Cecil Moore wrote:
Roger wrote:
Cecil Moore wrote:
Roger wrote:
Stored in the 1/4 WL between the short and mouth. No more current
needed once stability is reached.

EM RF current is stored in the stub? In what form?

Come on Cecil! Let's not go around in circles! You know very well how
it happens.

Here's an example using a circulator and load in
a 50 ohm system. Please think about it.

SGCL---1---2------------------------------+
\ / | 1/4
3 | WL
| everything is 50 ohms | shorted
R | stub

Are there any reflections at point '+'?

If not, how is energy stored in the stub?

If so, what causes those reflections?

I am not sufficiently familiar with circulators to respond. My present
level of understanding is that they can only be built using ferrite
inductors which have an ansiotropic (non-linear) magnetic response. If
so, they could not be compared to transmission lines without adding that
non-linear factor. Apparently energy is stored in these inductors only
if the power is moving in one direction, so it never reaches one branch.
I don't understand how a ferrite could do that.

Is there such a thing as a "all transmission line" circulator? If so,
where could I find the circuit?

Thanks,

73, Roger, W7WKB

Roger wrote:
Cecil Moore wrote:
Are there any reflections at point '+'?

If not, how is energy stored in the stub?

If so, what causes those reflections?

I am not sufficiently familiar with circulators to respond.

If the circulator is bothering you, forget it and assume the
following lossless conditions:

Ifor = 1 amp --
------------------------------+
-- Iref = 1 amp | 1/4
| WL
All Z0 = 50 ohms | shorted
| stub

Please think about it and answer the questions above.
The main point to remember is that there is no physical
impedance discontinuity at '+'.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Roger wrote:
Cecil Moore wrote:
Are there any reflections at point '+'?

If not, how is energy stored in the stub?

If so, what causes those reflections?

I am not sufficiently familiar with circulators to respond.

If the circulator is bothering you, forget it and assume the
following lossless conditions:

Ifor = 1 amp --
------------------------------+
-- Iref = 1 amp | 1/4
| WL
All Z0 = 50 ohms | shorted
| stub

Please think about it and answer the questions above.
The main point to remember is that there is no physical
impedance discontinuity at '+'.

OK. Let's begin by recognizing that this circuit is identical to a
straight transmission line. The purpose of identifying the stub is to
clearly locate the point 1/4 wavelength from the end of the line. The
line is shorted at the end.

We further assume that the peak current is 1 amp.

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.

Is it important to decide this issue? Yes, if it will affect the answer
to questions such as what is the voltage or current at this point.

Will it affect the answers? No. Under the conditions described, the
waves passing in opposite directions will have equal voltages and
opposite currents. If they pass through one another, the voltages will
add, but the currents will subtract. If they reflect, the voltage of
each component (Vf and Vr) will add on itself, and the individual
currents will reverse on themselves and therefore subtract. Either way,
the total voltage will double, and total measured current would be zero.
There is no reason to decide the issue.

How is energy stored in the stub? We have defined current as entering
an leaving the stub. Current is thought of as movement of charged
particles, but not as a concentration of particles. A concentration of
charged particles exhibits voltage. Energy is present when EITHER
current or voltage are shown to be present. Here, current is defined as
one amp so energy must be present some place on the line. The stub is
1/4 wavelength long physically, but it is 1/2 wavelength long
electrically, so that if we have energy present in the time-distance
shape of a sine wave, we would have an entire 1/2 wave's worth of energy
present on the stub at all times. The location of peak voltage (or peak
current) will depend upon the time-distance reference used to describe
the moving wave. (We would have equal voltage(but opposite polarity)
peaks located at the point {+} if we assumed the center of the forward
and reflected wave each to located 90 degrees from the shorted end.)

The circuit shows forward current Ifor and reflected current Iref as if
each were only one current. When we consider traveling waves, we need
to remember that Ifor and Iref can be measured on either of the two
wires composing a transmission line. The forward wave exists on both
wires, but the sides display opposite polatity and direction of current
despite both moving in the same direction. It is best to consider the
forward traveling wave as two waves, each carrying half the power, with
one wave per wire.

Does this match your own concept of the traveling waves acting at the
{+} point Cecil? If not, where do we differ?

73, Roger, W7WKB

Is this the kind of answer you were looking for? The answer could be
given mathematically but that might be even more confusing.

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.

Does this match your own concept of the traveling waves acting at the
{+} point Cecil? If not, where do we differ?

Where we differ is that you allow traveling waves to "reflect
off one another". There are no laws of physics which allow
that in the absence of a physical impedance discontinuity.
EM waves simply do not bounce off each other.
--
73, Cecil http://www.w5dxp.com

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

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.

I completely concur with your analysis.

No doubt you have fine tuned the analysis to notice that the current
stops (meaning becomes unobservable) at the identical instant that the
voltage spike (to double) is observed. You would have noticed that the
zone of unmeasurable current spreads equally both ways from the
collision point at the velocity of the wave(s). The voltage spike
spreads in lock step with the loss of current detection. The maximum
width of the loss of current and voltage spike is the width of either of
the pulses.

Now did the two pulses reflect, or pass through one another? I have
considered the question and can not discern a difference in my analysis
either way. IT SEEMS TO MAKE NO DIFFERENCE!


