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

```Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Tesla list wrote:

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

> matter what the shape and flat spirals have no special and desireable
> electrical properties.

This is not my experience.  Wind a flat spiral coil to 300 kHz, then wind a
solenoid coil to 300 kHz.  Coat each coil with 1/8" epoxy.  The flat spiral
coil will rise in frequency, the solenoid will drop in frequency.  The
geometries definitely do make a difference.

Further, there are two separate Wheeler equations for calculating the
inductance of flat spirals and solenoids.  The reason there are two
different equations, rather than just one, is because the geometry *does*
make a difference in the coil's properties.

Also, flat spiral coils have a greater range of resonance than do solenoid
coils.  For the same amount of wire and same average radius, a flat spiral
coil will have an output with lower potential and higher current than a
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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. I can't understand how the flat spiral can rise in frequency with the epoxy coating and find it hard to believe.
```Haven't tried it myself so am no expert.

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

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