Keith Dysart wrote:

On Dec 29, 2:31 pm, Cecil Moore wrote:

Roger wrote:

Are there reflections at point "+"? Traveling waves going in opposite
directions must pass here, therefore they must either pass through one
another, or reflect off one another.

In the absence of a real physical impedance discontinuity,
they cannot "reflect off one another". In a constant Z0 transmission
line, reflections can only occur at the ends of the line and only
then at an impedance discontinuity.


Roger: an astute observation. And Cecil thinks he has
the ONLY answer. Allow me to provide an alternative.

Many years ago, when I first encountered this news group
and started really learning about transmission lines, I
found it useful to consider not only sinusoidallly
excited transmission lines, but also pulse excitation.
It sometimes helps remove some of the confusion and
clarify the thinking. So for this example, I will use
pulses.

Consider a 50 ohm transmission line that is 4 seconds
long with a pulse generator at one end and a 50 ohm
resistor at the other.

The pulse generator generates a single 1 second pulse
of 50 volts into the line. Before and after the pulse
its output voltage is 0. While generating the pulse,
1 amp (1 coulomb/s) is being put into the line, so
the generator is providing 50 watts to the line.

After one second the pulse is completely in the line.
The pulse is one second long, contains 1 coulomb of
charge and 50 joules of energy. It is 50 volts with
1 amp: 50 watts.

Let's examine the midpoint (2 second) on the line.
At two seconds the leading edge of the pulse arrives
at the midpoint. The voltage rises to 50 volts and
the current becomes 1 amp. One second later, the
voltage drops back to 0, as does the current. The
charge and the energy have completely passed the
midpoint.

When the pulse reaches the end of the line, 50
joules are dissipated in the terminating resistor.

Notice a key point about this description. It is
completely in terms of charge. There is not a single
mention of EM waves, travelling or otherwise.

Now we expand the experiment by placing a pulse
generator at each end of the line and triggering
them to each generate a 50V one second pulse at
the same time. So after one second a pulse has
completely entered each end of the line and these
pulse are racing towards each other at the speed
of light (in the line). In another second these
pulses will collide at the middle of the line.

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. At the center
of the line, for one second the voltage is 100 V
(50 V from each pulse), while the current is
always zero. No charge crossed the mid-point. No
energy crossed the mid-point (how could it if
the current is always zero (i.e. no charge
moves) at the mid-point.

It is a minor extension to have this model deal
with sinusoidal excitation.

What happens when these pulses arrive back at the
generator? This depends on generator output
impedance. If it is 50 ohms (i.e. equal to Z0),
then there is no reflection and 1 joule is
dissipated in each generator. Other values
of impedance result in more complicated
behaviour.

So do the travelling waves "reflect" off each
other? Save the term "reflect" for those cases
where there is an impedance discontinuity and
use "bounce" for those cases where no energy
is crossing a point and even Cecil may be
happy. But bounce it does.

...Keith

It's fairly safe to make this argument when both pulses are identical.
I challenge you to obtain this result when they are not. :-)

73, Jim AC6XG

Jim Kelley wrote:

It's fairly safe to make this argument when both pulses are identical.
I challenge you to obtain this result when they are not. :-)

73, Jim AC6XG

I proposed this some time ago, and got the response that dissimilar
pulses would still bounce off each other, yet appear exactly as though
they were passing through without interaction. I haven't been able to
understand why this would be, but there are no mathematics to explain it.

Roy Lewallen, W7EL

On Jan 2, 4:57*pm, Jim Kelley wrote:
Keith Dysart wrote:
On Dec 29, 2:31 pm, Cecil Moore wrote:

Roger wrote:

Are there reflections at point "+"? *Traveling waves going in opposite
directions must pass here, therefore they must either pass through one
another, or reflect off one another.

In the absence of a real physical impedance discontinuity,
they cannot "reflect off one another". In a constant Z0 transmission
line, reflections can only occur at the ends of the line and only
then at an impedance discontinuity.

Roger: an astute observation. And Cecil thinks he has
the ONLY answer. Allow me to provide an alternative.

