From: a-kevinp@microsoft.COM (Kevin Purcell, Rho)
Newsgroups: rec.radio.amateur.misc
Subject: Lightening strikes
Date: 16 Nov 92 20:30:00 GMT
Article-I.D.: ingate.9211161339.aa03159
Organization: ucsd usenet gateway
NNTP-Posting-Host: ucsd.edu
Originator: daemon@ucsd.edu

Hit the wrong key and sent the message without any of my comments...

Mike, N4PDY says:

Charles Rissmeyer writes:
>... do those in-line lightning arrestors really work?

Nothing will protect you from a direct strike; if your
antenna gets a direct lightning zap, all bets are off.  The
real purpose of a lightning arrestor is to bleed off static
charges from near-strikes before they reach dangerous
levels.  The good ones (i.e., expensive) work a lot better
than the cheap ones.

I say:

This is not entirely true -- athough it is difficult to survive a 
direct hit with a little effort you can survive very damaging close in strikes.

The purpose of a bleed resistor is to bleed off static. A lightning 
arrestor (the modern gas-filled axial spark gaps) are designed to stop 
the lightning getting into your equpiment (if they're used properly).

The main trick is getting everything in the operating area (e.g. your 
house, your repeater) at the same potential by having them all well 
bonded together by heavy conductive straps. Potential differences are 
what cause the damage (and fire in some cases). Make sure all the metal 
in you house is bonded together, this includes giving a good external 
ground on the incoming power lines, and means that any ducting you have 
in the house should be strapped together to keep it at the same 
potential. This will avoid induced currents having potential difference 
between diffrent items and flashing over to those lower potential items.

A close in lighting strikes will also inductivly couple into the wiring 
-- this is were gas-filled tubes can stop problem with excessive 
voltages -- they turn on very quickley and can cope with massive 
instantaneous currents, but they must have a very good path to ground 
to get rid of all that junk. If it can find an eaier path it will take 
that path.

Use antennas that can be grounded via the mast rather than having 
isolated elements (e.g. use J-pole rather than an monopole/dipole or a 
gamma matched driven element rather than a dipole element). All the 
elements of the antenna should be bonded together and at DC ground.

The gas-filled tubes are axial spark gaps and are designed to prevent 
spikes coming down the centre conductor of the coax. You must also take 
the braid of the coax to ground via a spark gap, its a seperate path to 
ground (via your radio!). All of these gaps should be outsode the 
house, on the other side of a metal bulkhead which itself is well grounded.

I saw mentioned in QST a while back of a repeater site on the top of a 
mountain which got a ridiculous number of lightening strikes a year. 
This was a commerical and amateur site (in CO?) placed over a rocky 
poorly conductive ground. The solution to this problem was essentially 
to accept you were going to get hit but to minimise the potenital 
differences around the site by laying a large ground plane. Everything 
is conntected to that ground plane (all the equipment, structures, 
antennas and so on) which keeps all the equipment at a constant 
potential. If there is a nearby strike or even a direct hit, everything 
on the groundplane floats up to match the potenital near the strike. 
But as everything floats there are no pds, so nothing arcs over and the 
charge makes it safely to ground via the metal work rather than some 
expensive semi-conductor.

Check out the Sept 1992 QEX for a good article on lightening protection 
(why was this in QEX and not QST?) and read the H'book!

You can do a lot to protect yourself against strikes -- remeber 
commerical and military sites cannot afford to be taken out though a 
lightning hit.

73 de Kevin Purcell N7WIM/G8UDP (Seattle, WA)          a-kevinp@microsoft.com
Do your interests match mine? If so call me. I'd like to hear from you.
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Satellites, Antenna design, Propagation, VHF/UHF weak signal, Microwave, QEX
Newsgroups: rec.radio.amateur.misc