How to optimize parameters for making a coil with high-Q?

[Phil Allison]

"Bob Eld = Fucking Jerk Off "

True but ferrites have core losses not found in air. Of course it depends
on inductance range At one MHz it pretty much a toss up.

** But you said " in general ....".

Which is complete BULLSHIT !!!.

Even at 1MHZ , for practical sizes, ferrite cored inductors are way ahead.

Wot a fucking PITA jerk you are.

Must be a radio ham - right ?

Or just a total cunt head.

FOAD you vile pig.





...... Phil

 

[John Popelish]

Arthur C. wrote:

I want to make a simple coil (inductor) with a Q value as high as possible.
Now, I would like to know if there are simple guidelines on how to choose
the parameters to obtain this. Parameters are:

- thickness of the wire
- number of rounds
- length of the coil
- area of cross-section

I assume you are talking about an air core coil. Things get
even more involved when you include a high permeable core
material, but higher Q is often possible in a smaller volume
with a core.

Also, I assume you are talking about a lumped inductor,
where the length of the wire is very short compared to the
signal wavelength (this implies that the current is
instantaneously the same in every part of the wire).
Otherwise, the effective inductance involves waves
reflecting back and forth along the wire.

I understand that a realistic description of a coil involves (at least) a
parallel parasitic capacitance C and a series resistance R.
The resulting Q value would then be Q = (1/R) sqrt (L/C). Is this indeed the
relevant expression for Q?

Only if you are using the inductor as a self resonant
system, since that is the formula for the Q of a resonance.
If you use the inductor well below its self resonant
frequency, the effect of the stray capacitance is to just
reduce the total inductance a bit, and the formula for Q is
more closely, Q=w*L/R, where w (omega or frequency in
radians per second) is 2*pi*frequency in hertz.

If the coil is operated at a frequency of the order 1 MHz, can we assume for
R just the 'DC' series resistance of the wire, or does it change with
frequency?

Unfortunately, it is not so simple. Any time the magnitude
or direction of magnetic flux penetrating a conductive
material changes, a current is induced to circulate around
that changing flux. The magnetic field produced by that
circulating (eddy) current bucks the field that is causing
the flux to change, slowing the change. The effect in wire
is that the current first changes along the surface, and
those changes sink into the conductor over time. This "skin
effect" causes the current to use less than the full cross
section of the wire, raising the effective AC resistance
above the DC resistance (which produces no changing flux).
http://en.wikipedia.org/wiki/Skin_effect
At 1 MHz, the effective conductor depth is only 66 um, so
wire that is progressively more than twice that distance in
diameter has progressively higher AC resistance than its DC
resistance.

This wire table:
http://www.pupman.com/listarchives/1998/April/msg00222.html
shows dimensions of AWG sizes. AWG 35 wire has a diameter
of 0.143 mm, or 132 um, so any wire larger than about that
size wastes progressively more of its cross section as far
as its resistance at 1 MHz. This is why high Q RF coils are
often made with Litz wire, a woven bundle of fine, insulated
strands.

And, how can C be calculated?

Not simply. Either you find an empirical formula for the
winding style you are using (i.e. uniformly spaced,
straight, single layer solenoid) that someone else has
produced, or you use a finite element analysis program that
models the surface of the winding to approximate the
effective capacitance. Or you make a series of variations
and measure their properties at several frequencies and
calculate the lumped capacitance resistance and inductance
that best fits those measurements.

For inductance L I have found some useful information, but for the others
not yet...


Any suggestions? Thanks for your time,

Google will find you lots of good references, probably
starting with:
http://en.wikipedia.org/wiki/Inductor

The subtlety of inductors has kept me entertained for years
and years, so don't feel too bad, if you don't answer every
question in an afternoon.

--
Regards,

John Popelish

 

[Michael A. Terrell]

Bob Eld wrote:

"Phil Allison" <philallison@tpg.com.au> wrote in message
news:6c4k0uF3e0o40U1@mid.individual.net...

Which is complete BULLSHIT !!!.

Even at 1MHZ , for practical sizes, ferrite cored inductors are way
ahead.


Wot a fucking PITA jerk you are.
...... Phil

Hey Phil. Want to come to California and get married? Kiss Kiss. You know I
love you!

I think both parties still have to be human. Looks like Phil is still
out of luck.


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