Many years ago, when I first encountered this news group
and started really learning about transmission lines, I
found it useful to consider not only sinusoidallly
excited transmission lines, but also pulse excitation.
It sometimes helps remove some of the confusion and
clarify the thinking. So for this example, I will use
pulses.

Consider a 50 ohm transmission line that is 4 seconds
long with a pulse generator at one end and a 50 ohm
resistor at the other.

The pulse generator generates a single 1 second pulse
of 50 volts into the line. Before and after the pulse
its output voltage is 0. While generating the pulse,
1 amp (1 coulomb/s) is being put into the line, so
the generator is providing 50 watts to the line.

After one second the pulse is completely in the line.
The pulse is one second long, contains 1 coulomb of
charge and 50 joules of energy. It is 50 volts with
1 amp: 50 watts.

Let's examine the midpoint (2 second) on the line.
At two seconds the leading edge of the pulse arrives
at the midpoint. The voltage rises to 50 volts and
the current becomes 1 amp. One second later, the
voltage drops back to 0, as does the current. The
charge and the energy have completely passed the
midpoint.

When the pulse reaches the end of the line, 50
joules are dissipated in the terminating resistor.

Notice a key point about this description. It is
completely in terms of charge. There is not a single
mention of EM waves, travelling or otherwise.

Now we expand the experiment by placing a pulse
generator at each end of the line and triggering
them to each generate a 50V one second pulse at
the same time. So after one second a pulse has
completely entered each end of the line and these
pulse are racing towards each other at the speed
of light (in the line). In another second these
pulses will collide at the middle of the line.

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. At the center
of the line, for one second the voltage is 100 V
(50 V from each pulse), while the current is
always zero. No charge crossed the mid-point. No
energy crossed the mid-point (how could it if
the current is always zero (i.e. no charge
moves) at the mid-point.

It is a minor extension to have this model deal
with sinusoidal excitation.

What happens when these pulses arrive back at the
generator? This depends on generator output
impedance. If it is 50 ohms (i.e. equal to Z0),
then there is no reflection and 1 joule is
dissipated in each generator. Other values
of impedance result in more complicated
behaviour.

So do the travelling waves "reflect" off each
other? Save the term "reflect" for those cases
where there is an impedance discontinuity and
use "bounce" for those cases where no energy
is crossing a point and even Cecil may be
happy. But bounce it does.

...Keith

It's fairly safe to make this argument when both pulses are identical.
* I challenge you to obtain this result when they are not. *:-)

The example was carefully chosen to illustrate the
point, of course. But that is the value of particular
examples.

When the pulses are not identical, the energy that crosses
the point is exactly sufficient to turn one pulse
into the other. The remainder of the energy must bounce
because it does not cross the mid-point.

...Keith

Keith Dysart wrote:
When the pulses are not identical, the energy that crosses
the point is exactly sufficient to turn one pulse
into the other. The remainder of the energy must bounce
because it does not cross the mid-point.

All you have proved is that you cannot tell one photon
from another. Your whole charge repulsion argument
falls apart when dealing with photons (which constitute
EM waves). I suggest you study and discover what is
possible with photons and what is not possible. You
seem to be concentrating on the carriers of the waves
rather than the EM waves themselves. Photons do NOT
and cannot bounce off of each other under ordinary
circumstances. You are simply illustrating the limitations
of ignoring the basic physics of the situation and wasting
a lot of time and effort in the process.

I have sat on a cliff overlooking the Pacific Ocean
at Fitzgerald's Marine Reserve north of Santa Cruz, CA
and have seen waves rolling in, reflecting off the beach,
and rolling back out to sea. Those waves pass through
each other as if the other wasn't there. The wave energy
is moving in both directions. The H2O carriers move
hardly at all. You can argue that the energy in the
waves is equal and therefore no average energy is being
transferred, but I still see the waves with people
riding on those waves. I do not see waves bouncing off
of each other although one could, as you have, delude
oneself into creating a mental illusion of such.

When I look out into my back yard, I am seeing reflections.
If there were a thousand people here, they would all be
seeing different reflections all passing through each other.
Photonic waves pass through each other unimpeded. It would
be a weird looking world if they bounced off each other.

