[Amps] wires through center of coax

Dan Zimmerman N3OX n3ox at n3ox.net
Mon Dec 20 16:11:46 PST 2010

> By adding a conductor on the inside of the hollow center conductor that is
> not at the same potential as the holllow center conductor you change the
> unit length capacitance.
>     If the wire is at the same potential as the shield the character
> impedance will be lowered.

If you put a conductor grounded on each end inside the hollow center
conductor, and the hollow center conductor wall is many skin depths thick
(which it always will be for hollow-core coax, so it doesn't matter if you
delete the rest of the metal) , you end up with a second transmission line
in parallel  with the first, with its own characteristic impedance.

That's not the same as lowering the characteristic impedance of the original

The EM energy inside the original coax is well shielded from the interior of
the center conductor just like it's shielded from the outside world.
 Energy can certainly couple into the new inner transmission line at the
ends, but not right through the wall of the hollow center conductor.

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