Roy Lewallen wrote:
John Popelish wrote:

That's easy. RMS current is an AC measurement of current along the
conductor. Over any integer number of cycles, the total movement of
charge is zero. The current spends half the time going one way, and
half the time going the other way. This applies to both standing and
traveling wave induced currents. The only current that describes a
net movement of charge in a single direction is DC.


I see that Cecil is still having trouble with RMS, as well as with
current. Otherwise he couldn't have come up with the nonsense question

He seems to confuse energy in the wave traveling along a conductor
with the current it induces along that conductor, as it travels. I
have had a few such mental blocks and made a fool of myself a couple
times because I was sure I was right. But when the light finally came
on, lots of related things suddenly crystallized in my mind and I
jumped to a better understanding. One of my regrets is that I didn't
go back and apologize to my 7th grade science teacher for arguing with
him with so little tact, when I found out a year later that he had
been right and it was I who had been laboring under a misconception.
Same thing happened, on a different topic, in 8th grade science. So I
think I understand his attitude. I just hope that he sees that my
intentions are honorable, in this discussion. I am not attacking him,
but working for his understanding. I may be mistaken and end up
having another seventh grade moment here, but I'm not trying to
embarrass him.

In what direction is the RMS value of standing wave current flowing?

The RMS value of current doesn't flow. Charge flows, and current is the
rate at which it flows. RMS is one way of expressing the magnitude of a
time-varying current. In a steady state environment of pure sinusoidal
waveforms, any current can be expressed as Ipk * cos(wt + phi) where Ipk
is the peak value of the current, w (omega) is the rotational frequency,
and phi is the phase angle. This gives you precisely the value of
current at any instant in time, t. You can equally well express it as
Irms * cos(wt + phi) where Irms is the RMS value of the current. Nothing
is lost or gained by choosing one convention or the other, and using RMS
doesn't require abandoning the time varying or phase information. (In
EZNEC I chose to use RMS; NEC uses peak. They differ only by a constant
factor of the square root of 2. Both report phase angle along with
amplitude.) In either case, if you know or assume w, the current at any
instant is known if you know phi and either Ipk or Irms.

A point of clarification to John's posting:

When a standing wave exists on a transmission line, the phase of the
voltage or current is fixed (other than periodic phase reversals) with
position only if the end of the line is open or short circuited.
Otherwise, the phase of voltage and current will change with position.

Is that because the result is not a pure standing wave (superposition
of two equal and oppositely traveling waves), but a superposition of a
pair of traveling oppositely traveling waves of different amplitudes?

John Popelish wrote:
He seems to confuse energy in the wave traveling along a conductor with
the current it induces along that conductor, as it travels.

It's not confusion, John. It is engineering convention. Every
engineering reference book I have refers to current flow at
one point or another. Most of them also refer to power flow.
"Transmission Lines and Networks", by Walter C. Johnson even
refers to "The Conservation of Power Principle". Since there
is no such thing as an RF battery, we know exactly what Mr.
Johnson meant.

You are discussing the conventions used by physicists. Since
this is basically an RF engineering convention newsgroup, you
need to adjust your concepts accordingly or tell everyone that
you are nit-picking based on the conventions from the field of
pure physics.

In the engineering world: Power companies generate power and
transfer the power to the consumers over transmission lines.
RF transmitters generate power which is transferred over the
transmission line and radiated by the antenna.

There is always a convention for placing an arrow on a wire
to indicate direction of current flow, whether RMS AC or DC
or RMS RF. The AC conventions are left over from the DC
conventions. If you are trying to change those conventions,
please say so.

Food for thought: If an electron can pass through two different
holes at the same time, can it also travel in two directions
at the same time? Quantum physics says that is a possibility.

Is that because the result is not a pure standing wave (superposition of
two equal and oppositely traveling waves), but a superposition of a pair
of traveling oppositely traveling waves of different amplitudes?

Yes, but the definition of a standing wave is that the two waves
are of equal amplitudes. The wave you are describing is a hybrid
wave containing both a traveling wave and a standing wave. Any
real-world system contains hybrid waves in various ratios of
traveling waves to standing waves.
--
73, Cecil http://www.qsl.net/w5dxp

John Popelish wrote:
Roy Lewallen wrote:
John Popelish wrote:

A point of clarification to John's posting:

When a standing wave exists on a transmission line, the phase of the
voltage or current is fixed (other than periodic phase reversals) with
position only if the end of the line is open or short circuited.
Otherwise, the phase of voltage and current will change with position.

Is that because the result is not a pure standing wave (superposition of
two equal and oppositely traveling waves), but a superposition of a pair
of traveling oppositely traveling waves of different amplitudes?

