Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL

Cecil Moore wrote:
Walter Maxwell wrote:

Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.

On Sat, 04 Oct 2003 18:00:39 -0700, Roy Lewallen wrote:

Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL

Well, Roy, if what you say above is true then why does the phase of reflected
voltage change 180 degrees and reflected current does not change when the
forward waves encounter a perfect short-circuit termination?

And on the other hand, why does the phase of reflected current change 180
degrees and reflected voltage does not change when the forward waves encounter a
perfect open-circuit termination?

How then can the reflected voltage and current be other than 180 degrees
regardless of the load?

If what you say is true then my explanation in Reflections concerning the
establishment of the standing wave must be all wrong. Is this what you're
saying?

Walt, W2DU

Cecil Moore wrote:
Walter Maxwell wrote:

Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.

Roy Lewallen wrote:
Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Optics doesn't have the luxury of only two directions so Kirchhoff's
conventions are meaningless for light. With light scattering in any
number of directions in 3D space, 3D optical engineers must be a little
more careful than 1D RF engineers. :-)

Obviously, light and RF waves obey the same physics but an RF transmission
line is essentially a one-dimensional environment with a plus and minus
direction. I assume fiber-optics is subject to that same simplification.
--
73, Cecil http://www.qsl.net/w5dxp



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Walter Maxwell wrote:
How then can the reflected voltage and current be other than 180 degrees
regardless of the load?

Consider a transmission line driven by two identical sources which are
signal generators with circulators+loads designated by SGCL.

SGCL1-----------50 ohm coax----------SGCL2

All the power sourced by SGCL1 is dissipated in SGCL2 and all the power
sourced by SGCL2 is dissipated in SGCL1. The system is perfectly
symmetrical. Will there be ordinary standing waves? Of course. The voltage
and current from SGCL1 are in phase. The voltage and current from SGCL2
are in phase. The only difference between the two currents is
Kirchhoff's convention. When the voltages are maximum at the same point,
they superpose to 2*V. When the two currents are maximum at the same point,
they superpose to zero because they are traveling in opposite directions.

We would say that SGCL2's voltage and current are 180 degrees out of
phase.

Someone looking at the experiment from the other side of the screen
would say that SGCL1's voltage and current are 180 degrees out of phase.

It is only a convention, one that doesn't exist for 3D light.
--
73, Cecil http://www.qsl.net/w5dxp



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I'm not at all saying your explanation is wrong. I'm just pointing out
the effect of defining the direction of current. You say that the phase
of the reflected current changes 180 degrees. Another way to say the
exactly the same thing is that the current doesn't change phase but
reverses direction. The phase of the current depends on its direction,
so is affected by how we define "positive" direction. The phase of
voltage, on the other hand, isn't.

Consider a short circuit. At that point, Vr = -Vf. That, we know from
the requirement that the total V, the sum of Vr and Vf, has to be zero.
But how about the current? The magnitude of the reflected current equals
the magnitude of the forward current. At the short, the current isn't
zero -- it's twice If. Assuming that If is always defined as being
positive toward the load, let's first define the positive direction of
Ir as also being toward the load. Then the total current at any point is
If + Ir. At the short, it's If + Ir = 2 * If, which says that Ir = If.
You can say that the phase of the current hasn't changed as a result of
the reflection. But if we define the positive direction of Ir as being
toward the source, then the total current at any point on the line is If
- Ir. At the short it's If - Ir = 2 * If, so Ir = -If. So the phase of
Ir is 180 degrees relative to the phase of If. Of course, it's also
traveling in the opposite direction, by definition. So you have your
choice. You can say that the reflected current is flowing in the same
direction as the forward current, and with the same phase. Or you can
equally correctly say that the current has reversed both direction and
phase due to the reflection. They're exactly equivalent, both give
correct mathematical results, and are equally valid.

The same reasoning applied at an open circuit, where the total current
is zero, shows that when Ir is defined as positive toward the load, the
total current = If + Ir = 0 means Ir = -If. In other words, the
reflected current, defined as being in the same direction as the forward
current, has undergone a 180 degree phase shift. But if Ir is deemed
positive toward the source, then the total current is If - Ir = 0, so we
say that the reflected current has undergone a reversal of direction but
no change in phase.

As long as we always calculate the total current by using Kirchoff's
principle as If + Ir if Ir is positive toward the load, or If - Ir if
it's positive toward the source, all results are valid.

Among the consequences of the two possible definitions of positive
direction for Ir is that the current reflection coefficient Ir/If can be
either equal to the voltage reflection coefficient, or its negative.
And, as in earlier postings, one can conclude that Vr/Ir can equal
either Z0 or -Z0. Both depend on the definition of the positive
direction of Ir (assuming that If is consistently defined as positive
toward the load, which is a good assumption).

