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

It's reasonable, though. Looking at demo 4 with the TLVis1 program, you
can see that there's power all along the line except at specific nodal
points (where I or V is always zero), yet there's no power at all being
delivered to the load.

Roy Lewallen, W7EL

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

It's reasonable, though. Looking at demo 4 with the TLVis1 program, you
can see that there's power all along the line except at specific nodal
points (where I or V is always zero), yet there's no power at all being
delivered to the load.

Roy, my though was that on anything but a lossless line with VSWR=1,
instantaneous power (being the rate of flow of energy) varies with time
and location, so to make the statement that "power is present" and to
quantitatively compare it with the power at a point (being the end of the
line where the load is attached) seems to not be so reasonable.

If the statement is about average power in both cases, then it is
reasonable, obvious even, that power decreases with distance from the
source.

Perhaps "power is present" is an avoidance of the somewhat tautological
form "power flows to the load".

Owen

Owen Duffy wrote:
Perhaps "power is present" is an avoidance of the somewhat tautological
form "power flows to the load".

Want to muddy the waters even more? Ramo & Whinnery say:
"Another very important case is that of a perfect conductor,
which by definition must have a zero tangential component
of electric field at its surface. Then ^P^ [Poynting vector]
can have no component normal to the conductor and there can
be no power flow through the perfect conductor."
--
73, Cecil http://www.w5dxp.com

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

It's reasonable, though. Looking at demo 4 with the TLVis1 program, you
can see that there's power all along the line except at specific nodal
points (where I or V is always zero), yet there's no power at all being
delivered to the load.

Roy, my though was that on anything but a lossless line with VSWR=1,
instantaneous power (being the rate of flow of energy) varies with time
and location, so to make the statement that "power is present" and to
quantitatively compare it with the power at a point (being the end of the
line where the load is attached) seems to not be so reasonable.

If the statement is about average power in both cases, then it is
reasonable, obvious even, that power decreases with distance from the
source.

Perhaps "power is present" is an avoidance of the somewhat tautological
form "power flows to the load".

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.

73, Roger, W7WKB

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. I don't
think that is a sheer coincidence. :-)
--
73, Cecil http://www.w5dxp.com

On Fri, 25 Jan 2008 16:15:33 GMT
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. I don't
think that is a sheer coincidence. :-)
--
73, Cecil http://www.w5dxp.com

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.

The transmission line was a dynamic power storage device for two microseconds after the power source was disconnected.

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 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