Yes, but I wouldn't put it quite that way. I prefer to say that this is
simply a special case of the more general result you get when you sum
forward and reverse waves. Nothing magical or abrupt happens when the
two traveling waves are equal in amplitude -- if they're slightly
different, you get a little phase shift of the total current with
position along the wire, the current minima aren't quite zero, and the
spatial shape of the amplitude of the total current -- that is, the
shape of the standing wave -- isn't quite sinusoidal. Making the
amplitudes more and more different smoothly transitions the nature of
the total current until in the special case of the reverse traveling
wave being zero you have the distribution of a pure traveling wave.

Roy Lewallen, W7EL

Speaking as a lurker, I find Roy's and Tom's postings very educational and I
appreciate the time they take to do it.

I am a little dense, but I think I have learned four key points (at least,
key for me) from this material:

1. One can discuss transmission lines and antennas using pulse analysis or
steady-state analysis. When these two are mixed together the results can be
a mess.

2. When discussing "phase difference" we need to specify the two components
that have the difference. (I.e., phase difference between the current into
and out of an inductor is a different animal than the phase difference
between current and voltage at a specific point.)

3. Superposition ("adding together") of power computations is not valid in
reactive circuits.

4. Displacement current is as real as any other current when dealing with
antennas and their components. (I cannot remember "displacement current"
ever being mentioned back in the dark ages when I was in EE school. Perhaps
the school should remain nameless.)

Bill - W2WO

I'm very glad to hear that our postings are being read and considered.

Bill Ogden wrote:
Speaking as a lurker, I find Roy's and Tom's postings very educational and I
appreciate the time they take to do it.

I am a little dense, but I think I have learned four key points (at least,
key for me) from this material:

1. One can discuss transmission lines and antennas using pulse analysis or
steady-state analysis. When these two are mixed together the results can be
a mess.

True. You can actually translate from one to the other, but it requires
an FFT or its inverse. Attempts to mix the two nearly always leads to
invalid conclusions.

2. When discussing "phase difference" we need to specify the two components
that have the difference. (I.e., phase difference between the current into
and out of an inductor is a different animal than the phase difference
between current and voltage at a specific point.)

Yes, although we can use an arbitrary reference as long as it's the same
for all components. For example, if one current has a phase angle of 50
degrees relative to some arbitrary reference and the other has a phase
angle of 30 degrees relative to that same reference, we know that the
phase of the first relative to the second is 20 degrees.

3. Superposition ("adding together") of power computations is not valid in
reactive circuits.

It's never valid. Let me give you an example. Consider two AC or DC
voltage sources, each of 10 volts amplitude, with their negative
terminals connected together. (If they're AC, have them be of the same
frequency and in phase.) Connect a 10 ohm resistor between their
positive terminals. Superposition says that we can analyze the circuit
with each source individually and the other one turned off (short
circuited in the case of a voltage source), and add the results. What we
get should be the same answer as a full analysis with both the sources
on at the same time. So let's do it. Turn off source #2. The current
from source #1 through the resistor is 1 amp. The voltage across the
resistor is 10 volts. Now turn source #1 off and #2 on. The current
through the resistor is 1 amp going the other way than before, or -1
amp. The voltage across the resistor is 10 volts, but in the opposite
direction as before, or -10 volts. Adding the results gives a total of 0
amps through and 0 volts across the resistor. That's the right answer --
it's what we have when both sources are on. But now look at the power
dissipated by the resistor. With only source #1 on, it's I^2 * R = 1^2 *
10 = 10 watts. With only source #2 on, it's (-1)^2 * 10 = 10 watts. The
sum of the two is 20 watts, which is not the dissipation with both
sources on. Superposition does not apply to power, period. If it ever
seems to, it's only because of coincidence.

Don't be confused by the "forward" and "reverse" power concept. This is
not superposition and the concept must be used with great care to avoid
reaching invalid conclusions.

4. Displacement current is as real as any other current when dealing with
antennas and their components. (I cannot remember "displacement current"
ever being mentioned back in the dark ages when I was in EE school. Perhaps
the school should remain nameless.)

It's a useful concept, but also has to be used with care because it
isn't a real current consisting of movement of electrons. Current in one
conductor creates a field which induces current in another conductor,
making the current appear to have "flowed" from one conductor to the
other. The classic example is of course current flow "through" a
capacitor. "Displacement current" is a widely used term; it's in the
index of the first four EM texts I grabbed from the bookshelf. Of an
example of a parallel RC circuit in Kraus' _Electromagnetics_, he says,
"The current through the resistor is a *conduction current*, while the
current 'through' the capacitor may be called a *displacement current*.
Although the current does not flow through the capacitor, the external
effect is as though it did, since as much current flows out of one plate
as flows into the opposite one."

