Current across the antenna loading coil - from scratch
 

Richard Harrison wrote:


Displacement current which is the a-c current through a capacitor, that
has no d-c conduction, is not the "ONLY" thing that allows a conductor to
have a current taper along its length..


Richard,

That is incorrect, and even Terman never said such a thing.

Charge is not created or destroyed. It either keeps moving as current,
or it is stored. Charge storage is the equivalent of displacement
current. The terminology is slightly confusing at times, but it has not
changed for over 100 years.

Detailed discussions of this topic are found in virtually every
intermediate and advanced textbook on electricity and magnetism.

73,
Gene
W4SZ

Current across the antenna loading coil - from scratch
 

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

Current across the antenna loading coil - from scratch
 

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

Current across the antenna loading coil - from scratch
 

Not that I could fan the flames any more anyhow, but just what was the
original discussion about anyhow?

As in Cecil says what, and those disagreeing with him say what?

I'm curious how something that doesn't seem that complex can generate
so many weeks of acrimony and vitriol! I don't know the answer - but
then again, I'm not really sure what the question is. But I do know
where to look it up....

- 73 de Mike KB3EIA -

Current across the antenna loading coil - from scratch
 

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

Current across the antenna loading coil - from scratch
 

Check my article that describes the controversy, shows some proof of reality
and then efforts of the "gurus" to deny it and "reason" why it can't be so.
http://www.k3bu.us/loadingcoils.htm
The problem is that back in 1953 in QST article there was erroneous
conclusion/statement made, which propagated through the books, until W9UCW
measured the current across the loading coils and found that there is
significant drop from one end to the other, and the rest is (ongoing)
history

Yuri, K3BU.us


"Michael Coslo" wrote in message
...
Not that I could fan the flames any more anyhow, but just what was the
original discussion about anyhow?

As in Cecil says what, and those disagreeing with him say what?

I'm curious how something that doesn't seem that complex can generate so
many weeks of acrimony and vitriol! I don't know the answer - but then
again, I'm not really sure what the question is. But I do know where to
look it up....

- 73 de Mike KB3EIA -

Current across the antenna loading coil - from scratch
 

Richard Harrison wrote:
A conductor can lose energy
through dissipation and radiation forever, not just relocate it
temporarily through storage in a reactance.

The dissipation line at the end of a rhombic antenna does not handle the
entire output of the transmitter at its other end. Most of the energy is
already radiated by the time it reaches the dissipation line.

For instance, consider 100 ft. of 50 ohm coax with losses of
3 dB driving a 50 ohm load from a source of 200 watts.

At the source, we have 100 volts at 2 amps. At the load, we
have 70.7 volts at 1.414 amps. The current dropped by exactly
the same amount as the voltage. Hint: The V/I ratio must be
maintained at 50 ohms for flat lines.

Anyone who doesn't understand RF H-field (current) drop in
a lossy transmission line has probably been so seduced by his
lumped circuit model that he thinks the model dictates reality
instead of vice versa.
--
73, Cecil http://www.qsl.net/w5dxp

Current across the antenna loading coil - from scratch
 

Gene Fuller wrote:
Sorry, you cannot pick and choose which displacement currents to consider.

Why not? All I (and probably Yuri) have ever been considering
are displacement currents to earth ground from the coil. That
is the only current flowing sideways from the coil to ground.
--
73, Cecil http://www.qsl.net/w5dxp

Current across the antenna loading coil - from scratch
 

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

Current across the antenna loading coil - from scratch
 

Michael Coslo wrote:
Not that I could fan the flames any more anyhow, but just what was the
original discussion about anyhow?

As I realized what the actual misconception really is, the discussion
shifted from coils to standing waves. Seems to me, W8JI and W7EL do
not understand the difference implied by these two different equations
(assuming |Ifor|=|Iref|).

Ifor = I1*cos(kx+wt) and Iref = I1*cos(kx-wt)

Istnd = I1*cos(kx+wt) + I1*cos(kx-wt) = I2*cos(kx)*cos(wt)

Gene Fuller has kindly explained the difference but W8JI and W7EL
seemed to have ignored his explanation. Gene says there is no
phase information in standing wave current phase and I agree.

As in Cecil says what, and those disagreeing with him say what?

Looking at: http://www.qsl.net/w5dxp/travstnd.GIF

I say the RMS standing wave current's unchanging phase on the right
hand graph, cannot be used to measure phase. W7EL continues to report
those phase measurements as valid indicators of delay through coils
when installed in standing wave environments. I say all the phase
information in the standing wave current is in its magnitude which
is a cosine function as explained in Kraus and Terman. W8JI and W7EL
both dismiss the phase information in the standing wave magnitude
and insteadtrust the standing wave phase to yield valid delay
measurements.

I'm curious how something that doesn't seem that complex can
generate so many weeks of acrimony and vitriol! I don't know the answer
- but then again, I'm not really sure what the question is. But I do
know where to look it up....

Now you know what the argument is about. Seems to me, W8JI, W7EL,
and others possess misconceptions caused by assuming the unproven
presuppositions of their lumped circuit model. They "prove" their
misconceptions by making measurements known to be invalid. I can't
tell if they are aware of what they are doing or not.

If you know where to look for the answer, please tell us. I have
looked and only found a clear explaination in "Optics" by Hecht.

A side argument is whether standing wave current can drop to
zero at a node in an unterminated transmission line. W8JI continues
to assert that current cannot drop without some imagined third path.
--
73, Cecil http://www.qsl.net/w5dxp