Current through coils
 

Cecil Moore wrote:

For more information, take a look at:

http://lists.contesting.com/archives.../msg00540.html

Very interesting, but not enough information to allow me to repeat
their measurements.

I also note that the opening statement:

"For closewound coils, with length to diameter ratios around 5:1, a
series of fairly careful measurements have been made with the coils
arranged vertically above a ground plane, fed at the base, with a
capacitive load on the other end, and the driving frequency arranged
to be at the resonant frequency of the whole assembly."

This definitely specifies only a single frequency for the test. While
it is not the self resonant frequency of the coil alone, it is
definitely a resonant situation, where there will be a considerable
standing wave through the coil. So I don't see how this reference
supports your claim that measuring the delay at resonance tells you
the delay at other frequencies. It also contradicts your claim about
how a standing wave makes it difficult to measure the current delay
through the coil. What have I missed?

Current through coils
 

Richard Harrison wrote:
The wavelength of a line is the distance a wave must travel for one
complete cycle (360-degrees). If you want the phase shift for a line,
take the length of line required for one degree of phase retardation and
multiply it by the length of line you have.

If you want to know the velocity factor of a piece of
transmission line, the easiest thing to do is find
its first self-resonant frequency. A little math
will yield the VF which allows prediction of the
phase shift through any reasonable length of
tranmission line.

If you want to know the velocity factor of a coil,
the easiest thing to do is find its first self-
resonant frequency. A little math will yield the
VF of the coil which allows prediction of the
phase shift through any reasonable length of coil.

Not disagreeing - just expanding.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Cecil Moore wrote:

If you want to know the velocity factor of a piece of
transmission line, the easiest thing to do is find
its first self-resonant frequency. A little math
will yield the VF which allows prediction of the
phase shift through any reasonable length of
tranmission line.

If you want to know the velocity factor of a coil,
the easiest thing to do is find its first self-
resonant frequency. A little math will yield the
VF of the coil which allows prediction of the
phase shift through any reasonable length of coil.

If the inductor in question does not take much advantage of mutual
induction across its length nor has much capacitance across its length
(say, a straight conductor, strung with ferrite toroids), then I can
see the similarity with a transmission line. But as the inductor
approaches a lumped inductance with significant inter winding
capacitance and mutual inductance coupling the current across a
significant part of its winding length, I see on reason to assume the
transmission line method (delay independent of frequency) strictly
applies. It might, but it would take more than you saying so to
assure me that it is a fact.

In other words, transmission line concepts like uniform inductance per
length and uniform capacitance per length get rather muddled in a real
inductor.

Current through coils
 

Richard Clark wrote:
Well, I for one note that your call for a reference to that one point
(coil self resonance) was met by a "link" to a mailing list on another
point (assembly self resonance).

Give us a break, Richard. Those two subjects were in different
paragraphs and completely unrelated. I looked up the reference
and it is, "The Great Physicists From Galileo To Einstein,
Biography of Physics", by George Gamow, (ISBN: 0486257673)
I was going to furnish that information on Thursday when I
go back to work.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

John Popelish wrote:

If the inductor in question does not take much advantage of mutual
induction across its length nor has much capacitance across its length
(say, a straight conductor, strung with ferrite toroids), then I can see
the similarity with a transmission line. But as the inductor approaches
a lumped inductance with significant inter winding capacitance and
mutual inductance coupling the current across a significant part of its
winding length, I see on reason to assume the transmission line method
(delay independent of frequency) strictly applies. It might, but it
would take more than you saying so to assure me that it is a fact.

In other words, transmission line concepts like uniform inductance per
length and uniform capacitance per length get rather muddled in a real
inductor.

Tom W8JI posted a good description and summary of inductor operation a
little while ago, but it looks like it could bear repeating, perhaps
with a slightly different slant.

In a transmission line, a field at one end of the line requires time to
propagate to the other end of the line. As the EM fields propagate, they
induce voltages and currents further down the line, which create their
own EM fields, and so forth. These propagating fields and the currents
and voltages they produce make the whole concept of traveling voltage
and current waves useful and meaningful.

But in a tightly wound inductor, a field created by the current in one
turn is coupled almost instantly to all the other turns (presuming that
the coil is physically very small in terms of wavelength). Consequently,
output current appears very quickly following the application of input
current. The propagation time is nowhere near the time it would take for
the current to work its way along the wire turn by turn.

Once again it's necessary to point out that I'm speaking here of an
inductor which has very good coupling between turns and minimal field
leakage or radiation, for example a toroid. If you make an air wound
inductor and slowly stretch it out until it's nothing more than a
straight wire, it'll begin by resembling the toroid -- more or less,
depending on how well coupled the turns are and how much its field
interacts with the outside world -- then slowly change its
characteristics to resemble a straight wire. There's no magic transition
point. So by choosing the inductor, you can observe behavior anywhere
along this continuum.

Roy Lewallen, W7EL

Current through coils
 

John Popelish wrote:

Cecil Moore wrote:
(snip)

Other multiple measurements by independent sources agree with
me and disagree with you, Tom. Wonder why you neglected to post
this reference from your own server?

http://lists.contesting.com/archives.../msg00540.html

It is a posting to TowerTalk by Jim Lux, W6RMK. I'll just extract
some excerpts.

