Current through coils
 

Roy Lewallen wrote:
John Popelish wrote:

You keep going back to how lumped components can mimic actual
distributed ones (over a narrow frequency range). I get it. I have
no argument with it. But why do you keep bringing it up? We are
talking about a case that is at least a border line distributed device
case. I am not interested in how it can be modeled approximately by
lumped, ideal components. I am interested in understanding what is
actually going on inside the distributed device.


I'm sorry I haven't explained this better. If we start with the inductor
in, say, the example antenna on Cecil's web page, we see that the
magnitude of current at the top of the inductor is less than at the
bottom of the inductor. Cecil has promoted various theories about why
this happens, mostly involving traveling wave currents and "replacement"
of "electrical degrees" of the antenna. He and others have given this as
proof that the current at the two ends of an inductor are inherently
different, regardless of its physical size.

I agree up till you add, "regardless of physical size". I have seen
him talk only about large air core space wound coils. I came to the
discussion late, but this is what I have seen.

My counter argument goes
something like this:

1. If we substitute a lumped component network for the antenna, there
are no longer traveling waves -- along the antenna at least -- and no
number of "missing electrical length" for the inductor to replace. Or if
there is, it's "replacing" the whole antenna of 90 degrees. Yet the
currents in and out of the inductor are the same as they were before. I
feel this is adequate proof of the invalidity of the "replacement" and
traveling wave arguments, since I can reproduce the same results with
the same inductor without either an antenna or traveling waves. This is
shown in the modified EZNEC file I posted.

But what is the need for such an argument? Just to prove that lumped
component networks can model real, distributed things? I get that.
As I see Cecil's point (and I hate to say this with him absent), it is
that real, large coils with all their poor turns coupling and stray
capacitance both turn to turn and more important, to ground, take a
lot of those lumped components to model, accurately, but only their
own self, described by distributed network concepts to model, accurately.

2. The argument that currents are inherently different at the ends of an
inductor is shown to be false by removing the ground in the model I
posted and replacing it with a wire. Doing so makes the currents nearly
equal.

But the ground is there, in the application under discussion. All
components act differently if you connect them to something else.
This coil is connected to ground by its capacitance.

3. Arguments have then been raised about the significance of the wire
and inductor length, and various theories traveling waves and standing
waves within the length of the coil. Let's start with the inductor and
no ground, with currents nearly equal at both ends. Now shrink the coil
physically by shortening it, changing its diameter, introducing a
permeable core, or whatever you want, until you get an inductance that
has the same value but is infinitesimal in physical size. For the whole
transition from the original to the lumped coil, you won't see any
significant(*) change in terminal characteristics, in its behavior in
the circuit, or the behavior of the whole circuit.

Sounds reasonable to me. But it is not the application in question.

So I conclude there's
no significant electrical difference in any respect between the physical
inductor we started with and the infinitesimally small lumped inductor
we end up with. And from that I conclude that any explanation for how
the original inductor worked must also apply to the lumped one.

But only if you reduce the capacitance to ground to a low enough value.

That's
why I keep bringing up the lumped equivalents. We can easily analyze the
lumped circuit with elementary techniques; the same techniques are
completely adequate to fully analyze the circuit with real inductor and
capacitance to ground.

(*) I'm qualifying with "significant" because the real inductor doesn't
act *exactly* like a lumped one. For example, the currents at the ends
are slightly different due to several effects, and the current at a
point along the coil is greater than at either end due to imperfect
coupling among turns. But the agreement is close -- very much closer
than the alternative theories predict (to the extent that they predict
any quantitative result).

I have no argument with any of that.

(snip)
Or start with a less simplified theory that covers all cases, so you
don't have to decide when to switch tools.


That's fine, too. Will Cecil's theory explain the behavior of a lumped
constant circuit? Or everywhere along the transition between the
physical inductor and lumped circuit I described above?

Distributed network theory includes the possibility of lumped
components, it is just not limited to them.

(snip)
(if you add to that model, the appropriate lumped capacitors at the
appropriate places)


No. The inductor itself can be adequately modeled as a lumped inductor
without any capacitors at all.

Not if it is located in close proximity to ground, as this coil in
question is located. It does not act like any kind of pure
inductance, but as a network that contains some inductance and also
some other effects.

When you add ground to the model, you
have to add the equivalent shunt C to the lumped model. The C isn't a
property of the inductor itself; it's the capacitance between the
inductor and ground.

That is a very strange statement to my mind. Stray capacitance is an
unavoidable effect that any real inductor in any real application will
have as a result of it having non zero size. A thing made of wire
that takes up space has inductive character and capacitive character,
and transmission line character, and loss, all rolled into one. You
can set the situation up that it finds itself in, is that some of
those properties not very significant, but that are all part of the
effect of a real, physical inductor. I don't understand why you keep
pretending that these non ideal effects are the fault of something
else. They are a result of the device taking up space and being made
of metal.

