On Sun, 13 Aug 2006 22:15:38 +0100, "Reg Edwards"
wrote:


"Walter Maxwell" wrote
Reg, I had never given much thought to the series relationship of
the
capacitance between turns. I had always considered them as being in
parallel,
thus the honeycomb, or the basket-weave configurations to minimize
the interturn
capacitance. Have I misconstrued the purpose of those
configurations?

Do I also understand you correctly that with a specified length of
the solenoid,
and a given diameter, the total interturn capacitance is independent
of the
number of turns, because the capacitance between turns adds in
series to the
same value regardless of the number of turns?

====================================
Walt,

As I said, I was referring only to the solenoid form.

Below the self-resonant frequency and for some way above it, the
distributed self-capacitance is equivalent to a lumped capacitor
across the ends of the coil. Coi

Because capacitances between adjacent turns are in series with each
other, the capacitance between turns only matters when there are only
one or two turns. So, for ordinary proportioned coils, when there are
more than a few turns, the self-capacitance tends to become
independent of the number of turns, wire diameter and wire spacing.

The wire turns can be considered to form the outside of a Faraday
cage.

To calculate self capacitance, consider wire spacing to be zero. When
isolated in space we have the capacitance between the two fat halves
of a dipole. Which is calculable from length and diameter of the coil,
and is equivalent to a lumped capacitance between its ends, which may
be used to calculate the self-resonant frequency.

Or the self-resonant frequency can be calculated directly from
dimensions and number of turns.

In the past I have measured the self-resonant frequency of coils of
all sorts of dimensions. From antenna loading coils, coax choke
coils, to 6 feet long, 1 inch diameter, 1000 turns, 160-meter helical
antennas. In all cases measurement results agree with the calculating
formula within the uncertainties of the measured input data.
----
Reg.

Thanks, Reg, for the valuable insight. It does pay to read the posts made by one
G4FGQ.

Walt