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


"Arthur C." <nospam@xxxxxxxxxxxxxxxx> wrote in message
news:485b86ff$0$14348$e4fe514c@xxxxxxxxxxxxxxxxx
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 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?
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?
And, how can C be calculated?
For inductance L I have found some useful information, but for the others
not yet...


Any suggestions? Thanks for your time,

Arthur C.

To maximize Q, the wire should be a short as possible.

In general air core coils will have a higher Q than those with magnetic
materials. But the core material depends on the frequency range consistent
with the short statement. High inductance is not feasible in air core coils.

Wire should be litz wire made up of multiple strands of thin wire insulated
from each other. Especially true at 1MHz

High power, high frequency coils can be solid wire or tubing. 30MHz up.

To minimize winding capacitance, wire should be lace wound, layered back and
forth to minimize the proximity of one turn to the next. Necessary for High
Q at 1 MHz

The models of an inductor is the inductance in series with the resistance
and shunted by the stray capacitance. Below self resonance, the Q = Xl/R.
But, the R is the AC resistance of the wire at frequency, not the DC
resistance. At one MHz this value is much higher than the DC resistance
because of skin effect. That's the reason for Litz wire.

Lace winding is best done on a coil winding machine. It's probably a good
idea to go buy the inductor you need.



.

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