RadioBanter - Current through coils

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Current through coils

Roy Lewallen wrote:
If Cecil's analysis shows, or his theory requires, that the result be
different when adding the responses to traveling current waves than it
is by calculating the response directly from the total current, then the
analysis or theory is wrong. Superposition requires that the two results
be identical.

There is a phase shift through the coil for the individual
phasors. When the phasors are superposed, that phase shift
information disappears. That's what I meant by my statement.
The results are the same but information is lost.

Please see http://www.qsl.net/w5dxp/current.htm "Why the net
current is not constant through a loading coil" and take a look
at the phase of the net current. It is unchanging. Those phasors
are copied directly from "Optics" by Hecht.

What I was talking specifically about is the phase shift through
the coil so let's discuss that one limited technical subject.

The form of the forward traveling wave current is function(kz+wt)
The form of the reflected traveling wave current is function(kz-wt)
When we superpose those two waves we get the standing wave current.
The form of the standing wave current is function(kz)*function(wt)
A lot of information, including all phase information, has been lost
in that superposition process.

The standing wave current is obviously not like the traveling wave
currents because the equations are different. As Gene Fuller said
earlier, it has been stripped of all phase information by the
superposition process. You pointed out a couple of days ago that the
phase of the standing wave current is virtually constant from feedpoint
to the tip of the antenna while the phase of the traveling waves are
certainly not constant.

I have asked this technical question before and no one has answered
it. Given that the standing wave current would indicate a phase
shift of zero in 45 degrees of a wire antenna, what does that imply
for using the standing wave current to measure the number of degrees
of the antenna occupied by the loading coil?

If the standing wave current cannot determine the phase shift in
a wire, why does anyone think it can determine the phase shift
in a wire formed into a coil?

Kraus and EZNEC tell us that the standing wave phase shift is zero
from tip to tip in a 1/2WL thin-wire dipole. Why is it a surprise
that if we replace part of that antenna with a loading coil the
standing wave phase shift doesn't change and is still zero? What
useful information does knowing that provide?

Since the standing wave current phase is unchanging, how can it
be used to determine how much of an antenna has been replaced
by a loading coil?

You and Tom have used standing wave current for your measurements.
Delays and phases cannot be measured using standing wave current
because standing wave current doesn't contain any phase related
information. As Gene said, it lost all phase information in the
superposition. All we can gather from the standing wave current is
that the forward current and reflected current phasors are rotating
in opposite directions. The delay experienced by the traveling waves
is hidden by the superposition process.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils

On Sun, 19 Mar 2006 04:57:54 GMT, Cecil Moore
wrote:

Please see http://www.qsl.net/w5dxp/current.htm

I refer to the diagram in the section entitled "What EZNEC Says About
Current Distribution Using Inductive Loading Stubs"

You use the diagram to assert that there is "not a lot of difference
between inductive loading stubs and loading coils" by comparing the
current distribution with another case.

You show graphically the current on each side of the stub. You do not
show the current in each wire of the stub or the sum of the currents
in the stub.

EZNEC calculates the currents in each wire of the stub? Aren't those
currents a relevant detail that you have omitted from the diagram.

Owen
--

Current through coils

Owen Duffy wrote:
On Sun, 19 Mar 2006 04:57:54 GMT, Cecil Moore
wrote:

Please see http://www.qsl.net/w5dxp/current.htm

I refer to the diagram in the section entitled "What EZNEC Says About
Current Distribution Using Inductive Loading Stubs"

You use the diagram to assert that there is "not a lot of difference
between inductive loading stubs and loading coils" by comparing the
current distribution with another case.

You show graphically the current on each side of the stub. You do not
show the current in each wire of the stub or the sum of the currents
in the stub.

EZNEC calculates the currents in each wire of the stub? Aren't those
currents a relevant detail that you have omitted from the diagram.

I don't quite follow the theory on the web page, but what does it
predict should happen if there were no antenna at all, and the inductor
were connected to a simple series RC circuit instead of the whip?

I've taken the EZNEC model available there and modified it by replacing
the whip with a wire to ground from the top of the coil
(http://eznec.com/misc/test316_modified.EZ). I added a lumped impedance
in that wire to represent the impedance of the vertical wire I
deleted(*). The feedpoint impedance is the same as for the original
model, and the currents at the top and bottom of the inductor are almost
exactly the same as for the original model. Can the traveling wave
analysis be used to explain the inductor currents in this model? Is
traveling wave analysis necessary to explain them?

(*) The impedance inserted in the new wire isn't equal to the impedance
of the top wire driven against ground. The reason is that the new wire
to ground does radiate some, does have significant impedance itself, and
does interact with the inductor. The modified system, however, is quite
obviously very different in radiating properties from the original, and
isn't too different from a lumped RC load.

Roy Lewallen, W7EL

Current through coils

Cecil Moore wrote:
Please turn your technical expertise on this example which I
have asked you about many times with no response from you:

http://www.qsl.net/w5dxp/current.htm

At the bottom of the page, the coil is seen to have 0.17 amps
at the bottom and 2.0 amps at the top. With your lumped inductor
way of thinking, how is that possible?

Yes it is possible. There is no difference between doing things as
lumped components or standing wave models.

The only disagreement is you seem to claim some very odd things about
current not flowing, unless in the course of 400 posts you have
corrected that.

This entire thread reminds me of the Fractenna threads of years ago.

