David Robbins wrote:

"Andy Cowley" wrote in message
...
CW wrote:

Effective lightning protection can be done in the amatuer station for a
reasonable cost. Most though, don't do so.
"Andy Cowley" wrote in message
...

As I understand it, there is nothing that can work if
a direct lightening strike occurs.

How? How do you deal with thousands of amps? It's for certain sure that
a simple spark gap will be blown to kingdom come in the first millisecond,
so what happens in the next millisecond? and the one after...........

I think your method must be untried, untested and 'whistling in the dark'.

Andy, M1EBV

lightning doesn't go on for milliseconds, 50 micro-seconds is a relatively
long stroke. 30kA can go through a 12ga copper wire with no damage for
10-20 microseconds. in most cases there will actually be very little
voltage between wires of a coax or twin lead just because their insulation
will break down or the feedpoint of the antenna will arc over... both are
naturally occuring spark gaps that actually work very well to protect
equipment from direct strikes. assuming of course the tower and feedline
have good grounds. where people have problems is they don't ground the
shield of the coax to a single point ground along with the power lines, so
they get differential voltages between grounds that has no place to go but
through the equipment. properly grounded installations with relatively
small arresters to limit voltage on the center conductor of the coax
relative to the shield are very effective. for tube type receivers a simple
spark gap is adequate, for transistorized stuff you may need lower voltage
protection and should probably get something commercially made for the job.

I had not realised that the current was so 'spikey' I was aware that the
duration of a stroke, including restrikes, was of the order of a second
for large strikes.

I found this info:
0.2 MA for 200 uS, then ~10 kA for another 200 uS, then 300-500 A for ~0.75
seconds. This followed by an average of 3 to 4 (max 26) restrikes at 0.1 MA
(possibly decaying to 25 kA) for 200 uS for a big stroke.

There is a MIL standard for this - MIL-STD-464.

If your Coax breaks down then you have a PD in the tens of kV range for
common coax and arc over in solid dielectric is permanently damaging.

Sorry if I mislead anyone but we don't really suffer badly from
lightning in G-land.

Here are some interesting links.

http://www.weighing-systems.com/Tech...Lightning1.pdf
http://www.lightningsafety.com/nlsi_lhm/NFP_780.html
http://www.mil-std-464.com/
http://www.kolacki.com/MIL-STD-464.htm

I'm a little wiser now. Thanks

vy 73

Andy, M1EBV