Displacement current appears in Ampere's law, one of the four Maxwell
equations. In one formulation it has the quantity i + d(phi)e/dt on one
side. The i is conduction current, and the derivative quantity is known
as the displacement current.

Roy Lewallen, W7EL

Roy Lewallen wrote:
I'm very glad to hear that our postings are being read and considered.

Bill Ogden wrote:

Speaking as a lurker, I find Roy's and Tom's postings very educational
and I
appreciate the time they take to do it.

I am a little dense, but I think I have learned four key points (at
least,
key for me) from this material:

1. One can discuss transmission lines and antennas using pulse
analysis or
steady-state analysis. When these two are mixed together the results
can be
a mess.


True. You can actually translate from one to the other, but it requires
an FFT or its inverse. Attempts to mix the two nearly always leads to
invalid conclusions.

2. When discussing "phase difference" we need to specify the two
components
that have the difference. (I.e., phase difference between the current
into
and out of an inductor is a different animal than the phase difference
between current and voltage at a specific point.)


Yes, although we can use an arbitrary reference as long as it's the same
for all components. For example, if one current has a phase angle of 50
degrees relative to some arbitrary reference and the other has a phase
angle of 30 degrees relative to that same reference, we know that the
phase of the first relative to the second is 20 degrees.

3. Superposition ("adding together") of power computations is not
valid in
reactive circuits.


It's never valid. Let me give you an example. Consider two AC or DC
voltage sources, each of 10 volts amplitude, with their negative
terminals connected together. (If they're AC, have them be of the same
frequency and in phase.) Connect a 10 ohm resistor between their
positive terminals. Superposition says that we can analyze the circuit
with each source individually and the other one turned off (short
circuited in the case of a voltage source), and add the results. What we
get should be the same answer as a full analysis with both the sources
on at the same time. So let's do it. Turn off source #2. The current
from source #1 through the resistor is 1 amp. The voltage across the
resistor is 10 volts. Now turn source #1 off and #2 on. The current
through the resistor is 1 amp going the other way than before, or -1
amp. The voltage across the resistor is 10 volts, but in the opposite
direction as before, or -10 volts. Adding the results gives a total of 0
amps through and 0 volts across the resistor. That's the right answer --
it's what we have when both sources are on. But now look at the power
dissipated by the resistor. With only source #1 on, it's I^2 * R = 1^2 *
10 = 10 watts. With only source #2 on, it's (-1)^2 * 10 = 10 watts. The
sum of the two is 20 watts, which is not the dissipation with both
sources on. Superposition does not apply to power, period. If it ever
seems to, it's only because of coincidence.

Don't be confused by the "forward" and "reverse" power concept. This is
not superposition and the concept must be used with great care to avoid
reaching invalid conclusions.

4. Displacement current is as real as any other current when dealing with
antennas and their components. (I cannot remember "displacement current"
ever being mentioned back in the dark ages when I was in EE school.
Perhaps
the school should remain nameless.)


It's a useful concept, but also has to be used with care because it
isn't a real current consisting of movement of electrons. Current in one
conductor creates a field which induces current in another conductor,
making the current appear to have "flowed" from one conductor to the
other. The classic example is of course current flow "through" a
capacitor. "Displacement current" is a widely used term; it's in the
index of the first four EM texts I grabbed from the bookshelf. Of an
example of a parallel RC circuit in Kraus' _Electromagnetics_, he says,
"The current through the resistor is a *conduction current*, while the
current 'through' the capacitor may be called a *displacement current*.
Although the current does not flow through the capacitor, the external
effect is as though it did, since as much current flows out of one plate
as flows into the opposite one."

Displacement current appears in Ampere's law, one of the four Maxwell
equations. In one formulation it has the quantity i + d(phi)e/dt on one
side. The i is conduction current, and the derivative quantity is known
as the displacement current.

Roy Lewallen, W7EL

Not everyone is happy with the term "displacement current." Albert
Shadowitz, in his book _The Electromagnetic Field_, has a chapter
entitled "The So-called Displacement Current." The term isn't in
the index to Feynman's _Lectures on Physics_. (At least I couldn't
find it.) All that is academic to the fact that AC current seems to
be able to make its way through a capacitor with no more opposition
than the capacitive reactance. Fortunately, no one on this
newsgroup has any objection to the way the term is commonly used.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:

Not everyone is happy with the term "displacement current." Albert
Shadowitz, in his book _The Electromagnetic Field_, has a chapter
entitled "The So-called Displacement Current." The term isn't in
the index to Feynman's _Lectures on Physics_. (At least I couldn't
find it.) All that is academic to the fact that AC current seems to
be able to make its way through a capacitor with no more opposition
than the capacitive reactance. Fortunately, no one on this
newsgroup has any objection to the way the term is commonly used.
73,
Tom Donaly, KA6RUH

