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.