# Re: Measuring self-capacitance directly (Re: flat secondary)

```Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Paul, All -

Tesla list wrote:
>
> Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <paul-at-abelian.demon.co.uk>
> Referring to
>
> <http://www.abelian.demon.co.uk/tssp/formulae.html>http://www.abelian.demo
> n.co.uk/tssp/formulae.html
>
> Use the first formula to work out Fres, use the second formula to
> work out the effective series inductance Les from the DC inductance.
>
> Then Ces = 1/(4*pi*pi*Fres*Fres*Les)
>
> The formulas should give you an answer to within 10% for coils with
> h/d > 1.5 and coil former thickness less than 5% of the coil
> diameter, which covers most coils except the small table-top
> variety.  It'll probably get you closer than attempting to measure
> Cdc and applying a 50% rule of thumb.  Maybe someone will be nice
> and code the arithmetic into one of those web page calculator
> things.
> --
> Paul Nicholson
> --

Paul, I checked this on my new helical. For reference, the following
measurements were tripple checked for detail: NOTE: I rechecked turns per inch
over and over. Kept hitting 45. I had my son count for me a while back and he
came up with 42. This time was under a magnifying glass, so I know it's right.

Secondary (inches):
8.4375 diameter
40.375 length
45 turns per inch
1817 turns
24 awg
bottom turn is 17.5 inches off the floor.

Measured resonant frequency and inductance in garage:
Fr=111.9kHz, L=134.6mH

Then measured outdoors (no walls within 15 feet and scope probe over 6 feet
away). I used Al sheet under the coil for a ground plane. This time,
Fr=112.5kHz

Your calculations show Fr=113.24kHz (only 0.66% error!). Interestingly, Wheeler
would give something around 106kHz, roughly 5.75% error. Anyway, Ldc=133.78mH,
Les=112.52mH. This puts Ces at 17.56pF (Medhurst would be just under 17pF).

I simply pumped your equations into Excel to run the numbers and was (once
again) gleeming with excitement of computers telling the tale!

Very cool!

Take care,
Bart

```