Current through coils
 

Richard and everybody,

Let's try again from scratch, fresh, I will try to go step by step, so there
are no ambiguities, twists and turns to each own's la-la land.

Cecil is on well deserved break, so I am am on my own, stuck on whatever it
might be.

I will not continue, unless there is an agreement at each point, I go
sloooow, for the benefit of mine and others who duntgetit.
The "camp" think is to signify two groups claiming the different behavior of
the current in the antenna loading coil. No intent to punish anyone.

Please go to the new thread that I started.
"Current across the antenna loading coil - from scratch"
If needed I will post pictures on my web site, unless there is a way to do
it here.

Thank you!

Yuri, K3BU.us

Current through coils
 

Cecil Moore wrote:

But its propagation speed will be slower than it would be if the wire
were straight. don't know if that qualifies it for a "slow wave" line
or not.

A velocity factor of 0.0175 for a 75m bugcatcher seems to qualify.

I guess this depends on the official definition if "slow wave". It
may already be taken for something specific and limited. It may be
something like high frequency has come to mean an arbitrary frequency
band. This group is the first place I have come across the term.

Current through coils
 

Cecil Moore wrote:
John Popelish wrote:

If there is a standing wave on a wire, and you have a tiny current
transformer sensor you can slide along the wire, you can measure the
instantaneous current (or the RMS) at any point along the wire. If
the sensor sits at a single point and sees an AC current, you have no
way, from this one measurement, if this current is the result of a
standing wave (two oppositely traveling equal waves adding), or a
single traveling wave, or any combination of traveling waves of
different amplitudes. You know only the net current at that point.


But if one it smart enough to slide the sensor up and down the wire
and note the phase is fixed and unchanging, one knows he is dealing
with a standing wave.

Another point, entirely. My point is that current has a point
definition, and standing wave current is certainly indistinguishable
from traveling wave current, at a point. Current is current.

Patterns of current over length is another subject. But you keep
saying that there is something different about current in a standing
wave. There isn't. It is the pattern of current distribution over
time and distance along a conductor that is different with a standing
wave.

It is a nit, but it is snagging other people in the discussion, too,
so I thought it would help to clear it up.

Current through coils
 

Cecil Moore wrote:
Ian White GM3SEK wrote:

- and yet again Cecil snips the statement he is replying to. For the
second time in a day, I have to put back what I actually said:

The human observer sees a larger picture of the whole antenna, and can
choose many different ways to theorize about it. But a theory cannot
be correct if it requires that components behave in different, special
ways according to the way a person happens to be thinking about it at
the time.

If you cannot see that statement as a fundamental principle of
scientific logic, then I have run out of ways to tell you.

That statement was not innuendo at all. It means nothing more than
what it literally says.
It applies to any and every observer who attempts to construct a
theory about something. Everybody is included; but nobody is exempt.

It means the lumped-circuit model works where the distributed-
network model fails. That is false. It is just the opposite.
the distributed-network model works where the lumped-circuit
model fails.

No...

The difference is that my views join up with the rest of human
knowledge about antennas and circuit behaviour.

Only up to where the coils are 15 degrees long. Then the distributed
network model must be engaged to avoid blunders exactly like you
and others are making.


You are missing the point still.

Yours don't. They fail that crucial test.

Distributed network analysis fails the test??? Please provide
an example. The IEEE would probably publish a paper on such.

Every time I say that you are not applying established concepts and
techniques correctly, you twist it to make me say I am denying the
validity of the concepts themselves.

For the very last time: the basic concepts are valid; but the way that
you are applying them is not. Can you really and truly not see the
difference?



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Current through coils
 

This thread belongs back in the original place, so it flows in context.

Yuri Blanarovich wrote:
OK, I have been accused of being wrong, claiming that current across the
antenna loading coil is or can be different at its ends.

No one said that.

I and "my camp" say that we are seeing somewhere 40 to 60 % less current at
the top of the coil, than at the bottom, in other words, significant or
noticeable drop.

Quit trying to make it a gang war. It is antenna theory, not a bar
room brawl with a bunch of drunks.

W8JI and "his camp" are claiming it can't be so, current through the coil
has to be the same or almost the same, with no significant drop across the
loading coil.

