# RE: Flat coils & undamped waves (was Wire Length)

Original poster: "David Thomson" <dwt@xxxxxxxxxxxx>

Hi Ed,

> > >      Answer to your question is that "inductors are inductors" no
> > > matter what the shape and flat spirals have no special and desireable
> > > electrical properties.
...
>     Too much to comment on in one note.  Of course the geometries
> make a difference, the question is whether one is superior to the
> other and I voted for the solenoid.

Neither coils is superior to the other.  They both have different uses.  It
is a matter of applying the right coil to the right usage.  If your goal is
to spill electrons all over the place with a spectacular display, and you
plan to use only one coil, then yes, the solenoid would be your choice.
However, some people are a little more sophisticated and choose to
investigate the many different qualities of coils.

> I can't understand how the flat
> spiral can rise in frequency with the epoxy coating and find it hard
> to believe.

The flat spiral coil is naturally disposed to lower potentials and higher
currents than is the solenoid coil.  This is the inherent difference due to
their geometries.  When coating is first applied to a flat spiral, instead
of increasing the capacitance, the dielectric causes the potential to
increase, thus raising the resonance.  With a solenoid coil, the potential
is already maxed out so that coating it increases the capacitance, and thus
lowers the resonance.

I just performed these measurements last week.  I used the Wheeler equation
and resonance equation to predict how many turns will produce a flat spiral
coil of 299kHz.  The equation was spot on.  I verified this using a
frequency generator.  I also wound a tall solenoid to the same resonance and
tested it.  Both coils were resonant with each other.

Then I coated the flat spiral and insulated the tall solenoid.  The flat
spiral coil increased to 430kHz, the tall solenoid dropped to 280kHz.  I
spent two days constantly rechecking the measurements and working out the
mechanics involved.  If you don't believe it, try it for yourself.

As a result of the above experience, I have had to wind a new flat spiral
coil.  This time I wound it to .753 of the target frequency, anticipating
that insulating the coil will bring it up to the frequency I want.

> Haven't tried it myself so am no expert.

Normally these roles are reversed between us.

>     I'm not sure what you mean by "greater range of resonance" -
> that's not a familiar term.

Depending upon the treatment of the coils, all things being as equal as
possible, a tall solenoid could have a resonance range of 15kHz, while a
flat spiral coil of similar dimensions would have a range of 30kHz.  You
were just having a discussion with Gary Peterson on spread spectrum coils.
A wide resonance range is not the matter you were discussing, but a coil
with a wider resonance range could certainly resonate at more frequencies
than a coil with a narrower range.  Tesla could easily have transmitted
several different frequencies simultaneously on a flat spiral secondary
system.

Once again, this is something you can check for yourself.  Wind a flat
spiral coil and a solenoid coil to the same resonance.  See how broad their
peaks are.

> As for lower potential and higher
> current, that is a function of the surge inpedance of the circuit and
> not the geometry of the inductor.  For the same L/C ratio the
> performance should be the same.

If you wind flat spiral and solenoid coils with the same gages of wire, and
with the same average radius and wire length, you will find not only are the
potential/current ratios different, but also the inductance/capacitance
ratios are different, too.  Similar coils with different geometries *do not*
have the same L/C ratio.

Without even building the coil, and using just a coil modeling program, you
can make these calculations and see for yourself.  Coil geometry *does* make
a difference.

Dave