Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.


As I responded
before, the inductance of air coils varies very little with frequency.

That statement is true only at relatively low frequencies. Get near the
self-resonant frequency of an air core coil and you'll find otherwise.
Designers using relatively large coils over a wide frequency range run
into this problem all the time. As I mentioned in another post, the
classic example for Amateur Radio is the plate choke in a tube type
amplifier. Designing such a choke that has enough inductance to work
over the entire HF spectrum without self-resonances is nearly
impossible. Many amplifier designers don't even try; they just switch
inductance in and out of the choke depending on frequency.


Youall seem to be hitting all around the 'problem'. A coil has 3
components, the resistance of the wire, the inductance, and the stray
capacitance. As the frequency is changed from DC to low AC to RF each
component has more or a less effect on how it acts in a circuit. The actual
value of each does not change, just the effect on an external circuit.

For small coils at DC the reisitance is the major item that will be seen by
an external circuit. At low to medium frequencies the inductance will be
the major factor. At very high frequencies the capacitance may be the major
factor. At self resonant frequencies , the tuned circuit effect takes
over.

Bill Turner writes:

On Sun, 07 Dec 2003 10:24:02 +0000, John Devereux
wrote:

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

__________________________________________________ _______

Yes, it really is true. If you graph the reactance vs frequency of any
coil, starting just above DC, it will rise in a near-linear fashion for
a while, but will begin to steepen and when approaching the
self-resonant frequency, will quickly rise to maximum, and at that point
will suddenly drop to the opposite (negative, or capacitive) extreme and
then diminish back to near zero as the frequency continues to increase.

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

If you model a real-world "coil" as a perfect capacitor in parallel
with a perfect, *fixed*, inductor, it will behave as you
describe. (Well you need a resistor too if you don't want infinite
"Q"!)

At that self-resonant frequency, the coil is behaving like a parallel
resonant circuit, which of course it is, due to the parasitic
capacitance between each winding. This parasitic capacitance is
unavoidable and ALL coils exhibit this characteristic. The truly
strange thing is that above the self-resonant frequency, the coil
actually behaves exactly like a capacitor, believe it or not.

Real "Inductors" do indeed have a self-capacitance too, which will
make the component deviate from that of an ideal inductor in the way
that you describe. But this in itself does not make the inductance
(i.e. the inductive part of the reactance), vary.

SNIP

--

John Devereux

Bill Turner writes:

On Sun, 07 Dec 2003 10:24:02 +0000, John Devereux
wrote:

Well, just about anything is "non-linear" if you measure it accurately
enough! But is it really true that the *inductance* of a "small air
coil" is "dramatically" non-linear with frequency as you stated?

__________________________________________________ _______

Yes, it really is true. If you graph the reactance vs frequency of any
coil, starting just above DC, it will rise in a near-linear fashion for
a while, but will begin to steepen and when approaching the
self-resonant frequency, will quickly rise to maximum, and at that point
will suddenly drop to the opposite (negative, or capacitive) extreme and
then diminish back to near zero as the frequency continues to increase.

No, you are talking about the *reactance* ("reactive impedance"). We
have been talking about the *inductance* ! They are not the same
thing.

If you model a real-world "coil" as a perfect capacitor in parallel
with a perfect, *fixed*, inductor, it will behave as you
describe. (Well you need a resistor too if you don't want infinite
"Q"!)

At that self-resonant frequency, the coil is behaving like a parallel
resonant circuit, which of course it is, due to the parasitic
capacitance between each winding. This parasitic capacitance is
unavoidable and ALL coils exhibit this characteristic. The truly
strange thing is that above the self-resonant frequency, the coil
actually behaves exactly like a capacitor, believe it or not.

Real "Inductors" do indeed have a self-capacitance too, which will
make the component deviate from that of an ideal inductor in the way
that you describe. But this in itself does not make the inductance
(i.e. the inductive part of the reactance), vary.

SNIP

--

John Devereux

Bill Turner writes:

On 7 Dec 2003 04:21:04 -0800, Winfield Hill
wrote:

Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.


No, it does not. I'm afraid you are using the word "inductance" in a
different way from everyone else

It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Yes, because at high frequencies the current goes through the
capacitance of the coil rather than the *fixed* inductance.

(Uh, by the way, you do know who you are arguing with, right?)...



