On Fri, 25 Jan 2008 19:33:41 GMT
Owen Duffy wrote:

Roger Sparks wrote in
:

On Fri, 25 Jan 2008 05:32:33 GMT
Owen Duffy wrote:

Roy Lewallen wrote in news:13pirk5h1cpt4f5
@corp.supernews.com:

Owen Duffy wrote:
Roger Sparks wrote in
:

... The reader can
also see that more power is present on the transmission line than
is delivered to the load.

The notion that "power is present" is a different one.

Owen

...
Nothing mysterious was hinted with the words "power is present".

As I finished writing the post, I wanted to call attention to the
assumption that the reflected power is true power and adds to the
amount of energy "stored" on the transmission line. But "stored" is
a word that implies static conditions, and static conditions are not
found on a transmission line. So I substituted "present" for "stored.

Roger,

If you were wanting to mean "stored", perhaps it is energy that is stored
(over a non-zero length of line) rather than power. In that sense, energy
is "present" on the line, and the load may store energy (only if it has
reactive elements, and irrespective of whether it looks resistive at its
terminals).

Owen

I think everyone agrees that energy is stored on a transmission line in the sense that energy enters at time one and does not exit until some time later at time two.

It is important to be aware that the time shape of the energy package is preserved on a transmission line. The time shape information is contained in the power term for every instant of passing time.

I can understand why many hesitate to think of power as being "stored" on a transmission line, because at best, such storage is dynamic and fleeting. Thinking of power being "present" on a transmission line is better than "stored" because the concept of a time component is not lost.

--
73, Roger, W7WKB

Roger Sparks wrote:
If the steady-state forward power is 200 watts,
the reflected power is 100 watts, and the lossless
transmission line is one microsecond long, it
contains 300 microjoules of energy. I don't
think that is a sheer coincidence. :-)

Yep, and if we quickly replaced the source with a termination having the impedance of the transmission line, 100 watts of power would continue to be delivered to the load for one microsecond, delivering 100 microjoules of energy. 100 watts of power would be delivered to the reflected wave termination for two microseconds, delivering 200 microjoules of energy.

Can we consider the old wives' tale of no energy in reflected
waves to be laid to rest?
--
73, Cecil http://www.w5dxp.com

On Jan 25, 11:15*am, Cecil Moore wrote:
Roger Sparks wrote:
As I finished writing the post, I wanted to call attention to the assumption that the reflected power is true power and adds to the amount of energy "stored" on the transmission line. * But "stored" is a word that implies static conditions, and static conditions are not found on a transmission line. *So I substituted "present" for "stored.

The amount of energy existing in a transmission
line is exactly the amount required to support
the measured forward power and reflected power.

If the steady-state forward power is 200 watts,
the reflected power is 100 watts, and the lossless
transmission line is one microsecond long, it
contains 300 microjoules of energy.

Are you sure?

Check your answer by trying 100 kHz sinusoidal
steady-state excitation.

Even easier, ignore the reflected power and
test your assertion just for the forward power.

For fun, work out the line lengths for which your
claim is true.

...Keith

Keith Dysart wrote:
Check your answer by trying 100 kHz sinusoidal
steady-state excitation.

Good grief, Keith, get real. I guess I forgot to
say the assertion was for an integer multiple of MHz.
--
73, Cecil http://www.w5dxp.com

On Jan 24, 10:33*pm, Roger Sparks wrote:
[snip]
By examining this derivation, the reader can see that power and energy
is reflected when a wave encounters a discontinuity. *The reader can
also see that more power is present on the transmission line than is
delivered to the load.

This is the conventional phraseology for describing the behaviour at
the impedance discontinuity.

Allow me to offer a specific example for which this phraseology is
inappropriate.

Consider a 50 V step function generator with an output impedance of
50 ohms driving a 50 ohm line that is 1 second long terminated in an
open circuit.

Turn on the generator. A 50 V step propagates down the line. The
generator is putting 50 J/s into the line. One second later it
reaches the open end and begins propagating backwards.
After two seconds it reaches the generator. The voltage at the
generator is now 100 V and no current is flowing from the
generator into the line. In the 2 seconds, the generator put
100 joules into the line which is now stored in the line.
The line is at a constant 100 V and the current is zero everywhere.