In a wire, photons do not bounce off each other. However,
superposition can cause a redistribution of photon energy
at an impedance discontinuity. We call that redistribution
of energy a "reflections".
--
73, Cecil http://www.w5dxp.com

On Jan 3, 11:10*am, Cecil Moore wrote:
Keith Dysart wrote:
When the pulses are not identical, the energy that crosses
the point is exactly sufficient to turn one pulse
into the other. The remainder of the energy must bounce
because it does not cross the mid-point.

All you have proved is that you cannot tell one photon
from another. Your whole charge repulsion argument
falls apart when dealing with photons (which constitute
EM waves). I suggest you study and discover what is
possible with photons and what is not possible.

Can photons explain the state of a transmission
line driven with a step function after the line
has settled to a constant voltage?

If not, there would seem to be some difficulty
with the applicability.

...Keith

Keith Dysart wrote:
Can photons explain the state of a transmission
line driven with a step function after the line
has settled to a constant voltage?

Of course, photons can be used to explain all
EM wave action. A step function accelerates
electrons which then emit photons as EM waves.
Hint: electrons cannot move at the speed of
light. EM waves move at the speed of light.
--
73, Cecil http://www.w5dxp.com

On Jan 3, 1:48*pm, Cecil Moore wrote:
Keith Dysart wrote:
Can photons explain the state of a transmission
line driven with a step function after the line
has settled to a constant voltage?

Of course, photons can be used to explain all
EM wave action. A step function accelerates
electrons which then emit photons as EM waves.
Hint: electrons cannot move at the speed of
light. EM waves move at the speed of light.

Please describe the final state of the step
excited open circuited line using photons.

Thanks,

Keith

Keith Dysart wrote:
Please describe the final state of the step
excited open circuited line using photons.

Photons are emitted and absorbed by the electrons
as the electrons lose/gain energy. Photons are not
conserved. Only the energy in photons is conserved.

In a DC system with no accelerating or decelerating
electrons, all of the photons have been absorbed
back into the electrons (or lost to radiation).
Of course, this describes an ideal system.
--
73, Cecil http://www.w5dxp.com

On Thu, 3 Jan 2008 11:19:26 -0800 (PST), Keith Dysart
wrote:

Hint: electrons cannot move at the speed of
light. EM waves move at the speed of light.

I love these built-in failures of argument. :-)

Shine the sun on a pie pan. How fast is light moving in getting
through it? How fast is an electron moving in getting through it?

Is light traveling at the speed of light? Would it travel faster than
an electron if we took out the pie? Would it travel faster than an
electron if we kept the pie and took out the pan?

73's
Richard Clark, KB7QHC

Keith Dysart wrote:
The example was carefully chosen to illustrate the
point, of course. But that is the value of particular
examples.

When the pulses are not identical, the energy that crosses
the point is exactly sufficient to turn one pulse
into the other.

The remainder of the energy must bounce
because it does not cross the mid-point.
...Keith

So it really is almost as though the pulses travel through one
another, rather than bounce off one another.

I have seen the concept that energy doesn't cross nodal points alluded
to in some texts. However there are so many exceptions to it found in
physical systems as to render it a dubious notion at best. Useful
perhaps for illustration purposes.

In the discussion of standing waves on a string, Halliday and Resnick
says "It is clear that energy is not transported along the string to
the right or to the left, for energy cannot flow past the nodal points
in the string, which are permanently at rest. Hence the energy
remains "standing" in the string, although it alternates between
vibrational kinetic energy and elastic potential energy."

So the idea is valid for a simple harmonic oscillator in which there
are no losses. In such a case, once the system begins oscillating, no
further input of energy is required in order to maintain oscillation.
Clearly there is no flow of energy into or out of such a system.
What is clear is that energy doesn't pass through the nodes. It is
less clear that there exists an inherent mechanism which prevents the
movement of energy.

And so it appears in cases where there is no transfer of energy that
one might claim that waves bounce off of one another. There are no
other examples, and no supporting mechanism for it of which I am
aware, and so one might be equally justified in claiming that waves
pass through each other in all cases.

73, ac6xg