[TowerTalk] Ground resistance

Pete Smith n4zr at contesting.com
Thu Jun 8 16:55:22 EDT 2006

At 04:43 PM 6/8/2006, Jim Brown wrote:
>And remember that the nature of coax is to act as a common mode 
>choke at RF, so when you short the shield to ground, the center 
>conductor is pretty close to ground too. It isn't a PERFECT common 
>mode choke, so perhaps 1% of the lightning strike is present 
>between the center conductor and ground. So if the lightning is 50 
>kV, the center conductor might be 500 v above the shield. Enough 
>to blow something connected, but probably not to arc across the 

This is a very interesting point.  My grounding situation is so imperfect - 2nd floor shack, 150-year-old house - that I have opted to disconnect every conductor at the entry panel in the second floor window.  I had wondered what would happen if I ever took a direct hit, and finally did - on top of the tower about 200 feet from the house.  The lightning arced across between center and ground on my feedline bulkhead connector in the shack (scared the %^&* out of me), but that was all.  The rest of the damage in the house appears to have resulted from induced voltages on my telephone wiring and my (wired) computer network.

Perhaps this common mode choking effect explains why my shack wasn't full of leaping fireballs.

73, Pete N4ZR

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