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From: jhs%Mitre-Bedford@sri-unix.UUCP
Newsgroups: net.ham-radio
Subject: Re: Lightning protection
Message-ID: <12629@sri-arpa.UUCP>
Date: Mon, 20-Aug-84 11:53:00 EDT
Article-I.D.: sri-arpa.12629
Posted: Mon Aug 20 11:53:00 1984
Date-Received: Wed, 22-Aug-84 05:47:25 EDT
Lines: 59

According to Don Clark, a consultant used by FEMA, gas gaps are the best
available coaxial protectors.  They are advertised occasionally in QST from
one or more sources.

Air gaps are not necessarily fast enough in ionization to protect your
equipment.  Don't try to homebrew an air gap with a spark plug or something.
Special metals, gases, and geometries are necessary to get the fastest
ionization time.  This is especially important if you want to protect against
EMP as well as lightning.  (While some hams think this is silly, it is not.
There could well be limited nuclear exchange scenarios in which EMPs occur and
ham help is desperately needed afterward.  More info available on this
subject to interested parties.)  Incidentally, gas gap devices tend to "wear
out" after being activated many times.  They should be checked and/or replaced
occasionally.  Ask the manufacturer what means "check" and how often it should
be done.

Coaxial protection is not the whole story.  It just prevents large voltages
from developing between the coax shield and the center conductor.  You may
also want to keep the shield from conducting transient currents into the
building housing equipment.  One good way to do this is to run the coax down
to ground, make a SHARP bend (to insert inductance) and run the cable under
a foot or so of earth to the building, then making a SHARP bend upward to
bring the cable inside.  Inductance tends to make the lightning transient
jump off the cable to ground.  If the antenna is on a metal tower, running the
coax all the way to the bottom and including these sharp bends is a good idea.

Another trick which was mentioned in a course I took was to fasten some
welding cable (hich has very fine strands) to the tower sticking up in the air
with about 6 inches or so of the cable "frizzed" out like a head of hair so
the individual ends of strands are all separated as much as possible.  This
mass of sharp points causes intense E-fields which ionize the air and allow
dissipation of static charge well before a big strike occurs.  Supposedly if
you install such a protective device, you will find that many of the strands
develop beads of melted copper at their ends after a year or so of service.
Thus it would be wise to replace the "head of hair" protector every year or
so.  ****NOTE**** The claim was made that if you install this gadget, you will
NEVER take a lightning strike at that location.  It sounds like a very good
idea to me.  If you don't have a metal tower, the welding cable could be run
all the way down to the ground as a substitute, or maybe fastened to the coax
shield at the top and the shield grounded at the bottom.

Another strong recommendation I have heard is NEVER to depend on heating pipes
or water pipes or electrical conduit in your house for grounding for lightning
protection.  Instead, install a STRAIGHT VERTICAL CONDUCTOR going down to
ground, outside the wall.  Using water pipes or electrical conduit inside the
walls is said to be a good way to collect on your fire insurance policy.
Speaking of insurance, your insurance company might send an expert over to
advise you if you ask.  (Probably they will also raise your premiums when
they learn you are a ham!)

Good luck; hope some of the above is useful.  If you want a pointer to the
consultant I mentioned, I can probably get it.  Or you can call Russell Gates
at FEMA in Washington. (I don't recall his call sign, but he is a ham and
should be willing to help you find additional information.)

						73,
						John Sangster, W3IKG
						jhs at MITRE-Bedford
						(617) 271-7426w / 235-8753h