[TowerTalk] ground systems... ferrous conduit

[Tom Rauch]

> >I recall reading that this serves as a choke for current
> >flowing on those cables.
>
> For that matter.. how would one calculate the L and R of
such a choke?
>
> Consider it as a single turn inductor?  What's mu for mild
> steel?  Resitivity is 0.13 vs 0.017 for copper.

I don't understand that.

It seems to me eddy currents would prevent the conduit from
having much magnetic effect on conductors much above DC, so
I doubt the inductance increase is valid. As a matter of
fact it probably decreases inductance at frequencies much
above audio.

 When I measure common mode impedance of cables at MF and
higher and put them inside steel (or copper), impedance
drops. There was an article in QST and the Handbook about a
steel wool balun. The theory was similar, that the steel
would increase impedance. In reality, packing steel wool
around the cable actually decreased impedance of the cable
shield.

When you park your big steel car over those ac magnetic
loops in traffic sensors, the AC impedance of the loop
DECREASES. When you put a solid slug of steel or iron in or
around a coil carrying AC, the impedance decreases. It's
only when the particles are insulated or laminated to
prevent eddy currents that inductance increases. The size of
the area where eddy currents are formed limits the frequency
where opposing magnetic forces from the eddy currents
overtakes magnetic effects. That's why we use powdered iron
cores with insulated particles at radio frequencies, and why
we get away with laminating cores in inductors and
transformers at really low frequencies.

Why on earth would steel conduit in this application be any
different? I suspect it is a different reason or someone
seriously goofed. Maybe they think lightning is continuous
DC. Maybe they didn't understand inductors.

73 Tom