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Re: [TCML] Confusing Between 1/4 and 1/2 Wave Coil
On 7/29/11 4:20 AM, Cole Awesome-Jordan wrote:
Hi, I've had a bit of free time recently and i decided to go digging for
information on bipolar coils and the difference between 1/4 and 1/2 wave
coils.
Aieee... Do not think in terms of wavelength and coil configuration.
From a functional standpoint, lumped models are simpler, easier, and
more accurate. That whole "wind a 1/4 wavelength of wire" thing was a
coincidence for some shapes of coils that just happened to work sort of,
but confused everything.
And I read this
http://www.pupman.com/listarchives/2004/January/threads.html#00804
however this got me quite confused on the subject. When you calculate a
"normal" coil with kHz=1/(2*?*L*C) is this half or quarter wave? It's
half isn't it?
Neither.. that is the resonant frequency of the secondary LC combination.
And if you have a bipolar coil that is 1000 turns of 26 gauge magnet
wire (55.25 turns per inch) and the form is 2.375" by 18.125" (approx.)
what is the frequency? Because I've been getting all sorts of answers
some say multiple the frequency by 2, another said 1.38-1.4, and a
really odd answer I found of 2.6 somewhere.
When you say bipolar, do you mean a coil where both ends have HV
electrodes on them, as opposed to the more conventional ground plane on
one end, HV terminal on the other?
The inductance of that secondary is about 6.9 mH. It's the self C
that's going to be tricky. If it were vertical above a ground plane, it
would be about 6.5 pF (for a fRes of 750 kHz, with no topload).
The tricky thing is that the Medhurst formula
a) starts to break down for large H/D, and you're at 8:1
b) doesn't work for a "coil in free space"
If you have a "pancake" primary in the center of the coil, then, you can
probably use Medhurst, figure out the self C of a 9" long coil, and then
double it.
(that works out to about 4.2pF*2 or 8.4pF, by the way)
So now, you're looking at LC of 6.9mH and 8.4 pF, which will resonate at
662kHz..
I'm sure you'll have questions... ask away
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