# RE: How to Tune a Flat Spiral Coil

```Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Paul,

>I'm assuming here that you intend to ground the outer end(s) of the coil
during operation, so that the HV is developed at the center.

No, I find that grounding the outer end does not change the operation of the
coil much.  Only when the system is out of tune does a ground seem to make
any difference.

As I see it, there are three different types of pressure in a tuned flat
spiral secondary.  There is the transverse voltage that is completely
contained within the coil windings with the peak near the outer turns.  The
furthest turn outward has zero volts as it is a node.  Then there is the
electrostatic pressure that is caused along the pole of the coil.  The
electrostatic pressure is greatest directly above either pole and drops of
in the same slope as a damped wave.  The third pressure is the longitudinal
wave that emanates in a broad plane away from the outer turns of the coil.

My goal is to keep the transverse voltage completely within the coil turns
with the hope of maximizing the other effects produced be a flat spiral
secondary coil.

Start at a low frequency and sweep slowly upwards, monitoring
the AC voltage until you see a pronounced dip.  Note the
frequency at which the voltage dips lowest.  Then continue
sweeping upwards, to find the next resonance (won't be quite
such a low dip).  Continue until you've found the lowest
three resonances.

>> what I'm saying is that if I have a 35mH coil

>You've mentioned 35mH a couple of times recently - where does this come
same coil?

I have several coils.  The 35mH coil is a single wound flat spiral.  The
6.23mH coil is a trifilar coil in a wye formation.  You are correct, though,
I referred to the resonant frequency of the wye coil and the inductance of
the single wound coil.

I haven't had the time yet to measure the resonant frequency according to
your instructions, but I will do that this afternoon.  Using the method I
outlined, I used the frequency of the coil and the inductance to calculate
the primary capacitor.  For the 6.23mH coil, the calculated capacitance of
.2uF would make it resonate at 4.54KHz.  But this didn't work.  The spark
gap wouldn't fire.  So I used your earlier assumption that the inductance
would be 9 times the single wire inductance.  Calculating the capacitance
for an inductance of 56.07mH, the coil resonated nicely.  I was able to see
concentric layers within the plasma ball, indicating to me that a good
degree of resonance was obtained.

>From this early data, I would guess that your assessment of 9 times the
single wire inductance for a trifilar wound coil is accurate.

>In a way, that could be a reasonable statement.  It all depends on the
coupling coefficient (k-factor) between your primary and secondary.  If it's
very high, then both coils, along with their various respective
capacitances, will just combine their effects into a single resonance,
rather than the two needed for a conventional TC.

>Tell us about your primary: dimensions, turns, etc, and its position wrt
the secondary.

On the wye coil mentioned above, the primary is 1/4" above the secondary.
The last turn of the secondary is 1/2" radius away from the inside turn of
the primary.  The primary is composed of 3.5 turns of 1/4" OD copper tubing
with about 1/4" space between turns.

Dave

```