Faraday shield does not apply to insulated wires (including coax
shields) run for extents outside the cage (tower) as well. Models of
my entire tower (all the members) show current on the interior. So
does my sniffer. In every way I have been able to determine, tower as
faraday shield is an urban myth. (Regardless of what may be in print.)
Having an insulated conductor taped to a boom or tower modifies but
does not per se PREVENT current. This too with both model and sniffer.
The effect of the stub is just a capacitor on one side if the shield
is extended in any way.
And it IS in the field. Show me a model where it isn't?
Back to the original rules: All conductors are in play unless
overwhelmingly proved otherwise. When in doubt, it conducts. Not the
other way around.
The free space model is an abstraction only good for gathering
conceptual and isolated performance data. The real world is conductors
everywhere, all in the field and all conducting, just to degrees some
worth worrying about, some not.
The effect is that you have a spider web in the central field of the
antenna directly connected to half of all the elements. Put wires in
the model to show the outside of all the shields. Do NOT assume
faraday suppression. It's a myth.
Take a garden variety CB field strength meter and tape it to the
shield of a piece of coax running up inside the tower to an inverted
vee. Tape it so you can remove your hand and not be inducing current
from your body.
Do NOT use a balun. This will make the antenna want to put current on
the shield: a test setup to see if the tower stops current along the
shield induced at points outside the tower.
Key the transmitter.
Watch the meter jump.
The tower and the shield are acting like a piece of coax (tower is the
shield, and coax shield is the center conductor). Current from up top
can be found on shield of coax running away from the bottom of the
tower unless the coax shield is grounded at the bottom of the tower.
Even that merely reduces the current unless it's a really good *RF*
ground.
Guy.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: "Guy Olinger, K2AV" <olinger@bellsouth.net>;
<towertalk@contesting.com>
Sent: Saturday, May 31, 2003 10:51 AM
Subject: Re: [TowerTalk] Feeding one antenna in a stack
> At 09:48 AM 5/31/03 -0400, you wrote:
> >The shield has to be modeled as a wire in it's own right. It's in
the
> >other element's fields. If you break the shield connection at the
> >relays then it's short, nonresonant and of small effect. Otherwise
> >it's a wire connected to one of the same size (via the shield
> >connection) on the other side, connected to the feedline to the
shack,
> >possibly the tower, not short, and in play.
>
> In some instances being in the other elements' fields may not matter
much,
> particularly if the feedline is taped to the boom and mast, and then
goes
> to the bottom of the tower inside it. What I was wondering about is
the
> effect of having a conductor permanently linking one side of each
driven
> element (regardless of how the stack is switched) to all the others,
and
> secondarily whether a feedline thus connected but with center
conductor
> floated would still appear to its driven element like an open stub,
or
> something entirely different.
>
>
> 73, Pete N4ZR
> The World HF Contest Station Database was updated 9 May 03.
> Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
>
>
>
>
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