[TowerTalk] Fields and coax

[Tom Rauch]

> >Not So!   And RF signal consists of an electric field and
a magnetic
> >field.  The magnetic field can be influenced by ferous
materials
> >outside of the coax and can induce current in them.  The
copper or
> >aluminum shield cannot contain the magnetic field.  This
is why it
> >is recommended to route coax AWAY from conduit and water
pipes
> >through the house.  This is also why coiling coax is a
no-no.  You
> >concentrate the magnetic field and the coil becomes a
choke.
> >Especially in VHF and up.


He's totally wrong Bill. I hope you corrected him on that!!

The time-varying magnetic field is fully shielded by the
shield just as the time-varying electric field is. Both are
a "partner" of each other, and in the case of TEM mode
transmission in a transmission line if you take the electric
field to zero you take the magnetic field to zero you take
the electric field to zero, and vice versa.

That's why our transmitters can be shielded perfectly with
copper shield or aluminum shields as long as the shield is
several skin depths thick. That's also why a magnetic loop,
at a distance just over 1/8th wavelength from the loop, has
a dominant electric field (stronger than the magnetic
induction field) and why we can't make the electric field
change to zero near the loop by shielding it unless we also
take the magnetic field to zero...at which point the loop
would not radiate at all!!!

73 Tom