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December 20th 03, 03:51 AM

(Richard Harrison) wrote in message ...
Art, Kb9MZ wrote:
"---the local radio station has a line to ground with a large gap which
regularly arcs because of static build up. Most stations go off the air
momentarily when lightning strikes.'

AM broadcasters use unbalanced vertical radiators driven against a
ground radial system.

snip
Towers often get direct lightning hits. The paint remains pristene in
all the gaps I`ve seen. The arc to ground is always to the Faraday
shield between the tower coupling coils.

Can I assume then that broadcast coupling coils are always apart
to accomodate a faraday shield between them ? Is this an F,C,C, requirement?
Can't see how a Faraday shield can be used if they are link coupled
i.e. interleaved. I was contemplating an interleaf coupling until
I realised that I would have to do away with the Faraday shield !
Regards
Art

That picket fence between the
coils is pock marked like the face of the moon from tower strikes.
Splattered copper abounds.

You hear momentary disconnects during lightning strikes when listening
to an AM station during this kind of storm. This is a defense mechanism.
When lightning creates an arc, the conductive plasma path allows RF to
continue feeding the ionization. This allows an arc to keep alive that
the r-f is too feeble to strike for itself.

Transmitter output into the plasma short circuit is an overload capable
of transmitter damage.

To counter the arc problem, the coax is d-c isolated with capacitors at
the ends of the center conductor. The close-spaced coax usually gets an
arc when the antenna system is overloaded. The coax has a high
common-mode impedance.

A relay d-c power supply and a d-c relay coil are connected in series
and this series combination is connected between the center conductor
and coax shield.

An arc completes the d-c path for the relay coil. Relay activation is
used to momentarily kill the transmitter, extinguishing the arc.

Best regards, Richard Harrison, KB5WZI