Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!hao!hplabs!sri-unix!jhs@Mitre-Bedford 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