The need to be careful with the definition, and always making it clear,
is illustrated by the fact that of the first four fields/transmission
line texts I pulled off my shelf, two (Holt and Johnson) defined the
positive direction of If toward the load, and two (Johnk and Kraus)
toward the source. So you can't make an assumption that the definition
is even usually one way or the other.

Roy Lewallen, W7EL

Walter Maxwell wrote:
On Sat, 04 Oct 2003 18:00:39 -0700, Roy Lewallen wrote:


Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL


Well, Roy, if what you say above is true then why does the phase of reflected
voltage change 180 degrees and reflected current does not change when the
forward waves encounter a perfect short-circuit termination?

And on the other hand, why does the phase of reflected current change 180
degrees and reflected voltage does not change when the forward waves encounter a
perfect open-circuit termination?

How then can the reflected voltage and current be other than 180 degrees
regardless of the load?

If what you say is true then my explanation in Reflections concerning the
establishment of the standing wave must be all wrong. Is this what you're
saying?

Walt, W2DU


Cecil Moore wrote:

Walter Maxwell wrote:


Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.

Roy Lewallen wrote:
I'm not at all saying your explanation is wrong. I'm just pointing out
the effect of defining the direction of current. You say that the phase
of the reflected current changes 180 degrees. Another way to say the
exactly the same thing is that the current doesn't change phase but
reverses direction. The phase of the current depends on its direction,
so is affected by how we define "positive" direction. The phase of
voltage, on the other hand, isn't.

It might be easier to visualize using E & H fields and the right hand
rule. Point your thumb in the direction of wave travel (North). The orthogonal
index finger (up) represents the E-field and the orthogonal middle finger (East)
represents the H-field. Then turn the thumb in the opposite direction (South)
while keeping the index finger (E-field) pointed in the same direction (up).
The middle finger (H-field) will reverse direction by 180 degrees (West) but the
orthogonal relationships between the direction of travel and the fields are
still identical. Just another way of visualizing what you said above.
--
73, Cecil http://www.qsl.net/w5dxp



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On Sun, 05 Oct 2003 00:27:28 -0700, Roy Lewallen wrote:

I'm not at all saying your explanation is wrong. I'm just pointing out
the effect of defining the direction of current. You say that the phase
of the reflected current changes 180 degrees. Another way to say the
exactly the same thing is that the current doesn't change phase but
reverses direction. The phase of the current depends on its direction,
so is affected by how we define "positive" direction. The phase of
voltage, on the other hand, isn't.

Consider a short circuit. At that point, Vr = -Vf. That, we know from
the requirement that the total V, the sum of Vr and Vf, has to be zero.
But how about the current? The magnitude of the reflected current equals
the magnitude of the forward current. At the short, the current isn't
zero -- it's twice If. Assuming that If is always defined as being
positive toward the load, let's first define the positive direction of
Ir as also being toward the load. Then the total current at any point is
If + Ir. At the short, it's If + Ir = 2 * If, which says that Ir = If.
You can say that the phase of the current hasn't changed as a result of
the reflection. But if we define the positive direction of Ir as being
toward the source, then the total current at any point on the line is If
- Ir. At the short it's If - Ir = 2 * If, so Ir = -If. So the phase of
Ir is 180 degrees relative to the phase of If. Of course, it's also
traveling in the opposite direction, by definition. So you have your
choice. You can say that the reflected current is flowing in the same
direction as the forward current, and with the same phase. Or you can
equally correctly say that the current has reversed both direction and
phase due to the reflection. They're exactly equivalent, both give
correct mathematical results, and are equally valid.

The same reasoning applied at an open circuit, where the total current
is zero, shows that when Ir is defined as positive toward the load, the
total current = If + Ir = 0 means Ir = -If. In other words, the
reflected current, defined as being in the same direction as the forward
current, has undergone a 180 degree phase shift. But if Ir is deemed
positive toward the source, then the total current is If - Ir = 0, so we
say that the reflected current has undergone a reversal of direction but
no change in phase.

As long as we always calculate the total current by using Kirchoff's
principle as If + Ir if Ir is positive toward the load, or If - Ir if
it's positive toward the source, all results are valid.

Among the consequences of the two possible definitions of positive
direction for Ir is that the current reflection coefficient Ir/If can be
either equal to the voltage reflection coefficient, or its negative.
And, as in earlier postings, one can conclude that Vr/Ir can equal
either Z0 or -Z0. Both depend on the definition of the positive
direction of Ir (assuming that If is consistently defined as positive
toward the load, which is a good assumption).

