[TowerTalk] iron pipe "lightning chokes"

[Tom Rauch]

> .I am QUITE certain that any smart engineer would specify
that the
> .ground conductor be bonded to ferrous conduit at each
end. Why?
> .Because it it was not bonded, the conduit WOULD
significantly
> .increase the inductance of the ground wire running
through it.

I doubt it. Eddy currents would prevent much from happening
due to the fact it was ferrous.

There is a transformer effect that would be the same
mechanisim that occurs in a "shielded" loop. Current on
conductors INSIDE the conduit would induce an opposing
potential on the conduit inner wall, and that would cause a
parallel flowing current on the outer surface of the
conduit. Same thing that happens in concentric RF conductors
used in transformers as well as coaxial cables.

 But
> .by bonding it at both ends, the bonding conductor
consists of the
> .parallel combination of the copper wire inside it and the
conduit
> .itself. At low frequencies, the copper may contribute
significantly
> .to the conductance. As frequency increases, the current
will flow
> .almost entirely in the conduit.

True enough. Except the magnetic effects of a non-isolated
iron sheeth aren't what people seem to be thinking. At
higher frequencies (much above DC) the fact the conduit
isn't laminated or made from very tiny insulated iron
particles nulls any magnetic effects, other than an increase
in resistance obvious in skin depth formulas.

I don't understand why people think iron means increased
inductance for alternating or rapidly changing fields, yet
they gon out and buy powdered iron for inductors and
laminated transformers for power frequencies.

> The sense I had was the opposite.  The iron pipe would
present an
> inductance,

Not significantly different than a copper or aluminum pipe
would.

73 Tom