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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>"
> Referring to
> 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
> 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.
45 turns per inch
bottom turn is 17.5 inches off the floor.
Measured resonant frequency and inductance in garage:
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,
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!