On Thu, 22 Oct 2015 09:54:24 -0500, amdx wrote:

On 10/22/2015 9:43 AM, Brian Howie wrote:
On Wed, 21 Oct 2015 05:15:18 -0500, amdx wrote:


30pf per ft is a general number for capacitance of coax, but you know
it varies with type. I have some coax for automobile radio antennas
(AM/FM) that has 8pf per foot.
Mikek


Well its not real coax , but the diameter is about 5ft ( well its
octagonal ) say 15ft circumferance , There's 7 turns in close
proximity to the tube or each other, so the capacitance could be a few
hundred pf . I suppose could try measuring it. Self resonance with
12 turns was about 400KHz

http://www.angelfire.com/mb/amandx/loop.html


so effective minimumum capacitance works out around 100pf

Brian


I'm not sure if you're discussing 100pf of interwinding capacitance or
capacitance between the shield and the winding.

Mikek

It's pretty complicated .There's distributed capacitance between the
windings and distributed capacitance between the windings and the
shield. I know the inductance of the loop and the resonant frequency
with the variable C set to minimum, so I can work out an effective
capacitance 100pf that causes the resonance.

There's a further complication in that the coupling loop is also
capacitively coupled to the main windings and the shield, so
connecting that up changes the resonance as well.

I can't even begin to think how to model it.

The unshielded wide-spaced loop makes it easier to design, but you
get ( arguably) no electric field shielding, which is where we came
in.

I started out using the maths but ended up cut and try to get a
compromise solution.

Brian

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