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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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