# Re: [TCML] CW Tesla Coil calculations?

```Wil,

Java TC is fine and calculating the resonant frequencies of coils, however
it does not tell you what frequency that the coupled coils will resonate at
together.  Because there is a mutual inductance between the coils, the
operating frequency will be somewhat different than the resonant frequency
of either the primary or secondary.  The field of the 2 coils can add,
resulting in a lower operating frequency than the resonant frequencies
(often referred to as the "lower pole" mode), or the fields can cancel and
the operating frequency will be higher than the Fres of either coil (upper
pole mode).  The coupling coefficient determines the mutual inductance, and
consequently how far away the operating frequency will be from the
"resonant frequency".

I suggest using a circuit simulator like LTspice (which is freeware), and
making a model from the Ls and Cs that javaTC reports.  From here you can
run an AC sweep analysis and spot the possible operating frequencies, which
show up as peaks in the response (this is where the term "pole" comes from
as the frequency response has distinct peaks that resemble something like a
circus tent held up by a pole).

Richie Burnett has a good write up about frequency splitting, which is the
same phenomena im trying to describe:

http://www.richieburnett.co.uk/operatn2.html#splitting

The only real distinction to be made about a CW coil is that you are not
exciting both pole frequencies (like a SGTC does, for example, and the
result is a beating waveform... the sum of 2 frequencies).  CW coils will
"settle out" at one pole or the other, depending on tuning and how the
circuit operates.

As to what frequency produces the longest sparks, thats a really tough
question.  Higher frequency operation produces a hotter arc which can grow
longer for a given voltage, however its not free lunch since efficiency
often drops at higher frequency operation due to resistive loss, and losses
in the tube.  Id probably go for lower frequency operation if larger sparks
were the target, mainly to get the efficiency up so that more power could
be processed before melting down.

Steve

On Tue, Aug 5, 2014 at 11:56 PM, William Howard <snakeprior@xxxxxxxxx>
wrote:

> Dear Group,
> I'm having a dreadful time tuning my VTTC. I have been using Java TC as a
> guide for primary turns and using multi taps.
>
> My question is: is Java TC calculator accurate for CW Tesla Coils or would
> the frequencies be different?
>
> Further more what is the best frequency for spark length?  I've noticed the
> spark becomes shorter and more flame like above 1mhz until 6mhz it becomes
> a flame.
>
> If I were to design the optimum system what resonant frequency of the
> secondary should I use to get the longest CW spark?
>
> Cheers!
> -Wil
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