# Re: Number of turns for a secondary?

```Tesla List wrote:
>
> Original Poster: Hollmike-at-aol-dot-com
>
> In a message dated 1/3/99 3:00:04 AM Mountain Standard Time,
tesla-at-pupman-dot-com
> writes:
>
> >
> >  This thread has made me curious as I plan on winding a larger diameter
coil
> >  soon myself. I always thought that you decreased the gauge of the wire (
> > fewer
> >  turns per inch) to keep the H/D ratio compatible with the number of turns
> >  (ideally somewhere between 4:1 or 5:1 & under 1000 turns). I thought that
> > was
> >  why (aside from the current being carried) the larger coils always used a
> >  heavier gauge wire. Like I said I'm just curious & I know a lot of other
> >  factors come into play in the design. Thanks,  Jim
> >
> Jim,
>     According to one reference I have  the maximum inductance of a single
> layer solenoid coil occurs when the H/D ratio is 2.46:1.

That's for a fixed wire length.  Adding more turns always increases
both the inductance and, at least up to a limit, the Q as well.

>I once did a long
> drawn out calculation(based on the Wheeler equation) that showed this to
occur
> at a ratio of 2.22:1.   My calculation was based on a fixed length of
wire, so
> the number of turns went down as the diameter goes up.  It seems to be in
fair
> agreement with the text.
>     This doesn't mean that that is the best design for Tesla coils though.
> One still has to consider the coil's self capacitance and it's ability to
> withstand the high voltage difference between each end(probably the most
> important aspect in the design criteria).  This is probably why most coilers
> design them with H/D ratios from 4:1 - 6:1.
> Hope this helps,
> Mike

```