RadioBanter - Current through coils

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Current through coils

Cecil Moore wrote:
You measured standing wave current, Tom. Your measurements are
meaningless! Standing wave current has the same constant phase
whether the coil exists or not. Your measurements prove absolutely
nothing that is not already known.

Nothing I have said has changed from what I've said for years.

Now you have magnetic fields traveling slower than light speed in air,
and have gone right back to the same nonsense of standing wave current.

Please tell us all how you would measure the "traveling current" while
ignoring "standing wave current".

This ought to be good.....

Current through coils

John Popelish wrote:
When you talk about current flowing, you seem to be thinking of current
waves traveling along a conductor. Others seem to be saying "current"
and thinking of charge movement. I think that only the second is
technically correct ...

John, many thanks for some rationality from a cool head.

Conventions aside, that sounds about right. So would you agree
that if there's a forward current of one amp and a reflected
current of one amp, the net charge movement is zero and therefore
the standing wave current is not "going" anywhere? How can something
with a constant fixed phase angle of zero degrees "go" anywhere?

Standing waves involve no net wave travel in either direction, though
anywhere except at the current nodes, charge is certainly moving back
and forth along the conductor, during a cycle.

That's unclear to me. Why can't the E-field and H-field simply be
exchanging energy at a point rather than any net charge moving
laterally?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils

Cecil Moore wrote:

What's wrong with just reporting the measured the delay
through your test coils? Your measured data already
shows the current on one side of the coil to be
different from the current on the other side of the
coil. All we have to worry about now is the delay
through the coils.

What measurement are you talking about? The one I did over two years
ago that has been up on my web site since that time? Something on a
bench?

Current through coils

Cecil Moore wrote:
John Popelish wrote:

When you talk about current flowing, you seem to be thinking of
current waves traveling along a conductor. Others seem to be saying
"current" and thinking of charge movement. I think that only the
second is technically correct ...

John, many thanks for some rationality from a cool head.

Conventions aside, that sounds about right. So would you agree
that if there's a forward current of one amp and a reflected
current of one amp, the net charge movement is zero and therefore
the standing wave current is not "going" anywhere? How can something
with a constant fixed phase angle of zero degrees "go" anywhere?

Standing waves involve no net wave travel in either direction, though
anywhere except at the current nodes, charge is certainly moving back
and forth along the conductor, during a cycle.

That's unclear to me. Why can't the E-field and H-field simply be
exchanging energy at a point rather than any net charge moving
laterally?

Cecil,

I think I said all of that before the fun and games started. In any case
I agree 100% with John.

Let me try again to answer your question. This is all very basic
textbook stuff. I claim not the slightest bit of credit for any of this.

First, I hope we can agree that current is defined as the movement of
charge. In this case the charge moves only in the direction of the wire,
let's call it the z-direction.

The generic equation for a forward traveling wave is simply:

y = A cos (kz-wt)

The generic equation for a reverse traveling wave is:

y = B cos (kz+wt)

One can add constant phase offsets to the cosine arguments, but it does
not make any difference here. It just makes things look messy,
especially in ASCII. The parameters k and w are not independent either,
but again that does not really matter here.

In the case of current we can say:

If = Io cos (kz-wt)
Ir = Io cos (kz+wt)

I have set the "A" and "B" coefficients to the same value, Io, for
simplicity. If the currents are not the same the math gets a little
messier, but there is no fundamental difference. Keep in mind that the
If and Ir refer to the current that moves along the z-direction, i.e.,
charge moving in the back-and-forth direction along the wire. The "f"
refers to the forward "wave", and the "r" refers to the reverse "wave".
The current in both cases is not "forward" or "reverse" but simply
back-and-forth as in any AC condition. It is essential to separate the
concepts of wave and current. They may be connected, but they are not
the same, and they are not interchangeable.

OK, now lets add these two traveling waves together to make a standing
wave. This is a linear system, and superposition applies. We can simply
add the components. The basic equation is:

Isw = If + Ir = Io { cos (kz-wt) + cos (kz+wt) }

Through the use of a standard trigonometric identity this can be reduced to:

Isw = 2Io cos (kz) cos (wt)

What can be seen immediately is that the standing wave current still has
exactly the same time dependence that the traveling waves had. The
magnitude of the current is now a function of z, unlike the constant
magnitude in the traveling waves. The "current" is still defined as
above, namely the charge that moves back-and-forth in the z-direction.

The current oscillation factor (wt) is now decoupled from "z", unlike
the traveling wave case. The "wave" is stationary. The current itself,
however, behaves exactly the same as in the case of the traveling waves.

Of course there are important differences in radiation patterns for
traveling waves and standing waves. The magnitude of the current is
different along the wire. However, except at the standing wave nodes,
the standing wave current is very real and non-zero.

I am almost embarrassed to write this, since surely you and most readers
know all of this quite thoroughly. However, it appears you may have
overlooked something. I hope this helps.

73,
Gene
W4SZ

Current through coils

And, of course, since the net current is a function of distance along
the wire, it follows that in the case Gene described, charge in each
section of wire goes through a cyclic, sinusoidal increase and
decrease. In other words, the wire exhibits capacitance. See Reg's
second sentence in the posting that started this whole set of insanity
off. (One might call the current Reg mentions in the third sentence
"displacement current" as is often done.) Yawn. (It's kind of fun to
look at an animation of the case where the magnitude of If and Ir are
not equal. It's pretty straightforward to program in Matlab or
Scilab.)