73 Tom

Current through coils

Roy Lewallen wrote:
. . .
I've taken the EZNEC model available there and modified it by replacing
the whip with a wire to ground from the top of the coil
(http://eznec.com/misc/test316_modified.EZ). I added a lumped impedance
in that wire to represent the impedance of the vertical wire I
deleted(*). The feedpoint impedance is the same as for the original
model, and the currents at the top and bottom of the inductor are almost
exactly the same as for the original model. Can the traveling wave
analysis be used to explain the inductor currents in this model? Is
traveling wave analysis necessary to explain them?

(*) The impedance inserted in the new wire isn't equal to the impedance
of the top wire driven against ground. The reason is that the new wire
to ground does radiate some, does have significant impedance itself, and
does interact with the inductor. The modified system, however, is quite
obviously very different in radiating properties from the original, and
isn't too different from a lumped RC load.

Notice that the current into the grounded wire at the bottom of the coil
is about 1 amp, and the current going into ground at the grounded end of
the added wire is about 0.56 amp. So where is the extra current for the
coil bottom wire coming from? The answer is displacement current from
the coil. That is, the coil is capacitively coupled to ground, and this
causes displacement current from the coil to ground. The effect is
greatest at the end of the coil which is farthest from the source. A
decent model of the coil is an L network, with a series L, and a shunt C
to ground from the far end. This is all that's necessary to explain the
drop in current from the bottom to the top; no current waves, standing
or traveling, no transmission line analysis are required.

If you're not convinced, try this. Change the ground type to free space.
Then connect the bottoms of the two formerly grounded wires together
with another wire. You'll see that the current at the top of the coil is
now very nearly the same as at the bottom. We haven't changed any waves,
antenna lengths, or anything else related to antennas or waves. All
we've done is to eliminate the other side of the capacitor -- we've
removed the C in the equivalent lumped L network.

A simple lumped component model explains the difference between grounded
and free space models just fine. How well does the traveling wave theory
do at it?

Roy Lewallen, W7EL

Current through coils

Owen Duffy wrote:

Cecil Moore wrote:
Please see http://www.qsl.net/w5dxp/current.htm

I refer to the diagram in the section entitled "What EZNEC Says About
Current Distribution Using Inductive Loading Stubs"

You use the diagram to assert that there is "not a lot of difference
between inductive loading stubs and loading coils" by comparing the
current distribution with another case.

You show graphically the current on each side of the stub. You do not
show the current in each wire of the stub or the sum of the currents
in the stub.

The currents in stubs cannot be displayed very well at full size in
EZNEC just as the currents in coils cannot be displayed very well.
Maybe an enlarged view would show it. I will try to do that.

EZNEC calculates the currents in each wire of the stub? Aren't those
currents a relevant detail that you have omitted from the diagram.

Remember the present discussion is about the ability to use standing
wave current phase to measure the electrical length of a wire or a
coil. I have run the currents that you mention. The phase of the current
is almost constant through the stubs. The phase of the current is
almost constant through the coils. Would you like to see a list
of the current at points through the stub Vs the current at points
through the coil?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils

Roy Lewallen wrote:
Can the traveling wave
analysis be used to explain the inductor currents in this model?

In this new configuration, the traveling wave current
encounters a short-circuit to ground instead of the
open-circuit in a normal antenna. And that forward current
is reflected by that short circuit. In the shorted case
its phase doesn't change so the forward and reflected
currents add instead of subtrace. But their phasors are
still rotating in opposite directions. Please note that
the phase shift in the standing wave current is almost
zero throughout the system, i.e. standing wave phase
information has still been lost. We still don't know
the electrical length of the coil for the same reasons
we didn't know it before. Below 'func' stands for
'function of'.

The standing wave current reported by EZNEC is of the
form: func(kz)*func(wt) = fun(kz+wt) + func(kz-wt)

Is there any way in EZNEC to subtract out the func(kz-wt)
reflected term and leave just the forward term?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils

Roy Lewallen wrote:
In a linear system like an antenna or transmission line, superposition
applies. ... We must get exactly the same result, ...
If Cecil's analysis shows, or his theory requires, that the result be
different when adding the responses to traveling current waves than it
is by calculating the response directly from the total current, then the
analysis or theory is wrong. Superposition requires that the two results
be identical.

Superposition does not require that all information be preserved
through the superposition process. Here's an example:

Two people are across the room from each other with a coax cable running
between them. They each have two identical PSK modems but only one coax.
When they use one modem pair, any information one sends is received 100%
by the other. When they use the other modem pair, any information one
sends is received 100% by the other. When they superpose the signals
over the single coax line, all phase information is lost. The superposition
results are the same but all phase information is lost in the process.
The rules of superposition do not apply to the phase information content.
The phase information is lost in the process of superposition.

In like manner, the rules of superposition do not apply to the ability
of a standing wave to detect phase shift. The phase information is
lost in the process of superposition. One cannot use standing wave
current phase to measure the electrical length of a wire or a coil.

A 75m wire dipole is known to be close to 90 degrees long from the
feedpoint to the tip. EZNEC says the current changes phase by 2.5
degrees. How can current change phase by only 2.5 degrees in an
antenna wire known to be 90 degrees long?

Since EZNEC's standing wave current cannot detect a phase shift in
90 degrees of wire, why should it detect a phase shift in a coil?
If standing wave current measurements cannot detect a phase shift
in 90 degrees of wire, why should anyone's measurements detect a
phase shift in a coil?
--
73, Cecil http://www.qsl.net/w5dxp