
the existing equations and techniques for analysis are tractable
and produce accurate results, I am not motivated to develop an
alternate set with lower utility.

And yet the "no interaction" model, while accurately predicting
the behaviour has some weaknesses with explaining what is
happening. It is, I suggest, these weaknesses that help lead
some so far astray.

To illustrate some of these weaknesses, consider an example
where a step function from a Z0 matched generator is applied
to a transmission line open at the far end. Charge begins to
flow into the line. The ratio of the current to voltage on
the line is defined by the distributed inductance and
capacitance. The inductance is resisting the change in current
which causes a voltage to charge the capacitance. A voltage
step (call this V for later use) propagates down the line
at the speed of light. In front of this step, the voltage,
current and charge in the line is zero. After the step, the
capacitance is charged to the voltage and charge is flowing
in the inductance.

The step function eventually reaches the open end where
the current can no longer flow. The inductance insists
that the current continue until the capacitance at the
end of the line is charged to the voltage which will stop
the flow. This voltage is double the voltage of the step
function applied to the line (i.e 2*V). Once the
infinitesimal capacitance at the end of the line is
charged, the current now has to stop just a bit earlier
and this charges the inifinitesimal capacitance a bit
further from the end. So a step in the voltage propagates
back along the line towards the source. In front of this
step, current is still flowing. Behind the step, the
current is zero and voltage is 2*V. The charge that
is continuing to flow from the source is being used
to charge the distributed capacitance of the line.

The voltage that is propagating backwards along the
line has the value 2*V, but this can also be viewed as
a step of voltage V added to the already present voltage
V. The latter view is the one that aligns with the "no
interaction" model; the total voltage on the line is
the sum of the forward voltage V and the reverse
voltage V or 2*V.

In this model, the step function has propagated to the
end, been reflected and is now propagating backwards.
Implicit in this description is that the step continues
to flow to the end of the line and be reflected as
the leading edge travels back to the source.

And this is the major weakness in the model. It claims
the step function is still flowing in the portion of
the line that has a voltage of 2*V and *zero* current.

Now without a doubt, when the voltages and currents
of the forward and reverse step function are summed,
the resulting totals are correct. But it seems to
me that this is just applying the techniques of
superposition. And when we do superposition on a
basic circuit, we get the correct totals for the
voltages and currents of the elements but we do
not assign any particular meaning to the partial
results.

A trivial example is connecting to 10 volt batteries
in parallel through a .001 ohm resistor. The partial
results show 10000 amps flowing in each direction
in the resistor with a total of 0. But I do not
think that anyone assigns significance to the 10000
amp intermediate result. Everyone does agree that
the actual current in the resistor is zero.

The "no interaction" model, while just being
superposition, seems to lend itself to having
great significance applied to the intermediate
results.

Partially this may be due to poor definitions. If the
wave is defined as just being a voltage wave, then
all is well.

But, for example, when looking at a solitary pulse,
it is easy (and accurate) to view the wave as having
more than just voltage. One can compute the charge,
the current, the power, and the energy. But when
two waves are simultaneously present, it is only
legal to superpose the voltage and the current.
But it is obvious that a solitary wave has voltage,
current, power, etc. But when two waves are present
it is not legal to.... etc., etc.
The "no interaction" model does not seem to resolve
this conflict well, and some are lead astray.

And it was this conflict that lead me to look for
other ways of thinking about the system.

Earlier you asked for an experiment. How about this
one....

Take two step function generators, one at each end
of a transmission line. Start a step from each end
at the same time. When the steps collide in the
middle, the steps can be viewed as passing each
other without interaction, or reversing and
propagating back to their respective sources. We
can measure the current at the middle of the line
and observe that it is always 0. Therefore the
charge that is filling the capacitance and causing
the voltage step which is propagating back towards
each generator must be coming from the generator
to which the step is propagatig because no charge
is crossing the middle of the line.

Do you like it?

