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May 18th 06, 06:44 PM posted to rec.radio.amateur.antenna

On Thu, 18 May 2006 11:48:52 -0500, (Richard
Harrison) wrote:

A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.

Hi Richard, Yuri,

In regard to my last question that so stumped you two, there is
absolutely nothing inherent in non-ferrous vs. ferrous materials that
changes this one particular aspect of shielding. Both materials are
conductive, and both are selected for their lowest conductivity (hence
their lowest Ohmic loss). The only substantive difference is that a
ferrous material offers the prospect of using a thinner covering for
the same isolation. Above LF, this is hardly useful unless you are
planning on using very thin foils. Art's selection of mylar films
with conductive coatings is one such example that works with a
conductive surface thickness in the 100s of microns.

The "shielded dipole" observes the one principle requirement of
insuring that a break in continuity is maintained. Otherwise, the
shorted turn snuffs the antenna for any design held within it. This
prohibition in continuity is paramount to all designs and is driven by
both magnetic as well as electric considerations, as in the RF field
they are inextricably coupled.

To cut to the chase: there is no way to separate these fields and
select one of them over the other. If you choose to run your arc
welder within a loop's diameter of the antenna while also DXing; then
it is the balance of the antenna design, not the shielding that will
determine your success. Screw up the geometry of that gap, and you
will hear as much hash as if the "shield" never existed.

For the standard single turn "shielded dipole," the arms of the dipole
are the shield. There must be 10 million examples of this particular
model on 1 million repeater installations world-wide. There are also
tri-axial or twin-axial designs of the "shielded dipole" that wholly
divorce themselves of the exterior shield. All such designs, to work
effectively, exhibit a balanced configuration that is identical to the
standard single turn. The balance is the only consideration at issue,
and shielding is a means, but hardly a necessary ends to that
achievement. As such, shielding is simply insurance and a brute force
means to force balance through what in engineering terms is called
"swamping."

73's
Richard Clark, KB7QHC