Re: Spaced Coil measurements.
Hi Malcolm, All,
Using your measurements I get 18.25 pF from Medhurst's formula. I
plotted your data in Excel as 1/(f^2) VS C according to Howe. A
linear trend line fit to the data crosses the axis at about -50 pF.
50 pF seems a bit high. You gave Ns = 38 turns for the whole coil?
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
Sent: Monday, May 31, 1999 8:27 PM
Subject: Spaced Coil measurements.
Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
As promised, here are the measurements (and they do contain a
The Coil: Hs = 31.5", Ds = 12.3", Ns = 38 turns Dwire = 0.8mm
Winding pitch = 1.2 TPI, former = thinwall PVC tubing
According to Wheeler, Ls = 147.5uH
" " Medhurst, Cs = 17.8pF (or thereabouts - I estimated "H")
Measurement of L was rendered impossible by the stray electrical
signals flying round, mostly from local radio transmitters :( I was
bitterly disappointed by this and will figure out some other way to
get a direct reading.
However, if one takes Medhurst's Cself as gospel and adds that
to the shunt capacitances listed below, one gets a remarkable result:
Bare Coil, w. 100pF shunt w. 228pF shunt w. 336pF shunt
Fr 2.9MHz 1.113MHz 773kHz 644kHz
Ls 169.2uH 173.6uH 172.5uH 172.6uH
Now that I hadn't expected. Possible interpretations:
- Medhurst works and lumped rules hold good for this coil BUT Wheeler
- Suppose Wheeler is correct. Then Medhurst fails to give the
correct figure for Cself (if this were true, reading #1 suggests
Cself = 20.4pF, reading #4 suggests Cself = 78pF). This contradicts
Until Ls can be measured directly and definitiively, no firm result
but something clearly doesn't work properly for this case.