"For closewound coils, with length to diameter ratios around 5:1, a
series of fairly careful measurements have been made with the coils
arranged vertically above a ground plane, fed at the base, with a
capacitive load on the other end, and the driving frequency arranged
to be at the resonant frequency of the whole assembly."

Sure sounds like your 100 uH 10"x2" coil installed in a mobile ham
radio antenna environment.

(snip)

The tantalizing part from my perspective is this:

"The measurements were made with carefully designed fiberoptic probes
that were specifically designed to avoid perturbing the magnetic and
electric fields."

I would like to read a full description of this instrumentation.


Like many others I don't know everything. In line with reducing my
ignorance could you amplify on how the phenomena is measured with a
"fiber optic probe". What type of transducer is used to convert energy
of an electrical nature to energy of an optical nature with out
"perturbing the magnetic and electric fields".


Dave WD9BDZ

Current through coils
 

John Popelish wrote:
I also note that the opening statement:

"For closewound coils, with length to diameter ratios around 5:1, a
series of fairly careful measurements have been made with the coils
arranged vertically above a ground plane, fed at the base, with a
capacitive load on the other end, and the driving frequency arranged to
be at the resonant frequency of the whole assembly."

This definitely specifies only a single frequency for the test.

Yes, a 75m mobile base-loaded antenna is a single frequency
antenna. Why are you surprised? Those guys have figured out
something that I haven't, probably because they have better
tools at their disposal than I do. They seem to have a 1%
accurate model at frequencies other than the self-resonant
frequency. I, OTOH, am only sure of my accuracy at the
self-resonant frequency due to the limited tools at my
disposal.

So I don't see how this reference
supports your claim that measuring the delay at resonance tells you the
delay at other frequencies. It also contradicts your claim about how a
standing wave makes it difficult to measure the current delay through
the coil. What have I missed?

You missed the complete point, John. If one cannot eliminate
reflections from the measuring process, then use them to your
advantage in the measurements. Self-resonance means that the
forward wave is in phase with the reflected wave. The first
time that happens is when the wave has made a 180 degree round
trip to the tip of the antenna and back, i.e. it happens first
at self-resonance, when the coil is electrically 90 degrees
long. For a well-designed coil, like a well-designed transmission
line, it doesn't vary by much over HF frequencies. In short,
the self-resonance velocity factor should extend pretty well
to all HF frequencies below that self-resonance point. I need
to think about the frequencies above the self-resonance point,
but that doesn't apply to the present discussion.

I guess I should re-phrase my statement. Standing waves make
it difficult for *ME* and W8JI to measure the current delay
through the coil. I ran essentially the exact experiment that
W8JI ran with identical results. I even used the current pickups
that W8JI kindly furnished to me. The only difference between
W8JI and me is that I recognized the results to be bogus.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

David,

Could it be something as simple as the use of a fiber optic cable as an
alternative to a shielded coax cable? I suspect the "without perturbing
.. . ." part may be innocent overstatement. Wish I had a set of
high-frequency probes with fiber optic cables!

Chuck


David G. Nagel wrote:
John Popelish wrote:

Cecil Moore wrote:
(snip)

Other multiple measurements by independent sources agree with
me and disagree with you, Tom. Wonder why you neglected to post
this reference from your own server?

http://lists.contesting.com/archives.../msg00540.html


It is a posting to TowerTalk by Jim Lux, W6RMK. I'll just extract
some excerpts.

"For closewound coils, with length to diameter ratios around 5:1, a
series of fairly careful measurements have been made with the coils
arranged vertically above a ground plane, fed at the base, with a
capacitive load on the other end, and the driving frequency arranged
to be at the resonant frequency of the whole assembly."

Sure sounds like your 100 uH 10"x2" coil installed in a mobile ham
radio antenna environment.


(snip)

The tantalizing part from my perspective is this:

"The measurements were made with carefully designed fiberoptic probes
that were specifically designed to avoid perturbing the magnetic and
electric fields."

I would like to read a full description of this instrumentation.



Like many others I don't know everything. In line with reducing my
ignorance could you amplify on how the phenomena is measured with a
"fiber optic probe". What type of transducer is used to convert energy
of an electrical nature to energy of an optical nature with out
"perturbing the magnetic and electric fields".


Dave WD9BDZ

Current through coils
 

On Mon, 13 Mar 2006 19:26:10 GMT, Cecil Moore wrote:
Those two subjects were in different paragraphs and completely unrelated.
'xactly my point.

Current through coils
 

David G. Nagel wrote:
Like many others I don't know everything. In line with reducing my
ignorance could you amplify on how the phenomena is measured with a
"fiber optic probe". What type of transducer is used to convert energy
of an electrical nature to energy of an optical nature with out
"perturbing the magnetic and electric fields".

Like you (unlike W8JI) I don't know everything. :-) I have
hardly any idea how they used a "fiber optic probe" to make
their measurements. I suspect they superposed local RF phasors
and used a fiber optic system to report the results. That's
what I would do.

I have invited Jim, W6RMK, to join the discussion. Maybe
he can answer your questions.
--
73, Cecil http://www.qsl.net/w5dxp