Computing Pf and Pr will yield 50 W forward and 50 W reflected.
And yet no current is flowing anywhere. The voltage on the line
is completely static.

And yet some will claim that 50 W is flowing forward and 50 W
is flowing backwards.

Does this seem like a reasonable claim for an open circuited
transmission line with constant voltage along its length and
no current anywhere?

I do not find it so.

...Keith

On Jan 25, 5:31*pm, Cecil Moore wrote:
Keith Dysart wrote:
Check your answer by trying 100 kHz sinusoidal
steady-state excitation.

Good grief, Keith, get real. I guess I forgot to
say the assertion was for an integer multiple of MHz.

Yes, so it would seem.

And that would seem to narrow the applicability
of the original assertion rather severely.

...Keith

Keith Dysart wrote:
Computing Pf and Pr will yield 50 W forward and 50 W reflected.
And yet no current is flowing anywhere. The voltage on the line
is completely static.

Why would you compute Pf and Pr when no DC current is
flowing? It is an invalid thing to do and unrelated to
reality.

And yet some will claim that 50 W is flowing forward and 50 W
is flowing backwards.

I know of no one who will claim that for static DC.
There are obviously no photons being emitted and
therefore, no waves.

Your example is unrelated to standing waves on an
RF transmission line where energy is in motion,
photons are continuously being emitted and absorbed,
and current and voltage loops are active.

One must realize the limitations of one's model.
The wave model obviously fails where there are no
waves.
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:
And that would seem to narrow the applicability
of the original assertion rather severely.

What do you know? It narrows it to amateur radio,
the subject of this newsgroup.

To be entirely technically correct, since my assertion
was about average powers, the example transmission line
must be an integer multiple of 1/4 wavelength.
--
73, Cecil http://www.w5dxp.com

On Jan 26, 9:12*am, Cecil Moore wrote:
Keith Dysart wrote:
And that would seem to narrow the applicability
of the original assertion rather severely.

What do you know? It narrows it to amateur radio,
the subject of this newsgroup.

I was unaware that all Amateur transmission lines
were a multiple of 1 wavelength long. Are you sure?

To be entirely technically correct, since my assertion
was about average powers, the example transmission line
must be an integer multiple of 1/4 wavelength.

I would suggest 1/2 wavelength. For an intuitive proof,
consider a line with only forward power. Then think
of a quarter wave section with a voltage peak in the
middle. Then consider when the voltage 0 is in the
middle. Lots more energy in the former than the latter.
At 1/2 wavelength, the total energy in the line section
is constant.

...Keith

...Keith

On Fri, 25 Jan 2008 19:13:31 -0800 (PST)
Keith Dysart wrote:

On Jan 24, 10:33*pm, Roger Sparks wrote:
[snip]
By examining this derivation, the reader can see that power and energy
is reflected when a wave encounters a discontinuity. *The reader can
also see that more power is present on the transmission line than is
delivered to the load.

This is the conventional phraseology for describing the behaviour at
the impedance discontinuity.

Allow me to offer a specific example for which this phraseology is
inappropriate.

Consider a 50 V step function generator with an output impedance of
50 ohms driving a 50 ohm line that is 1 second long terminated in an
open circuit.

Turn on the generator. A 50 V step propagates down the line. The
generator is putting 50 J/s into the line. One second later it
reaches the open end and begins propagating backwards.
After two seconds it reaches the generator. The voltage at the
generator is now 100 V and no current is flowing from the
generator into the line. In the 2 seconds, the generator put
100 joules into the line which is now stored in the line.
The line is at a constant 100 V and the current is zero everywhere.

Computing Pf and Pr will yield 50 W forward and 50 W reflected.
And yet no current is flowing anywhere. The voltage on the line
is completely static.

And yet some will claim that 50 W is flowing forward and 50 W
is flowing backwards.

Does this seem like a reasonable claim for an open circuited
transmission line with constant voltage along its length and
no current anywhere?

I do not find it so.

...Keith

This is a reasonable observation for a static situation where energy is stored on a transmission line.

If the example contained an ongoing consideration, like "Where does the power move to?", then it would be reasonable to consider that the wave continued to move, simply to avoid the complication of what EXACTLY happens when a wave starts and stops.
--
73, Roger, W7WKB