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Old January 26th 04, 11:16 PM
Richard Harrison
 
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Jack Painter wrote:
"But I guess the currents in the coax weren`t "near enough equal" in
this case."

Yes, and I can`t guarantee common-mode or equal currents. If the folded
unipole gets zapped, equal or non-equal currents may flow in both
conductors. Induced currents are likely to be differential-mode. But if
they are differential-mode currents, something else likely happens. The
line flashes over.

I seem to be very lucky to never have damage with so many opportunities
for damage. We never lost a transistor radio front end with countless
strikes as evidenced by the pitted antennas. We know the coax arcs in
broadcast stations. Most stations have automatic circuits to kill the
transmitter when the coax arcs.


In medium wave broadcast stations there is almost always a Faraday
screen to keep down the harmonic radiation. It gets countless zaps as
evidenced by pock marks and metal splattered about its shield box.Even
so, the coax gets arcs. When you are on the air, transmitter energy
keeps the arc alive once a transient has struck the arc. Most
transmitters are equipped with a momentary kill relay whose d-c coil
circuit is completed by the coax arc. As soon as the transmitter is
killed, the relay is de-energized and the transmitter returns to the
air.

In the 2-way radio world, the transmitter is going to drop out in a
moment when the mike button is released, or the station was in the
receive mode when the lightning hit and there is no energy to sustain
the arc.

The arc prevents conveyance of the energy to the radio. I never saw a
broadcast transmitter with evidence of lightning inside the transmitter
and we have a good ides that these stations get struck almost every time
a dark cloud passes by.

Best regards, Richard Harrison, KB5WZI

 
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