[TowerTalk] Comments on Lighting from a non-expert

[Tom Rauch]

> Date:          Wed, 03 Jun 1998 08:35:10 -0500
> From:          Lee Buller <k0wa@southwind.net>

Hi Lee,

> Isn't it the goal of a good installation NOT to get hit by a lighting bolt?

Sure. You can do that by making sure your antenna is the shortest 
thing on the block. Of course that may run contrary to the goal of 
having the biggest signal on the band.

> That is my goal.  Gee, for years farmers have had lighting rods on the barn.
> Why not towers?

Barns are wood, towers are metal. The tower is already a lightning 
rod.

I think the mistake is thinking the rod or some other gizmo will 
prevent a strike by bleeding off charge. Hey, it's the cloud that's 
charged. The earth is so massive it's a perfect charge source or 
sink. Unless you bleed those charges off right into the cloud, 
nothing changes except the field gradient around the source of 
discharge.

If that lightning rod is spraying off charges into the air, the end 
effect is exactly the same as if  you had a bigger conductive object 
the shape of the ion cloud up there at the rod. 

The only difference is the corona makes noise. It certainly does not 
reduce the likelihood of a strike, because it does not lower the 
antenna height or reduce the potential difference between the cloud 
and the earth and everything attached to earth.

> Let me ask this.  What about the EMP of a lighitng strike.  Anyone have a
> close lighting strike, say a block or two away, and lose CMOS?

I never have. The only damage I've ever had was from a direct strike, 
and then it was confined to the connector area of the equipment, a 
VHF fiberglass covered antenna, a vacuum variable, and a few relays. 
All the CMOS stuff in the shack wasn't bothered, mostly because of 
good lead routing and bypassing.

I do get occasional resistor failures on Beverages (mostly the MOX 
types, the carbon compositions seem to handle the impulse better) 
but never a preamp or transformer failure to date.  

73, Tom W8JI
w8ji.tom@MCIONE.com

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