[hpsdr] lightning

[FRANCIS CARCIA]

When you have 50 amps of current flowing down your shield you may not see 2500 across the input. You have to look at the transfer Z of your coax vs the length to determine the transfered voltage. We use .033 ohms for twisted shielded pair. So say you have 30 meters of coax and use the same number. 50A X .033 X 30 = 49.5 volts induced on the center conductor.  This would be an open circuit voltage. Any load on the center conductor would reduce the voltage. Now say your dipole takes a common mode hit so 50 amps flows down the shield but the center conductor having received the same energy has no current path to ground or a high Z to ground. Now you could get 2500 volts on the 50 ohm input unless you have a clamp that could handle the same current to hold the voltage down. Ben Franklin was a lucky man. These old receivers with input transformers would blow out because the primary wire couldn't handle the lightning current. All about Ohm's law as it was taught to me. frank
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