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December 21st 03, 11:36 PM

Art Unwin KB9MZ

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(Richard Harrison) wrote in message ...
Art, KB9MZ wrote:
"Can I assume then that broadcast coupling coils are always apart to
accommodate a Faraday shield between them?"

This is not an FCC requirement, I Believe. The FCC sets a low allowable
harmonic content level for broadcast signals. A Faraday shield between
coils eliminates capacitive coupling between them.

Capacitive coupling between coils favors harmonics, as capacitive
reactance is inversely proportional to frequency. Killing capacitive
coupling is effective in eliminating harmonic radiation. Putting the
Faraday shield in the tower coupling makes a powerful lightning
deterrent, too.

The usual shield construction is a metal picket fence with the coils on
either side sharing an axis.

Best regards, Richard Harrison, KB5WZI

Just to be sure I understand this. Is one coil made from coax with the
braid grounded or is the Faraday shield of a corugated sheeting form
that provides a clearance groove in one or more turns to allow
entrance of the other
coil turns? Do any commercials use a rotating coil to vary the
coupling
with a fixed coil thus making the adition of a Faraday shield pretty
straight forward ? The latter is the form that I favour.
Regards
Art

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