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.

Roy and I are talking about this on other postings. I guess the purest
might point out that a 50 ohm generator only has a voltage to current
ratio of 50, but we don't know if it also has a resistor to absorb
energy. It is like a black box where the only thing we know about it is
that when we connect a 50 ohm resistor to it through a 50 ohm
transmission line, there are no standing waves.

In this case, a reflected wave could be used like a radar pulse to learn
what might be inside the box.

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


We certainly think similarly Keith. Thanks for the posting.

73, Roger, W7WKB

On Dec 29, 7:47*pm, Roger wrote:
Keith Dysart wrote:
[snipped]
I completely concur with your analysis.

No doubt you have fine tuned the analysis to notice that the current
stops (meaning becomes unobservable) at the identical instant that the
voltage spike (to double) is observed. *You would have noticed that the
zone of unmeasurable current spreads equally both ways from the
collision point at the velocity of the wave(s). The voltage spike
spreads in lock step with the loss of current detection. *The maximum
width of the loss of current and voltage spike is the width of either of
the pulses.

Now did the two pulses reflect, or pass through one another? *I have
considered the question and can not discern a difference in my analysis
either way. *IT SEEMS TO MAKE NO DIFFERENCE!

Agreed. Either view produces the same results.

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.

Roy and I are talking about this on other postings. *I guess the purest
might point out that a 50 ohm generator only has a voltage to current
ratio of 50, but we don't know if it also has a resistor to absorb
energy. *

All true.

It is like a black box where the only thing we know about it is
that when we connect a 50 ohm resistor to it through a 50 ohm
transmission line, there are no standing waves.

In this case, a reflected wave could be used like a radar pulse to learn
what might be inside the box.

Or slightly more precisely... An equivalent circuit that
will provide the same 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

We certainly think similarly *Keith. *Thanks for the posting.

...Keith

Roger wrote:
Roy and I are talking about this on other postings. I guess the purest
might point out that a 50 ohm generator only has a voltage to current
ratio of 50, but we don't know if it also has a resistor to absorb
energy. It is like a black box where the only thing we know about it is
that when we connect a 50 ohm resistor to it through a 50 ohm
transmission line, there are no standing waves.

Good for you, Roger, every reference I have on Thevenin
equivalent boxes say that what is happening inside the
box is irrelevant because it bears no resemblance to
reality.
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:
. . .
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.

. . .

That's an interesting and compelling argument. With the conditions you
describe, I don't see how it would be possible to tell whether the waves
reflected from each other or simply passed by without interacting.

But suppose we launch waves of different shapes from the two directions,
say a triangular wave and a rectangular one. Or perhaps make them
asymmetrical in some fashion. It seems to me that then we should be able
to tell which of the two possibilities happened. Being different, you
could argue that the reflection wouldn't be complete. But shouldn't we
expect some distortion of any part of a pulse that was acted upon by the
other?

I'll put my money on each of the waves arriving at the opposite end
unchanged. What do you predict will happen?

Roy Lewallen, W7EL

On Dec 30, 2:12*am, Roy Lewallen wrote:
Keith Dysart wrote:
. . .
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.

* . . .

That's an interesting and compelling argument. With the conditions you
describe, I don't see how it would be possible to tell whether the waves
reflected from each other or simply passed by without interacting.

But suppose we launch waves of different shapes from the two directions,
say a triangular wave and a rectangular one. Or perhaps make them
asymmetrical in some fashion. It seems to me that then we should be able
to tell which of the two possibilities happened. Being different, you
could argue that the reflection wouldn't be complete. But shouldn't we
expect some distortion of any part of a pulse that was acted upon by the
* other?

I'll put my money on each of the waves arriving at the opposite end
unchanged. What do you predict will happen?

I predict that the pulse arriving at the left end will
have the same voltage, current and energy profile as
the pulse launched at the right end and the pulse
arriving at the right end will be similar to the
one launched at the left.

They will appear exactly AS IF they had passed
through each other.

The difficulty with saying THE pulses passed
through each other arises with the energy. The
energy profile of the pulse arriving at the left
will look exactly like that of the one launched
from the right so it will seem that the energy
travelled all the way down the line for delivery
at the far end. And yet, from the experiment above,
when the pulses arriving from each end have the
same shape, no energy crosses the middle of the
line.

So it would seem that the energy that actually
crosses the middle during the collision is
exacly the amount of energy that is needed to
reconstruct the pulses on each side after the
collision.

If all the energy that is launched at one end
does not travel to the other end, then I am
not comfortable saying that THE pulse travelled
from one end to the other.

But I have no problem saying that the system
behaves AS IF the pulses travelled from one
end to the other.

On the other, it is completely intriguing that
a directional voltmeter could be placed anywhere
on the line and the voltage profile of the two
pulses can be recovered. And this is true even
at the middle of the line where, in the experiment
with identical pulse shapes, no current flows and
no energy crosses. But the shape of the two pulses
can still be recovered.

So in the end I say it is AS IF the voltage and
current pulses pass through each other, but the
energy does not necessarily do so. That way I
am not left with having to account for where the
reflected energy goes when it arrives back at
the source.

...Keith