This difference is the source of confusion and
misunderstanding about the current -- the current we see at the top of
the inductor is the current exiting the inductor minus the current going
via the shunt C to ground. It's not due to a property of the inductor
itself. We're seeing the *network* current, not the inductor current.

I agree. But a large, air core, spaced turn coil is a network, not a
pure inductance. This is just reality.

Removing the ground lets us see the inductor current by itself.

Or, emphasizes that particular aspect of its nature.

Another reason to avoid that model, unless you are just looking for
the least amount of math to get an approximation. But computation has
gotten very cheap.


The problem is that it obscures what's happening -- we can no longer
easily tell which effects are due to the radiation, which are due to the
capacitance, and which are inherent properties of inductance unless we
separately analyze separate simplified circuits (as I did with EZNEC).
And that's really what the whole disagreement has been about. Effects
due to shunt capacitance have been claimed to be inherent properties of
all inductors, and elaborately crafted theories developed to attempt to
explain it. If all you want is numbers, they're plenty easy to get
without the programmer needing to have the slightest understanding of
what's happening. And he will have learned nothing he can apply to other
situations.

Distributed analysis is just fine, but it should predict the same coil
currents with the antenna replaced by lumped components. And it should
predict nearly equal currents in the inductor ends when ground is
removed. And it should predict the same results when the coil and the
shunt C to ground are replaced by lumped components. Because that's what
really happens. My simplified lumped component analysis does all this. A
rigorous solution of the fundamental equations for distributed networks
does this also -- EZNEC does its calculations with just such equations
and reaches the correct conclusions. But I don't believe that Cecil's
theories and methods provide the correct results in all these cases.
(snip)

Sorry, here is where I have to withdraw. I can't say what Cecil is
thinking.

Current through coils
 

John Popelish wrote:

But what is the need for such an argument? Just to prove that lumped
component networks can model real, distributed things? I get that.
As I see Cecil's point (and I hate to say this with him absent), it is
that real, large coils with all their poor turns coupling and stray
capacitance both turn to turn and more important, to ground, take a
lot of those lumped components to model, accurately, but only their
own self, described by distributed network concepts to model, accurately.

Cecil's point is rather obscure, but as I read it Cecil thinks the ONLY
way to model a loaded antenna is through reflected wave theory.

As I understand what Cecil writes, he seems to be saying if we use a
current meter we cannot measure current. If we look at an inductor's
properties he seems to say they change in the presence of standing
waves. He also seems to be saying a loading inductor replaces a certain
number of electrical degrees through some reflection property.

What most others seem to be saying is an inductor is an inductor. It
behaves the same way and has the same characteristics no matter how it
is used, so long as we don't change the displacement currents by
varying capacitive coupling to surroundings.

That is where the difference is.

I can easily build a loading coil that has no appreciable change in
current from end-to-end. My measurements of typical loading coils shows
it is the ratio of load (termination) impedance to capacitance to the
outside world that controls any difference in current, and not the
"electrical degrees" the coil replaces. It is also not the reflected
waves that cause the unequal currents, but rather the fact the inductor
has distributed capacitance to earth or other objects besides the coil.

Capacitance from the coil to itself won't cause these problems. The
change in phase of current at each end of a coil would depend heavily
on stray C of the coil to the outside world as compared to reactance of
the coil, and it would also depend on less than perfect flux linkage
across the inductor.

I measured a typical inductor and found it did have more phase delay in
current at each terminal than the actual spatial length of the coil
form would indicate. I measured a delay about equal to double the
length of the 10 inch coil form length. If the inductor was perfect,
the delay would be about equal to light speed across the length of the
inductor form.

The only thing in all of this I can't find agreement with is what Cecil
is saying. I'm not disputing currebnt can be different, and phase can
be different. What I am disputing are Cecil's claims that an inductor
behaves differently in an antenna than in a lumped system that
represents the antenna, and that the cause of inequality in currents or
phase delay is caused by reflected waves and cannot be understood
without applying reflected wave theory.

In my experience, either lumped circuits or reflected waves will work
IF applied correctly.

This is my take of the disagreement.

73 Tom

Current through coils
 

On Fri, 24 Mar 2006 20:13:26 -0500, John Popelish
wrote:

He and others have given this as
proof that the current at the two ends of an inductor are inherently
different, regardless of its physical size.

I agree up till you add, "regardless of physical size". I have seen
him talk only about large air core space wound coils. I came to the
discussion late, but this is what I have seen.