Cheers,
Tom

Current through coils

At great risk, let me try this approach.

I have a 100 turn 2" diameter #18 gauge wire air core inductor. There
are 100 turns, so there is about 630 inches or 32 feet of wire in the
coil.

I have a Network Analyzer with port to port time delay measurement
capability. It measures coaxial cables very well, and even clip leads.

Cecil, please predict or guess the group delay of this inductor at 3.8
MHz. Tell us all what that group delay means for your wave theory.

Just come close, and I will tell you what it measures. I can even print
the plot just for you.

73 Tom

Current through coils

Cecil Moore wrote:
John Popelish wrote:

When you talk about current flowing, you seem to be thinking of
current waves traveling along a conductor. Others seem to be saying
"current" and thinking of charge movement. I think that only the
second is technically correct ...

John, many thanks for some rationality from a cool head.

Conventions aside, that sounds about right. So would you agree
that if there's a forward current of one amp

By this I assume you mean a traveling current wave with an RMS value
of 1 amp.

and a reflected current of one amp,

Meaning a returning current wave with an RMS current of 1 amp.

the net charge movement is zero and therefore
the standing wave current is not "going" anywhere?

Sorry, no. There is no net (average over one cycle) current, whether
the wave is traveling or standing. In both cases the instantaneous
current changes direction every half cycle at any given point. If
there is a standing wave made of a 1 ampere RMS current wave and a 1
ampere RMS returning wave, then the standing wave current will vary
from zero amperes RMS at current nodes to 2 amperes RMS at current
peaks. Looking just at just current, and at only a single point, a
traveling current wave and a standing current wave are
indistinguishable. You cannot tell if the measured RMS current is
made up of a wave traveling in one direction, or the sum of two waves
traveling in opposite directions.

How can something with a constant fixed phase angle of zero degrees "go" anywhere?

The only way to understand a standing wave having a phase of zero
degrees, that makes sense to me, is that it applies to all points
between one current node and the next. The points between the next
two nodes have a phase of 180 degrees (charge is moving in the
opposite direction at all times) with respect to the points between
the first two nodes. So, if you pick some point between a pair of
current nodes, all other points along the standing wave must be either
be in phase with the current at that point, or 180 degrees out of
phase with it. In a standing wave, charge sloshes back and forth in
opposite directions between alternate pairs of current nodes.
Likewise, where the charge piles up and sinks (at the current nodes),
voltage peaks occur because of the charge accumulation or shortage.

Standing waves involve no net wave travel in either direction, though
anywhere except at the current nodes, charge is certainly moving back
and forth along the conductor, during a cycle.

That's unclear to me. Why can't the E-field and H-field simply be
exchanging energy at a point rather than any net charge moving
laterally?

In an isolated EM plane wave, I think this is the case, and
displacement charge in space takes the place of conductor current.
But when a wave is guided by a conductor, we can measure the charge
sloshing back and forth in the conductor in response to those fields.

Take a look at:
http://galileo.phys.virginia.edu/cla...axwell_Eq.html
about half way down.

Here is an excerpt:

(begin excerpt)

"Displacement Current"

Maxwell referred to the second term on the right hand side, the
changing electric field term, as the "displacement current". This was
an analogy with a dielectric material. If a dielectric material is
placed in an electric field, the molecules are distorted, their
positive charges moving slightly to the right, say, the negative
charges slightly to the left. Now consider what happens to a
dielectric in an increasing electric field. The positive charges will
be displaced to the right by a continuously increasing distance, so,
as long as the electric field is increasing in strength, these charges
are moving: there is actually a displacement current. (Meanwhile, the
negative charges are moving the other way, but that is a current in
the same direction, so adds to the effect of the positive charges’
motion.) Maxwell’s picture of the vacuum, the aether, was that it too
had dielectric properties somehow, so he pictured a similar motion of
charge in the vacuum to that we have just described in the dielectric.
This is why the changing electric field term is often called the
"displacement current", and in Ampere’s law (generalized) is just
added to the real current, to give Maxwell’s fourth -- and final --
equation.

(end excerpt)

Current through coils

Cecil,

Earlier you made comments about the time delay through a 75 meter
loading inductor being somewhere around 60 nS or so.

You have consistently disagreed with me when I said time delay through
an inductor with tight mutual coupling from turn-to-turn is somewhat
close to light speed over the physical length of the inductor, rather
than the time it takes current to wind its way around through the
copper. You didn't like my measurement of a small 100uH choke, and said
a large inductor like a bug catcher coil is different. You predicted
standing waves in that inductor.

I have a 100 turn 2 inch diameter air wound inductor of pretty good
quality. It is 10 inches long.

Please tell all of us the time delay you expect in that inductor on 3.8
MHz. Please tell all of us what that delay means for your various
changing theories about waves standing in that coil.

I'll sweep the inductor from below the BC band up to 30MHz in a time
measurement mode and post the printout of the sweep with scale values
and markers that show time delays.

73 Tom