> I run my cables inside galvanized water pipe buried 8",
> the metal shield
> magnetic field and electrical field. PVC pipe or direct
> bury does not
> protect you from magnetic field, in special from lightning
> strikes.
The fact that a shield is steel or iron or not doesn't make
a bit of difference for energy above a few Hz, except it
might actually make it a poorer shield from increased
resistance. When the frequency is high enough and the area
large enough eddy currents cancel the magnetic field, that's
why we have to powder iron cores or laminate audio
transformers with insulated very thin slices. We have to
make it so there is no room for eddy currents to flow.
See some measurements at:
http://www.w8ji.com/skindepth.htm
Those great big steel cars that park on the magnetic sensor
loops in the pavement actually raise the frequency because
the eddy currents generate a strong counter mmf that cancels
any magnetic effects. The same is true if you stick a steel
or iron screwdriver blade into an inductor running at much
more than audio frequencies. Put the steel or iron blade of
a screwdriver in an RF coil and the inductance decreases,
just as it does with brass or aluminum slug.
That shield can be copper, aluminum, or iron and work the
same so long as it is a solid conductor. If the wall is
thick enough nothing that is time-varying goes through the
wall. Neither magnetic or electric. True for a small loop
antenna, and true for any shield.
73 Tom
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