That's interesting. It prompted me to look at my other electromagnetics
texts. Of the eight I have (Johnk, Jordan & Balmain, Kraus, Ida, Majid,
Holt, Ramo et al, and King), all include displacement current in the
index and all discuss the concept. Only King objects to its use,
although he notes that "The second term [in Ampere's law] was called the
'displacement current' by Maxwell, and this name continues to be used."
He goes on to say that "Actually this terminology is unfortunate because
the word displacement belongs to the old ether model and because the
word current means specifically moving charge." He adds further reasons
for his objection in the following paragraphs. With a copyright date of
1945, King's book (_Electromagnetic Engineering_, Vol. I) is the oldest
of the texts I have. Perhaps the term has become more acceptable as time
has passed. I do see why physicists such as Feynman wouldn't be
accepting of the term.

As I mentioned in my earlier posting, it does need to be used with care.
We have to always keep in mind that it isn't a real current and
therefore doesn't always behave like one. But it is a useful concept as
long as we stay aware of its limitations.

Roy Lewallen, W7EL

Tom Donaly wrote:
Not everyone is happy with the term "displacement current." Albert
Shadowitz, in his book _The Electromagnetic Field_, has a chapter
entitled "The So-called Displacement Current." The term isn't in
the index to Feynman's _Lectures on Physics_. (At least I couldn't
find it.) All that is academic to the fact that AC current seems to
be able to make its way through a capacitor with no more opposition
than the capacitive reactance. Fortunately, no one on this
newsgroup has any objection to the way the term is commonly used.

Here's an associated quote from "Electromagnetic Engineering"
by R.W.P King: "an adequate representation of the reactance
of a coil with a nonuniformly distributed current is NOT
POSSIBLE in terms of a coil with a uniform current [a lumped-
element inductance] connected in parallel with a lumped
capacitance."
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Tom Donaly wrote:

Not everyone is happy with the term "displacement current." Albert
Shadowitz, in his book _The Electromagnetic Field_, has a chapter
entitled "The So-called Displacement Current." The term isn't in
the index to Feynman's _Lectures on Physics_. (At least I couldn't
find it.) All that is academic to the fact that AC current seems to
be able to make its way through a capacitor with no more opposition
than the capacitive reactance. Fortunately, no one on this
newsgroup has any objection to the way the term is commonly used.


Here's an associated quote from "Electromagnetic Engineering"
by R.W.P King: "an adequate representation of the reactance
of a coil with a nonuniformly distributed current is NOT
POSSIBLE in terms of a coil with a uniform current [a lumped-
element inductance] connected in parallel with a lumped
capacitance."

I don't know what that has to do with displacement current, Cecil,
but if you're worried about it you can just use your coil at a frequency
where you get a more satisfactory current distribution. I made a coil
like you talk about (mine was 5.25 inches long,
27 turns, 6 inches in diameter) and it behaved pretty much like
a coil in parallel with a capacitor up to a few megahertz, at least.
Beyond that, it was a different story.
73,
Tom Donaly, KA6RUH

On Mon, 10 Apr 2006 20:52:06 GMT, "Tom Donaly"
wrote:

Not everyone is happy with the term "displacement current." Albert
Shadowitz, in his book _The Electromagnetic Field_, has a chapter
entitled "The So-called Displacement Current." The term isn't in
the index to Feynman's _Lectures on Physics_. (At least I couldn't
find it.) All that is academic to the fact that AC current seems to
be able to make its way through a capacitor with no more opposition
than the capacitive reactance. Fortunately, no one on this
newsgroup has any objection to the way the term is commonly used.

Hi Tom, and others,

The "labeled" currents span a much too small arena. There are also
the induced currents (no, not necessarily from flux linkage) and
convection currents (which IS the primary correlative to the induced
current).

The convection currents are possibly the only current that attain the
speed of light velocity. The others are so astronomically slow, that
it is arguable to say that any current (electron/hole transport) in a
wire is any more significant than that that is supposed to never cross
through the dielectric of a capacitor.

In other words, the displacement current is labeled fictitious because
no electron ever moves from one plate to the other. Now, if we simply
substitute solid gold for that dielectric (still maintaining the same
plates); then no electron ever makes it from one plate to the other -
and yet current flows in the entire AC circuit by proportion to the
impedance presented to it by either the dielectric capacitor, or the
gold capacitor.

This, of course, illustrates the corruption of usage in the term
"current."

73's
Richard Clark, KB7QHC