[TowerTalk] Low Inductance Grounded -Sleeve & bends

[Tom Rauch]

> > Are you saying the fact it is steel changes something
other
> > than resistivity?
>
> Yes, it's also ferromagnetic...
> A piece of pipe essentially acts similar to a lossy
ferrite.

Not so!

Ferrite materials have the magnetically soft iron particles
suspended in insulation, The insulation prevents the
particles from contacting each other. Omitting insulation
allows eddy currents to flow. Eddy currents generate a
countering magnetic field, so the net effect is not at all
like core materials.

As an example, if you place a solid steel rod inside an RF
coil the inductance DECREASES!

This is the reason cores in power or audio transformers are
laminated, and each lamination is insulated from the next. A
given lamination thickness establishes a cutoff frequency
where the material acts like "magnetic short" and
effectively has no external magnetic properties.

You can see this effect in core materials. A 73 mix material
has equal resistivity and inductance at 1 or 2 MHz. As
frequency is increased, the material provides less and less
inductance and eventually some materials will even change
signs!!

At 2MHz and higher, a conductor passed through a steel
tubing has LESS series impedance (inductance) than the same
conductor in open air, although loss resistance increases. I
suppose at some very low frequency near dc the steel could
increase magnetic effects, but it would have to be where the
eddy currents are small. That would be well below power line
frequencies.

I found this through measurement while playing with that
nonsense idea the Handbook published about using steel wool
to make a balun.

73 Tom