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Re: [RFI] Eliminating TVI

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Eliminating TVI
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Mon, 29 Dec 2003 08:34:10 -0500
List-post: <mailto:rfi@contesting.com>
> On Mon, 29 Dec 2003 03:01:38 +0000, n0tt1@juno.com wrote:
> >Earth-grounding the shields when they leave the shack helps that
> Well, maybe, and maybe not. Consider that many shacks are located such
> that a "ground" conductor will have considerable inductance and be some
> significant fraction of a wavelength at many frequencies of interest,

There is another problem also, and that is how the shield actually works.
Unless the shield has ZERO voltage differential between each grounded end
with BOTH ends grounded and the wires inside are treated properly in
termination, it might become another conductor that couples common mode
energy into the conductors inside.

That's why I mentioned the "shielded" loop antenna. It is a good example of
how the shield doesn't really change anything any distance (even a few feet)
from the conductor at higher frequencies. The shield simply becomes the
actual antenna, and the voltage differential across the gap (ends of the
shield) cause a current to flow INSIDE the shield IF the conductors inside
the shield allow a path for opposite direction currents.

We have to be very careful looking at shields and assuming they do at higher
frequencies what we think they do at low frequencies. In many cases they are
not any different than adding a small capacitor from the conductor to
ground, in other cases they might do nothing at all, and in some cases they
might help.

While the action of shields is pretty basic stuff, transmission line theory
in complex systems with time-varying HF fields isn't a casual topic. As I
said before, it is the root of "junk science" CFA antennas and other
nonsense are created from. The CFA antenna inventor cautions people NOT to
use a choke balun on the feedline or his antenna will "stop working". Think
about that one for a while, and you'll realize people who work with this
stuff often don't have a clue how a transmission line or antenna actually

73 Tom

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