wrote:
There is certainly no power at the zero crossings.

There is no NET power at the zero crossings. There exists
equal amounts of power flowing in opposite directions as
represented by the forward Poynting vector and the reflected
Poynting vector. Those two power flow vectors simply cancel
at the NET power equal zero point.
--
73, Cecil, W5DXP

W5DXP wrote:

wrote:
There is certainly no power at the zero crossings.

There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

73, ac6xg

Jim Kelley wrote:

W5DXP wrote:
There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

Well, since the NET voltage is always zero at a voltage node when
the forward power and reflected power are equal, the instantaneous
voltage is always zero, i.e. the steady-state voltage is always
zero. If we have equal power flow vectors in opposite directions,
the NET power is zero at all points up and down the line.
--
73, Cecil, W5DXP

W5DXP wrote:

Jim Kelley wrote:

W5DXP wrote:
There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

Well, since the NET voltage is always zero at a voltage node when
the forward power and reflected power are equal, the instantaneous
voltage is always zero, i.e. the steady-state voltage is always
zero.

I've never actually seen the voltage at a node in a standing wave
pattern referred to as an instantaneous voltage - especially considering
that it doesn't vary with time. Instantaneous usually means the
solution to a function f(t) at time t (not f(x) and position x.) Nodes
and zero crossings aren't necessarily the same thing.

73, Jim AC6XG

Jim Kelley wrote:
Nodes and zero crossings aren't necessarily the same thing.

They are for standing waves on lossless unterminated lines,
by definition.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

W5DXP wrote:

Jim Kelley wrote:
Nodes and zero crossings aren't necessarily the same thing.

They are for standing waves on lossless unterminated lines,
by definition.

I think not. Standing waves are spatial. At certain points
on the line the (NET) voltage is always zero: nodes.
At other points on the line, the (NET) voltage is sinusoidal
and has 2 zero crossings per cycle. The amplitude of these
sinusoids varies spatially along the line resulting in the
standing wave.

In an ideal line terminated by Zo, no matter where you attach
your oscillograph to the line, you will observe a sinusoid
of the same amplitude. This sinusoid will have zero crossings
and power at the time of these zero crossings will be zero;
no energy will be flowing at the time of the zero crossing.

And going back to the comment that started this sub-thread,
it is this cyclical variation in energy flow which prompted
the power dudes to invent three phase lines in which the
energy flow does not vary cyclically; power is constant.

....Keith

wrote:

W5DXP wrote:

Jim Kelley wrote:
Nodes and zero crossings aren't necessarily the same thing.

They are for standing waves on lossless unterminated lines,
by definition.

I think not. Standing waves are spatial. At certain points
on the line the (NET) voltage is always zero: nodes.

For the infinite number of times a snapshot of the voltage is
not zero, the node *IS* a zero-crossing point. That is more than
obvious from the JAVA applets on this web page.

http://www.gmi.edu/~drussell/Demos/s....html#standing

In an ideal line terminated by Zo, ...

That configuration is not covered by my statement above which
applies only to standing waves on lossless unterminated lines.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

Keith wrote:
"---it is this cyclical variation in energy flow which prompted power
dudes to invent three phase lines in which energy flow does not vary
cycillaly, power is constant."

It`s true the wires are shared by multiple phases which peak 120-degrees
apart in the 3-phase case. This distributes power flow more evenly with
respect to time and reduces peak loads since the phases never coicide.
Tesla figured this out 100 years ago.

Coincidences of incident and reflected waves are very different from the
cyclical variations of a-c.

Incident and reflected waves have cyclical voltages and currents. As the
reflected wave is just a delayed incident wave, the period is the same
for both.

In those line spots where the total reflected voltage is 180-degrees
different in phase from the incident wave, the net voltage is always
zero during the cycle if the reflection is complete on a lossless line.

The power is not zero at points where the voltage is always zero because
the voltages that add to zero are not zero. In the lossless line, these
voltages are full strength, as are the currents at a current null, some
90-degrees away in space from the voltage null on the line.

Fact is, both the forward power and the reflected power would measure
the same at any point along the line.

The wave action has been observed and documented for more than a
century. The explanations withstand all arguments, so far. Something new
will be needed to replace the ancient wave theory to win acceptance.
Keith`s zero power at zero null spots won`t persuade.

The power appearing to null is not the whole story when there are
forward and reflected powers, each having electric and magnetic fields
with phase differences all around.

Best regards, Richard Harrison, KB5WZI