Hi John,

One of the problems is the thread discussion is freely mixed with
practical observations and theoretical arguments - these can clash,
especially when mixed indiscriminately to prove one point.

First, several years ago, came the shocking observation that the
current into a coil is not the same as the current out of it.
Somewhere along the debate, this practical measurement was then
expressed to be in conflict with Kirchhoff's theories. However,
Kirchhoff's current law is for currents into and out of the same point
intersection, not component. The association with a point is found in
that the "lumped" inductance is a dimensionless load. The association
with Kirchhoff was strained to fit the load to then condemn the load
instead of simply rejecting that failed model and using the correct
one.

The problem came from incorrectly specifying the coil in EZNEC which
offers a coil generator (inductor) in the wires table as well as a
coil specification (inductance) in the loads table. This shocking
difference between model and observation would have been easily
resolved by simply using the coil generator (inductor) in place of the
lumped equivalent (inductance).

How do you know when you've made a mistake in application? You do two
designs and compare each to what nature provides. You discard the
model that does not conform to nature.

Want to know what the difference is between the two (the good and the
bad design) at the far receiver? ±.32dB Hence the name of my thread
"Current through coils - BFD."

....snip

But the ground is there, in the application under discussion. All
components act differently if you connect them to something else.
This coil is connected to ground by its capacitance.

Roy's point is that the proposed "theory," as Ian has also pointed
out, has to correctly answer all scenarios, not just one. We don't
have enough shelf space in libraries that prove the resistance of each
resistor constructed - one formula does quite well for 99.999% of
them, and a couple more formulas for those that don't (and those new
formulas will give the same answer for the first 99.999% as well).

When you add ground to the model, you
have to add the equivalent shunt C to the lumped model. The C isn't a
property of the inductor itself; it's the capacitance between the
inductor and ground.

That is a very strange statement to my mind. Stray capacitance is an
unavoidable effect that any real inductor in any real application will
have as a result of it having non zero size.

You are mixing an observational fact with a theoretical statement. The
lumped model contains ONLY inductance, to make it conform to nature,
as Roy is doing here, you have to add in all the nasty bits.

OR

Build a helix (inductor) in the wires table.

A thing made of wire
that takes up space has inductive character and capacitive character,
and transmission line character, and loss, all rolled into one.

These are all properties that reside in a helix (inductor) constructed
in the wires table. Some of these properties (like inductance) also
reside in the load table, but not the capacitance to earth. If it
matters, it is up to you to make the correct choice.

....snip

Well, the rest was more conflict between theory and practice that is
and has been resolvable for a long time. Even the conflict is
separable. For those who persist in making poor choices, they will
always have either a problem with a model, or the genesis of a new
theory, or rattle on beyond 500 posts - sometimes all three.

73's
Richard Clark, KB7QHC

Current through coils
 

Thanks, Tom and Richard. I've said what I want to say in just about
every way I can possibly think of, and without a great deal of success
in communicating to John what I mean. I hope you'll have better luck --
I've run out of different ways to say it. I hope some of the readers, at
least, have understood what I've been saying.

Roy Lewallen, W7EL

Current through coils
 

Roy Lewallen wrote:
Thanks, Tom and Richard. I've said what I want to say in just about
every way I can possibly think of, and without a great deal of success
in communicating to John what I mean. I hope you'll have better luck --
I've run out of different ways to say it. I hope some of the readers, at
least, have understood what I've been saying.

I want to thank you for the efforts and patience with me. I hope my
insight into the operation of large air core inductors has improved
because of it. Please do not think that I am taking up a defense of
everything Cecil thinks (since I am sure I don't know what that is,
anyway), but just found this technical discussion stimulating. It
isn't often I get to talk with such experienced people without getting
dismissed or ignored. It has been useful to me. I can't think of a
better compliment to offer.

Current through coils
 

Tom, the W8JI one, wrote, among other things,
"Capacitance from the coil to itself won't cause these problems. The
change in phase of current at each end of a coil would depend heavily
on stray C of the coil to the outside world as compared to reactance of
the coil, and it would also depend on less than perfect flux linkage
across the inductor."

I've lost track of exactly what "these problems" are, but I was
wondering about the "and it would also depend on less than perfect flux
linkage across the inductor" part. To help resolve that, I did a Spice
simulation; I modelled a transmission line with ten "L" sections
cascaded. Each was 1uH series, followed by 100pF shunt to ground. I
put a 100 ohm load on one end and fed the other end with a 2.5MHz sine
wave with 100 ohms source resistance. Sqrt(LC) is 10 nanoseconds per
section, so I expect 100 nanoseconds total delay, or 90 degrees at
2.5MHz. That's what I saw. Then I added unity coupling among all the
coils, and to keep the same net inductance, I decreased each inductor
to 100nH. The result was STILL very close to a 90 degree phase shift,
with a small loss in amplitude. In each case, the current in each
successive inductor shifts phase by about 1/10 the total. Although the
simulation is less than a perfect match to a completely distributed
system with perfect flux linkage (and just how you do that I'm not
quite sure anyway...), but it's close enough to convince me that
perfect flux linkage would not prevent behaviour like a transmission
line, given the requisite distributed capacitance.

