One thing that might be helpful to know about bipolar coils is that they
are actually two coils on the same former that are centerpoint grounded,
usually with a space in the center, so as far as tuning is concerned,
enter your parameters for the secondary based on 1/2 of the coil length,
minus the gap in the center...just like it was a single coil. That being
the case, bipolar secondaries need to be quite long to get good
performance. If you wind them the length of a standard coil, you end up
with the equivalent performance of two coils of half that length. So
overall ratios of a bipolar should be somewhere in the 1:6-1:9 range. As
far as winding the coil...you can just wind a continuous coil and do
your lacquer treatment, and then go back later and cut it in the center
and remove an inch or so of the middle windings, then tie the two ends
together and design some sort of a (well insulated)ground connection
from there.
Its also possible to design/build a horizontal coil that is continuous,
i.e. without center tap, but note that the primary to secondary coupling
must be very loose, or you will have serious issues with racing sparks.
The vintage ones I have seen were something like a 6" secondary and a
20-30 inch primary.
Coupling on bipolars is a consideration as well. I built mine with a 4"
secondary and an 8 inch primary former, and the coupling is still too
tight. 12" probably would have been perfect.
Hope that's helpful.
Shannon Weinhold
Klasdja Intelligent Innovations
-----Original Message-----
From: Jim Lux [mailto:jimlux@xxxxxxxxxxxxx]
Sent: Friday, July 29, 2011 7:51 AM
To: tesla@xxxxxxxxxx
Subject: 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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