LF: Lightning Story

[Bill de Carle <ve2iq@magma.ca>]

A couple of nights ago a thunderstorm came through here just after we 
had gone to bed.  We thought the storm had passed but then there was 
a very loud bang.  I heard the crackle and sizzle of electricity and 
the boom almost simultaneously.  A few minutes later fire trucks came 
so I got up to investigate.  Turns out the bolt hit a tall pine tree 
on my neighbour's property.  What remained of the tree was on 
fire.  Our telephone was not working but the mains power stayed 
on.  I thought for sure the next morning I'd have to replace blown 
out parts.  At the time of the lightning strike my antennas were 
disconnected (outdoors) from the house because we knew the storm was 
coming.  I was concerned about a preamp for some VLF loops back in 
the woods and a newly-installed mini-whip (J-310 front end) on the 
garage roof.  I measured the distance to where the lightning hit: 150 
M from the struck tree to the mini-whip and 200 M to the loop 
preamp.  I checked the preamp first: it was dead.  Burned IC (TL084, 
I could still detect the smell in the morning).  When I tested the 
mini-whip I was amazed it still worked fine!  The loop has protective 
diodes (1N4148) across the hi-Z ends and those diodes also survived 
the strike, which was unexpected.  Starting at the base of the struck 
pine tree one notices a newly-made straight line trench in the ground 
(about 30 M long) heading towards the nearby river.  Evidently the 
current was making its way to the water.  I'm wondering why the 
mini-whip (closer to the strike) survived but the TL084 did not.  The 
preamp drives a buried cable (about 50 M) to my radio shack.  That 
cable (no DC path to ground at either end) runs in a direction 
parallel to the trench made by the lightning but displaced at least 
100 M.  Could enough voltage be induced to burn up a TL084?
Bill VE2IQ