----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Wednesday, February 02, 2005 10:23 PM
Subject: Re: [TowerTalk] iron pipe "lightning chokes"
> On Wed, 2 Feb 2005 22:26:42 -0500, Jim Jarvis wrote:
>
> >The sense I had was the opposite.
>
> The literature disagrees with you. So does NEC. See the IEEE Emerald
> Book and Green Book. Also see Ott.
>
> The conduit DOES greatly increase the inductance of a wire within it
> that is NOT connected to the conduit at both ends. But when you
> connect the wire on both ends, you make the conduit part of the
> circuit, and the current flows on its skin, just like it would on
> any other conductor.
>
Sort of bringing us back full circle... how does one calculate the
inductance (and loss.. since that's pretty signficant).
Lightning impulses are pretty broadband, and I suspect that the L and R are
strongly frequency dependent, so coming up with a useful "loss" or
"attenuation" number would be quite complex, since you'd have to integrate
over all frequencies.
However, one should be able to do the analysis for a transient. Since you
can approximate lightning transients as the sum of two exponentials
(something easy to manipulate mathmatically), it might not be that hard to
come up with an analytical solution.
I think the problem is very similar to the classic lab demo of eddy current
braking. You have a conductive tube and drop a magnet through it. The
magnet takes much longer to fall through the tube than it would the
equivalent distance in free air, so energy is being dissipated as heat in
the tube.
As far a skin depth and electrical resistance of the conduit goes, it's
about 10 microns for 1MHz in steel. Even so, the RF resistance is something
like half an ohm per meter for 1MHz and 1" conduit.
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