wrote:
May I suggest that if you had read the posting to which I
responded and the rest of my response you would have found
exactly the example you are looking for: the forward
voltage and current on a transmission line when a standing
wave is present (and the reflected as well).

How did the standing wave get there in the first place? *POWER*
from the generator. You simply cannot have standing waves without
a power source, a forward wave, and a reflected wave.

You are asking us to completely ignore the cause of standing waves.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

wrote:
May I suggest that if you had read the posting to which I
responded and the rest of my response you would have found
exactly the example you are looking for: the forward
voltage and current on a transmission line when a standing
wave is present (and the reflected as well).

How did the standing wave get there in the first place? *POWER*
from the generator. You simply cannot have standing waves without
a power source, a forward wave, and a reflected wave.

You are asking us to completely ignore the cause of standing waves.

It is not obvious to me how you extrapolate my postings to these
outrageous assertions.

For sure there is power from the generator. It is needed to charge
the line and to provide whatever power is consumed in the load
and line losses.

When a standing wave is present, for sure there is a forward
and reflected voltage and current wave. After all it is called
a voltage standing wave.

But these voltage and current forward and reflected waves do not
have power. They are exactly the same as the voltages computed
using superposition in circuit analysis, they are superposed in
exactly the same way to find the resultant voltage, and it is
illegal, except in special cases, to assume that these constituent
voltage terms represent power.

May I suggest, for clearer understanding, that just for a few
moments (say 30 minutes), you set aside RF and consider how
a line is charged by a step function. Do the voltage and
current reflection diagrams. And then consider the energy
flow just in front and just following the voltage step
as it propagates down the line and back and down and back...

Take the time to do this for the following cases...
- matched generator
- line terminated in Z0
- line open
- line shorted

After the line has charged consider what happens when
the generator voltage is set back to 0.

Do it all again for a mismatched generator.

Then for a charged open termination line, consider what
happens when a load of Z0 is applied. And then when the
load is removed.

For each of these cases determine how the voltage fronts
propagate, the energy flow in front of and following the
step, the resulting energy distribution on the line and
whether this energy is stored in the capacitance or
inductance or H field or E field.

Because of the step function excitation, none of these
computations are difficult.

With this example it is easy to see when energy is flowing
and when it is not, and contrast this to the energy flows
computed using the forward and reflected voltages.

Well maybe the above is more than 30 minutes, but there
is much to be learned from a thorough understanding of
the behaviour with this simple excitation.

Now replace step excitation with sinusoidal; the principles
are the same, but the computations are more complex and
the resulting voltage and energy distributions on the line
are more interesting. But the fundamentals are the same.

The above thought experiment was the one that made clear
to me the fallacy of assigning power to the forward and
reflected voltage waves.

So there is some risk for you doing this thought experiment;
the results may conflict with some of your deeply held
beliefs. It is a risk worth taking.

....Keith

wrote:
But these voltage and current forward and reflected waves do not
have power.

The source puts out power. If that energy doesn't go into the
forward and reflected waves, where does it go?

I am not going to do any of your thought experiments until you stop
ignoring the questions I previously asked you about mine.

100W source---one second long 291.4 ohm lossless line----50 ohm load

During steady-state, the transmission line contains 300 joules that
have not made it to the load. Where did that 300 joules go?
--
73, Cecil http://www.qsl.net/w5dxp



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And then consider the energy
flow just in front and just following the voltage step
as it propagates down the line and back and down and back...

=======================

Kelvin had trouble with voltage steps when trying to predict signalling
speed on the first Atlantic telegraph cable. They went ahead, chartered the
Great Eastern steamship, steamed West and laid the thing anyway. Shortly
afterwards it broke. Kelvin was created a Lord for his un-finished services
and had a bridge named after him.

But it was indeed a difficult problem in that day and age. Twenty years
later, hard-of-hearing Heaviside invented a brand new branch of mathematics,
the Operational Calculus, to solve that particular, and a great number of
other problems. For HIS services to mankind, as a revolutionary, he was
derided by the pompous Establishment whose members resorted only to
plagiarised text books. Heaviside only had a Layer named after him.

To their own credit, it was American communications engineers who eventually
acknowledged his genius. But then, Americans always did have sympathetic
feelings towards revolutionaries.

What amazes me is the amount of trouble some modern American engineers still
appear to suffer from on the subject of propagation of an electric current
along a pair of wires. Wires have been around a long time now.

Instead of thinking in terms of frequency and waves why not do as Oliver did
and try time and waveshape. It worked for him. Exactly what, where and when
is being reflected becomes clear.

And if anybody enjoys playing with numbers just replace 'j-omega' with 's'
(It was 'p' when I first played with it.)

Incidentally, the concept of wire-gauges originated in my home city,
Birmingham, England. Faraday was familiar with it. It was internationally
known as the BWG. The Americans, just to be different, changed theirs to
AWG. Now (nearly) everybody has gone metric. ;o)
----
Reg, G4FGQ

Reg, G4FGQ wrote:
"Instead of thinking in terms of frequency and waves why not do as
Oliver did and try time and waveshape?"

The digital revolution is well underway and Reg has the right idea.
Radio is the domain of frequency and waves however. Terman`s first
words: "Electrical energy that has escaped into free space exists in the
form of electromagnetic waves." are still true.

Step functions lost interest with the demise of telegraph, but ones and
zeros are back bigger than ever.

Best regards, Richard Harrison, KB5WZI

There are a lot of TDR and oscilloscope users out there to contradict that.

Roy Lewallen, W7EL

Richard Harrison wrote:
. . .
Step functions lost interest with the demise of telegraph, but ones and
zeros are back bigger than ever.

Best regards, Richard Harrison, KB5WZI

Roy, WtEL wrote:
"There are a lot of TDR and oscilloscope users out there to contradict
that." (Step functions lost interest with the demise of telegraph, but
ones and zeros are back bigger than ever.)

Lord Kelvin, William Thomson suggested in the 19th century that life may
have arrived here from outer space. He died in 1907. Do you suppose he
was using TDR with an oscilloscope to determine subsea cable faults?

Best regards, Richard Harrison, KB5WZI