I have no camp. You are lifting what I say out of context and deleting
important things.

What I say, over and over again, is I can build an inductor in a short
mobile antenna that has essentially equal currents at each end. A
compact loading coil of good design has this type of performance.

The current taper across the inductor is not tied to the number of
"electrical degrees" the inductor "replaces". It is tied to the
distributed capaciatnce of the coil to the outside world in comparison
to the termination impedance at the upper end of the coil.


wrote in message
Let's focus on one thing at a time.

You claim a bug cather coil has "an electrical length at 4MHz of ~60
degrees". That concept is easily proven false, just like the claim a
short loaded antenna is "90-degree resonant". Both can be shown to be
nonsense pictures of what is happening.

Assume I have a 30 degree long antenna. If the loading inductor is 60
electrical degrees long, I could move it anyplace in that antenna and
have a 90 degree long antenna.

We all know that won't happen, so what is it you are really trying to
say?

OK lets get me some educating here.
I understand that, say quarter wave resonant vertical (say 33 ft at 40m) has
90 electrical degrees.
Is that right or wrong?

Right.

The current distrubution on said (full size) vertical is one quarter of the
wave of 360 deg. which would make it 90 degrees. Max current is at the base
and then diminishes towards the tip in the cosine function down to zero.
Voltage distribution is just opposite, min at the base, feed point and max
at the tip. EZNEC modeling shows that to be the case too.
Is that right or wrong?

Right. Although the distributed capacitance can change the shape.

If we stick them end to end and turn horizontal, we get dipole, which then
would be 180 deg. "long" or "180 degrees resonant".
If not, what is the right way?

Right.

If I insert the coil, say about 2/3 up (at 5 ft. from the bottom) the
shortened vertical, I make the coil size, (inductance, phys. dimensions)
such that my vertical will shrink in size to 8 ft tall and will resonate at
7.87 MHz.
I learned from the good antenna books that this is still 90 electrical
"resonant" degrees.
Maximum of current is at the feed point, minimum or zero at the tip.

What "good book"? It would help to see the context.

None of my engineering books use electrical degrees except to describe
overall antenna height or length.

They might say "60 degree top loaded resonant radiator" but they don't
say "60 degree tall radiator 90 degree resonant".

There might be a correct context, but I can't think of one off hand. So
I need an example from a textbook.

If you stick those verticals (resonant) end to end and horizontal, you get
shortened dipole, with current distribution equal to 180 degrees or half
wave. Max current at the feed point, minima or zero at the tips. (RESONANT
radiator)

The current distribution would not be the same as a half wave, becuase
the antenna is not 1/2 wave long.

Can we describe "pieces" or segments of the radiator as having proportional
amount of degrees corresponding to their physical length, when excited with
particular frequency?

Yes. It works fine for length. It does NOT work for loading inductors,
it does not work for short antennas which have anything form a uniform
distribution to triangular distribution, or any mix between including
curves of various slopes.

A 30 degree tall antenna with base loading simply has power factor
correction at the base, provided the inductor is not a significant
fraction of a wavelength long. It is a 30 degree base loaded radiator,
not a 90 degree antenna. And the inductor is not 60 degrees long.

73 Tom

Current through coils
 

John Popelish wrote:
. . .
Of course, it can't. But a lumped LC network made of perfect, ideal
components can be constructed that mimic the terminal conditions of the
coil in question to any degree of accuracy desired. The caveat is that
you may not explore much of a frequency range if you expect this
idealized model to remain a good mimic. At another frequency, you have
to rebuild it to copy the effects at that frequency. The broader the
frequency range of such a model, the more complexity it must have.

Yes, but you can use an arbitrarily large number of sections, each with
a small amount of L and C, and mimic a transmission line to any desired
degree, over any frequency range you want. And all with zero physical
size in the theoretical case, and arbitrarily small physical size in the
practical case. In the limit of an infinite number of sections of
vanishingly small L and C each, you arrive at the general equations for
a transmission line, valid at all frequencies.

The point I'm trying to make is that you don't need any particular
physical size or any particular length of wire to make something that
behaves like a transmission line to any degree of accuracy.