The need to be careful with the definition, and always making it clear,
is illustrated by the fact that of the first four fields/transmission
line texts I pulled off my shelf, two (Holt and Johnson) defined the
positive direction of If toward the load, and two (Johnk and Kraus)
toward the source. So you can't make an assumption that the definition
is even usually one way or the other.

Thanks, Roy, for the lucid explanation. I had not previously thought of the
interplay between direction and polarity, as you have so clearly pointed out. I
have always considered a minus rho for current when rho is positive for voltage,
bu I didn'[t carry the thought through far enough.

Walt, W2DU


Roy Lewallen, W7EL

Walter Maxwell wrote:
On Sat, 04 Oct 2003 18:00:39 -0700, Roy Lewallen wrote:


Whenever you're dealing with current, you have to pay attention to the
definition of positive direction. If you define the positive direction
of forward current as being toward the load and of reflected current
toward the source, then Vf is in phase with If and Vr is in phase with
Ir. I suspect that a similar caution needs to be heeded when dealing
with optics.

Roy Lewallen, W7EL


Well, Roy, if what you say above is true then why does the phase of reflected
voltage change 180 degrees and reflected current does not change when the
forward waves encounter a perfect short-circuit termination?

And on the other hand, why does the phase of reflected current change 180
degrees and reflected voltage does not change when the forward waves encounter a
perfect open-circuit termination?

How then can the reflected voltage and current be other than 180 degrees
regardless of the load?

If what you say is true then my explanation in Reflections concerning the
establishment of the standing wave must be all wrong. Is this what you're
saying?

Walt, W2DU


Cecil Moore wrote:

Walter Maxwell wrote:


Sorry, Cecil, the phase between reflected voltage and current is
always 180
degrees, not zero.


Yep, I know better, I just mis-spoke. Did you know that there is no
such convention for light? It's Kirchhoff's current convention that
dictates a 180 degree phase between reflected voltage and reflected
current. EM light doesn't follow Kirchhoff's convention.

For EM light, there is no phase shift in the reflection if the index
of refraction is higher. If the index of refraction is lower, there
is a 180 degree phase shift in both E and H fields.

On Sat, 04 Oct 2003 17:31:31 -0500, Cecil Moore
wrote:

everything can be explained by achieving a conjugate match at
one point on the transmission line when the reactance looking
in either direction is at a maximum.

Hi Cecil,

I don't know which is funnier: that you have a
one-solution-answers-every-question; or that you have so many of them.

Reach into your bag and present us the conjugate for:
source50---50 ohm feedline---+---150 ohm feedline---load150
or the rather more terse (and simpler - bound to confound):
source=200Ohm(resistive)---50 ohm feedline---load=200Ohm(resistive)



73's
Richard Clark, KB7QHC

On Sat, 4 Oct 2003 19:49:19 +0000 (UTC), "Reg Edwards"
wrote:

SWR meters are designed to operate and provide indications of SWR, Rho,
Fwd
Power, Refl.Power, on the ASSUMPTION that the internal impedance of the
transmitter is 50 ohms. It makes the same INCORRECT assumption as a lot
of
people do. This should not be surprising because it was people who
designed
it.

So SWR meters nearly always give FALSE indications about what actually
exists.

------------------------------------------------------
Reg,

BTW, I did force the SWR meter to see a different source impedance. There
was no difference in SWR readings for either the 1:1 or 2:1 case.

------------------------------------------------------

Tarmo,

And of course, as you and I know, on whatever line there is between the
meter and transmitter, the swr is neither the indicated 1:1 nor 2:1 because
the input impedance looking back towards the tranmitter is not the assumed
50 ohms. Both readings are false, even meaningless. There may in fact be no
standing waves to measure.

To avoid confusing novices and budding engineers, retarding education,
rename the meter the TLI (Transmitter Loading Indicator) which is what it
really is.
----
Reg, G4FGQ


Reg, can you furnish a a mathematical expression that includes source resistance
as a required parameter for determining SWR?

Walt, W2DU

Correction:

On Sun, 05 Oct 2003 00:27:28 -0700, Roy Lewallen wrote:
. . .

(Last paragraph):

The need to be careful with the definition, and always making it clear,
is illustrated by the fact that of the first four fields/transmission
line texts I pulled off my shelf, two (Holt and Johnson) defined the
positive direction of If toward the load, and two (Johnk and Kraus)
^^
toward the source. . .

That should be Ir, not If. I've never seen the positive direction of If
defined as anything other than toward the load.

Roy Lewallen, W7EL