[TowerTalk] Near Field Lightning Damage

[Roger (K8RI) on TT]

Years ago, while we were building a new home on the NW corner of the old 
family farm, we lived in a Mobile home as we had already sold our first 
home.

I'd set the rig (Kenwood 830) on the kitchen counter to operate. At 
other times the coax went into a broom closet.  One cold and windy day 
with heavy snow, I heard a loud snapping sound like a mouse trap.  We 
had neither the mice, or that many traps as the snap occurred about 
every 5 to 10 seconds.

I opened the closet to be greeted by a big fat arc across the end of the 
PL-259.  It was a fat, bright blue arc that curved out well away from 
the end of the connector.  With the closet door open the "snap" was even 
louder. It was the equivalent of a good, brisk, and efficient hand 
clap.  I never had an ignition system including mags that could equal 
that spark.

That spark was created by the heavy snow blowing across my quarter wave, 
40 meter vertical.  It was very cold, so you could call it a dry and 
very cold snow.  IOW: One whale of a Van der Graaf Generator!

I've had precipitation static as well as induced voltage from lightening 
create those snaps, but never at that strength.  OTOH I would normally 
disconnect AND GROUND those cables when any storm is near.  Now that I 
have a grounding bulkhead and arresters  where the cables enter the 
house, that is no longer a problem.

I would guess that if a thunderstorm is near, those snaps are from 
induced voltage caused by the lightening, rather than static buildup.  
OTOH lightening is just a huge static discharge containing thousands of 
amps.  Possibly hundreds of thousands of amps.  A lightening strike a 
mile away can induce a thousand volts (or more) per meter in a wire. As 
the 75 meter antenna is roughly 37 meters long, that's 19 KV per side.  
One of those that shakes the house could be many times that, but 
"typically" that induced voltage has very little power, but it can, so 
it's nothing to ignore and a good reason gor grounding the coax or 
throwing it outside.

A close, powerful stroke, could induce enough voltage and power to be 
lethal just from the shield to ground.  That's a good reason not to just 
disconnect the coax from the rig.

-- 

73

Roger (K8RI)


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