....Keith

Standing-Wave Current vs Traveling-Wave Current
 

Keith Dysart wrote:
Odd. Cecil has not named his "references" which is
quite unusual for he truly likes to name-drop: Ramo,
Whinnery, Hecht, IEEE, ...

I get ragged on for giving too many references and
not giving enough references. You guys please make
up your minds.
--
73, Cecil http://www.w5dxp.com

Standing-Wave Current vs Traveling-Wave Current
 

On Jan 1, 10:42*am, Cecil Moore wrote:
Keith Dysart wrote:
Odd. Cecil has not named his "references" which is
quite unusual for he truly likes to name-drop: Ramo,
Whinnery, Hecht, IEEE, ...

I get ragged on for giving too many references and
not giving enough references. You guys please make
up your minds.

A reference backing up your claim that a well-defined
source impedance can not be used to compute transient
reflections at the source would be entirely appropriate.

And you might find googling '"lattice diagram" reflection'
to assist in understanding the counter-claim.

...Keith

Standing-Wave Current vs Traveling-Wave Current
 

Keith Dysart wrote:
And yet the "no interaction" model, while accurately predicting
the behaviour has some weaknesses with explaining what is
happening. It is, I suggest, these weaknesses that help lead
some so far astray.

Everything can be understood within the context of
superposition and conservation of energy. No need
to invent any new laws of physics like EM waves
bouncing off each other in free space.
--
73, Cecil http://www.w5dxp.com

Standing-Wave Current vs Traveling-Wave Current
 

Roy Lewallen wrote:
But any analysis which isn't valid when driven
by a perfect voltage source in series with a resistance (or current
source in parallel with a resistance) is fundamentally flawed.

Any model that violates the laws of physics is fundamentally
flawed. Your model has EM energy sloshing around like water.
Your model has EM energy neither flowing into the source
nor being reflected. That is a violation of the conservation
of energy principle.

The SPICE simulation of the circuit I analyzed was, of course, a
transient analysis. The source was a perfect voltage source which
produced a sine wave beginning at t = 0 and continuously after that,
just as in my analysis. For anyone having SPICE, here's the netlist:

What is it that you think you have proved? That there
is no energy in reflected waves? That EM waves don't
move at the speed of light? That the conservation of
energy principle is invalid? What?
--
73, Cecil http://www.w5dxp.com

Standing-Wave Current vs Traveling-Wave Current
 

Keith Dysart wrote:
A reference backing up your claim that a well-defined
source impedance can not be used to compute transient
reflections at the source would be entirely appropriate.

I made no such claim. My claim is that *your* source
is *NOT* well-defined and is just a result of your
hand-waving fantasies. When you stop refusing to
provide a schematic, we can discuss whether it is
well-defined or not.

Again, I freely admit that you can leap tall buildings
at a single bound in your mind.
--
73, Cecil http://www.w5dxp.com

Standing-Wave Current vs Traveling-Wave Current
 

Keith Dysart wrote:
On Dec 30, 5:30 pm, Roger wrote:


I don't recall any examples using perfect CURRENT sources. I think a
perfect current source would supply a signal that could respond to
changing impedances correctly. It should solve the dilemma caused by
the rise in voltage which occurs when when a traveling wave doubles
voltage upon encountering an open circuit, or reversing at the source.

What do you think?

A perfect current source has an output impedance of
infinity, just like an open circuit. The reflection
coefficient is 1.

Similar to the reflected voltage for the perfect
voltage source, the reflected current cancels leaving
just the current from the perfect current source.

...Keith

This disagrees with Roy, who assigns a -1 reflection coefficient when
reflecting from a perfect voltage source.

The Norton or Thévenin equivalent circuits seem capable of positive
reflection coefficients. That is all that I am looking for.

Your search suggestion from a different posting '"lattice diagram"
reflection'yields some examples that demonstrate positive reflection
coefficients.

I must have missed something, because I can't understand why there is an
insistence that a negative reflection coefficient must exist at the
source for the 1/2 or 1 wavelength long transmission line fed at one end.

73, Roger, W7WKB