--

John Devereux

Bill Turner writes:

On 7 Dec 2003 04:21:04 -0800, Winfield Hill
wrote:

Bill, it's one thing to say a coil's reactance is non-linear, but it's
another to assert its inductance varies with frequency.

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency.


No, it does not. I'm afraid you are using the word "inductance" in a
different way from everyone else

It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Yes, because at high frequencies the current goes through the
capacitance of the coil rather than the *fixed* inductance.

(Uh, by the way, you do know who you are arguing with, right?)...



--

John Devereux

Bill Turner wrote:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Now you have gone and said something that is simply not true. The
small inductor has a nearly fixed inductance with a parallel with a
nearly fixed capacitance. The combined impedance of these two fixed
reactances produces a nonlinear impedance, but there is nothing about
that impedance that implies a large change in either the inductance or
capacitance of the combination, at least not until you get to so high
a frequency that the winding is a significant fraction of a cycle
long. The rise in impedance near resonance does not exhibit the same
phase shift that between voltage and current that a large inductance
would have.

--
John Popelish

Bill Turner wrote:

Both statements are true and easily provable. A simple air core coil
which measures one microhenry at a low frequency may have an inductance
of several millihenries (or even henries) when near its self resonant
frequency. It's a simple law of physics; there is no way around it.
And *above* the self-resonant frequency, the choke actually behaves like
a capacitor, believe it or not.

Now you have gone and said something that is simply not true. The
small inductor has a nearly fixed inductance with a parallel with a
nearly fixed capacitance. The combined impedance of these two fixed
reactances produces a nonlinear impedance, but there is nothing about
that impedance that implies a large change in either the inductance or
capacitance of the combination, at least not until you get to so high
a frequency that the winding is a significant fraction of a cycle
long. The rise in impedance near resonance does not exhibit the same
phase shift that between voltage and current that a large inductance
would have.

--
John Popelish

Bill Turner wrote:

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Agreed.

The inductance should be measured at whatever frequency you plan to use
the inductor, whether a single frequency or a wide band of frequencies.
Otherwise you risk a nasty surprise. To measure a coil at low frequency
and then label it as a "one microhenry" coil, for example, is asking for
trouble when that "one microhenry" coil is used at a higher frequency.
To be accurate, when you specify inductance you must also specify the
frequency of measurement.

Agreed. One usually specifies an inductor that is measured at a
higher frequency than the one being used.

(snip)

--
John Popelish

Bill Turner wrote:

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Agreed.

The inductance should be measured at whatever frequency you plan to use
the inductor, whether a single frequency or a wide band of frequencies.
Otherwise you risk a nasty surprise. To measure a coil at low frequency
and then label it as a "one microhenry" coil, for example, is asking for
trouble when that "one microhenry" coil is used at a higher frequency.
To be accurate, when you specify inductance you must also specify the
frequency of measurement.

Agreed. One usually specifies an inductor that is measured at a
higher frequency than the one being used.

(snip)

--
John Popelish

In article , Bill Turner
writes:

On Sun, 07 Dec 2003 13:55:35 +0200, Paul Keinanen
wrote:

One can still argue that the inductance and inductive reactance are as
well as the capacitance and the capacitive reactance are still there
as separate entities, but we can not measure them separately from
terminals of the coil. Thus, this is an artefact of the measurement
method.

Not only can you *not* measure them separately, they can not be
physically separated either, since the parasitic capacitance is always
present between adjacent windings. I would not call it an artifact of
the measurement method, but rather an artifact of the coil itself.

Nonsense. General Radio had a nice little formula way back
before 1956 for finding the distributed capacity of an inductor.
It was published in the Green Bible (ITT Reference Data for
Radio Engineers, small format, dark green hard cover). I used it
years ago and earlier this year and many times between.

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

Now if someone actually wants to WIND COILS, I have a little aid
for tiny ones wound on common screw thread forms that was
published in Ham Radio magazine. Has measured Qs over
frequency as well as basic inductance. I'll attach it to private e-mail
(PDF) to anyone that requests it.

Using common screw thread formers and solid wire allows a good
repeatability between bench and application. Forms can be anything
from a 4-40 bolt to a common screw-thread lamp base.

Folks in here are getting too wound up...and coiling to strike. :-)

Len Anderson
retired (from regular hours) electronic engineer person