Roy Lewallen, W7EL

Current through coils
 

Roy Lewallen wrote:
The point I'm trying to make is that you don't need any particular
physical size or any particular length of wire to make something that
behaves like a transmission line to any degree of accuracy.

and more important to this discussion, you don't need standing waves or
antennas. For any given load impedance, it behaves the same way.

It's a shame Cecil misses that point, and thinks it is standing waves
that affect the system.

73 Tom

Current through coils
 

Roy Lewallen wrote:
John Popelish wrote:

. . .
Of course, it can't. But a lumped LC network made of perfect, ideal
components can be constructed that mimic the terminal conditions of
the coil in question to any degree of accuracy desired. The caveat is
that you may not explore much of a frequency range if you expect this
idealized model to remain a good mimic. At another frequency, you
have to rebuild it to copy the effects at that frequency. The broader
the frequency range of such a model, the more complexity it must have.


Yes, but you can use an arbitrarily large number of sections, each with
a small amount of L and C, and mimic a transmission line to any desired
degree, over any frequency range you want. And all with zero physical
size in the theoretical case, and arbitrarily small physical size in the
practical case. In the limit of an infinite number of sections of
vanishingly small L and C each, you arrive at the general equations for
a transmission line, valid at all frequencies.

The point I'm trying to make is that you don't need any particular
physical size or any particular length of wire to make something that
behaves like a transmission line to any degree of accuracy.

Oh. Then never mind. :-)

Current through coils
 

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

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.

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. 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. 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).


The question, I think is whether large, air core coils act like a
single inductance (with some stray capacitance) that has essentially
the same current throughout, or is a series of inductances with
distributed stray capacitance) that is capable of having different
current at different points, a la a transmission line. And the
answer must be that it depends on the conditions. At some
frequencies, it is indistinguishable from a lumped inductance, but at
other frequencies, it is clearly distinguishable. You have to be
aware of the boundary case.


Yes. It's a continuum, going from one extreme to the other. As Ian has
pointed out several times, any theory should be able to transition
from one to the other.

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?

The example Cecil posted on his web page was one for
which the L could be modeled completely adequately as a lumped L, at
least so far as its current input and output properties were concerned.

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


Being a significant fraction of the antenna's total length, it of
course does a substantial amount of radiating which a lumped model
does not.
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.

. . .

A lumped inductor has no stray capacitance. Those also have to be added
to the model, before the effect would mimic the real coil (neglecting
radiation).

By removing the ground in the model on my web site, I found that a
lumped inductor mimics the real inductor very well without any C. Of
course, to model an inductor close to ground requires adding a shunt C.
Modeling an inductor connected to a resistor would require adding a
resistor to the model. But we shouldn't confuse what the inductor is
contributing to the performance of the circuit with what the other
components are. And that confusion has been common here.

. . .

But in the real world, the capacitance is always there. It varies,
depending on the location of the coil, but it never approaches zero.

It can get insignificantly small, as in the modified model. But that's
really beside the point. The point is that the shunt C isn't an inherent
property of the inductor, and the current difference between the top and
bottom of an electrically short coil is due to the current flowing
through the external shunt C, however big or small it is. It's not due
to waves bouncing around inside the coil or painstakingly winding their
way turn by turn from one end to the other, or by any inherent and fixed
property of the inductor or the antenna it's connected to.

Roy Lewallen, W7EL

Current through coils
 

Cecil,

You can be the master of brevity, at least when it serves your purposes.
You might take a look at the entire sentence rather than clip out the
portion that sets the context.

"Certainly these consolidating functions are useful for a general
overview, but how can you learn anything about the details of a complex
system by averaging and netting?"

By the way, "steady-state analysis" has nothing whatsoever to do with
averaging. Steady-state simply means the system does not have a defined
starting time. There are no remaining startup transients. It cannot be
determined whether operation started one second ago or one year ago.
Steady-state does not mean DC, averaged, or RMS.



73,
Gene
W4SZ

Cecil Moore wrote:
Gene Fuller wrote:

... how can you learn anything about the details of a complex system
by averaging and netting?


Because the conservation of energy principle is about
averaging and netting. Because steady-state analysis
is about averaging and netting. Because engineers
have 200 years of averaging and netting behind us
to prove that it works. When you try to track an
individual electron's velocity and position, guess
what happens?