(That was from a "transient" simulation, 10usec after startup so it
should be essentially steady-state; but I'll probably play with an AC
sweep of both cases as I find time.)

Cheers,
Tom

Current through coils
 

K7ITM wrote:
cascaded. Each was 1uH series, followed by 100pF shunt to ground. I
put a 100 ohm load on one end and fed the other end with a 2.5MHz sine
wave with 100 ohms source resistance. Sqrt(LC) is 10 nanoseconds per
section, so I expect 100 nanoseconds total delay, or 90 degrees at
2.5MHz. That's what I saw. Then I added unity coupling among all the
coils, and to keep the same net inductance, I decreased each inductor
to 100nH. The result was STILL very close to a 90 degree phase shift,
with a small loss in amplitude. In each case, the current in each
successive inductor shifts phase by about 1/10 the total. Although the
simulation is less than a perfect match to a completely distributed
system with perfect flux linkage (and just how you do that I'm not
quite sure anyway...), but it's close enough to convince me that
perfect flux linkage would not prevent behaviour like a transmission
line, given the requisite distributed capacitance.

Thanks Tom,

That's very interesting. My thought is the difference in
phase-of-current at each of the inductor would be affected by mutual
coupling, with perfect coupling preventing phase differences in
current, but maybe that is shortsighted.

I'll have to think about that a while and how it might affect what I am
saying.

73 Tom

Current through coils
 

The phase shift in degrees, along a coil or any other sort of
transmission line, is fixed rigidly by its physical dimensions and
test frequency.

Phase shift is entirely independent of the way it is used, the circuit
it is in and the circuit currents which flow.
----
Reg. G4FGQ

Current through coils
 

Of course, that's blatantly false, taken literally. A 2" diameter 10"
long solenoid coil coaxially inside a 2.5" ID grounded conductive tube
will not have the same phase shift as the identical coil inside a 5" ID
grounded conductive tube, and neither will behave the same as the same
coil included as a loading coil in Cecil's mobile antenna. It won't
even have the same inductance in each case.

Before you say, "Give us a break, Tom. Of course it won't and clearly
that's not what was meant," just consider how literally both the
posters and the lurkers here take things.

AND in fact, as shown in the simulation I just reported on, the
coupling between that coil and the magnetic fields of other nearby
components does affect the performance of that coil. In general, when
the fields, electric and magnetic, around any component interact with
their environment, a change in that environment will change the
behaviour of the component. Thankfully, we have a lot of components
where that effect is minimal at the frequencies of interest, but we do
need to take note of cases where the effect is important. I DAILY work
with tiny components that DO behave differently, depending on their
environment. At several GHz, seemingly small couplings can be very
important.

Cheers,
Tom

Current through coils
 

K7ITM wrote:
(snip)

To help resolve that, I did a Spice
simulation; I modelled a transmission line with ten "L" sections
cascaded. Each was 1uH series, followed by 100pF shunt to ground. I
put a 100 ohm load on one end and fed the other end with a 2.5MHz sine
wave with 100 ohms source resistance. Sqrt(LC) is 10 nanoseconds per
section, so I expect 100 nanoseconds total delay, or 90 degrees at
2.5MHz. That's what I saw. Then I added unity coupling among all the
coils, and to keep the same net inductance, I decreased each inductor
to 100nH. The result was STILL very close to a 90 degree phase shift,
with a small loss in amplitude. In each case, the current in each
successive inductor shifts phase by about 1/10 the total. Although the
simulation is less than a perfect match to a completely distributed
system with perfect flux linkage (and just how you do that I'm not
quite sure anyway...), but it's close enough to convince me that
perfect flux linkage would not prevent behaviour like a transmission
line, given the requisite distributed capacitance.

(That was from a "transient" simulation, 10usec after startup so it
should be essentially steady-state; but I'll probably play with an AC
sweep of both cases as I find time.)

I look forward to your tests with turn-to-turn capacitance added to
the model as well. It will no longer match the 100 ohm source and
load over as wide a frequency range, but it will look closer to the
real thing above the self resonant frequency. You may be able to
measure the phase shift versus frequency up to resonance, and test
Cecil's idea that you can measure the self resonant frequency and use
that to predict the phase shift at much lower frequencies.