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Author Topic: Lightning Protection Question  (Read 1384 times)

Posts: 40

« on: August 22, 2001, 09:17:17 PM »

OK, I have my Glen Martin tower grounded by #6 aluminum wire to an 8' ground rod.  Also, coax has an MFJ 270 lightning arrestor connected to #6 aluminum wire to a 6' ground rod.  Question is: Do I still need to disconnect the coax from the rig when storms approach, or is this sufficient?  I understand it will not hold up necessarily to a direct bolt, but the ground should dissipate static and corona as well as nearby strikes.  

Posts: 3585

« Reply #1 on: August 22, 2001, 10:18:55 PM »

Hi again Len. First of all, I live down here in the hurricane belt. I have a thunderstorm over my house one day out of three. When the big winds blow out of the Gulf I may have a week long thunderstorm. And there are times when I pretty much MUST operate while lightning is flashing in the vicinity. So I'm biased more than somewhat!
  Now - ground has a lot of meanings. In the case of lightning protection ground means a conductive body with enough current sinking ability to keep lightning induced surges to a safe voltage. If the resistance to ground is low enough, the voltage surge on the coax will be clamped at a very low level.
  Therefore, a lightning ground must have high current handling capacity and low resistance to the conductive layers under the surface of your soil. For lightning protection - ground starts at your local permanant water table.
  A single six foot rod in pretty typical soil probably has 10-12 ohms of resistance to actual ground. Use ohms law to figure what the voltage at your ground rod will be when a 50,000 ampere surge comes flashing down your coax!
  You don't think your coax will handle 50,000 amps? Copper vapor makes a very good plasma - with even less resistance than solid copper! Coax will carry 50,000 amps long enough to make everything within reach junk.
  So, how much separation between your coax and everything else are you going to need to keep that voltage from getting loose in your shack? And your house wiring!
  Yes - your high resistance ground rod will give you adequate protection from static charges, induced current surges if they are not too strong, etc., etc..
  Now, I use a mininum of three 8 foot ground rods sunk 8 feet apart. That may be overkill in your area - but ground rods are cheap. Rigs and appliances are expensive. Kapish?
  73  Pete Allen  AC5E

Posts: 527

« Reply #2 on: August 23, 2001, 06:06:34 AM »

No matter how good a ground system you have it is
adviseable to disconnect and ground your coax when
it is not in use. Static discharge from your coax
can destroy your equipment and if you ever have
the experience of ball lightning rolling through
your shack you will never again forget to ground
your coax. An expensive and near deadly experience.

Posts: 40

« Reply #3 on: August 23, 2001, 09:03:53 AM »

The only real problem is that my house is on a hill, about 10 feet above the street.  In essence, to reach any sort of water table, I would need a 24' rod.  Imagine trying to drive that puppy into the ground!  Also, our whole community is on a hill.  So, there's two strikes (no pun intended).  I understand the concept that grounding bleeds off the static that builds up from wind and can bleed off corona that may appear during electrical storms.  We had a storm last night, but I had the coax disconnected as well.

The six foot ground rod is actually an 8' rod, but it struck a large rock about 6 foot down.  

I have sufficient aluminum wire left to do two more rods at 8'.  

One reservation I had was laying the aluminum wire across the roof from the tower to the edge and then down to the 8' rod (this one is completely sunk in).  

The antenna is not the highest structure in the area.  In fact the house next door is even taller, and so are a few of the trees.  

73, Lenny KX8X

Posts: 3585

« Reply #4 on: August 23, 2001, 12:27:19 PM »

Yes, Lenny, do put down your two extra ground rods - 8 feet from your present rod, and at least 8 feet from each other. Preferably, eight feet to the right and eight feet to the left. This gives you the lowest possible resistance to your "real ground;" and keeps the surge voltage at a mininum.
  Do disconnect your rig whenever lightning is in your area. If it's at all possible, arrange for the center conductor to be positively shorted to the shield. That is, disconnect your coax from your rig and plug it into a shorted jack, or use a shorting antenna switch (with a separate heavy wire to ground) and disconnect your rig at the switch.
  The idea is to have redundant ground paths and keep damaging voltages as low as possible.
  73  Pete Allen  AC5E

Posts: 3585

« Reply #5 on: August 23, 2001, 12:31:11 PM »

Oops - sorry but I forgot to mention that back in the days when I used around Kansas and Nebraska most homes, barns, and any other structure with any appreciable value had a lightning rod near the gable ends. And a nice heavy copper wire across the roof, down the end, and down to a well grounded lightning rod. In fact - I have known of people coupling two or even three rods lengthwise and driving 16 to 24 feet of rod in the ground.
  73 again, Pete Allen  AC5E

Posts: 75


« Reply #6 on: August 23, 2001, 06:02:03 PM »

In short, yes, you should disconnect because you never know what might come your way.  Normally when not operating I left my coax switches in the grounded positions to protect from minor static and preciptation induced charges, etc. If I knew a storm was coming or leaving on an extended trip I'd disconnect most everything.

I have two fairly long and thorough docs on my web site regarding grounding and lightning if you care to read them.


Posts: 22

« Reply #7 on: November 07, 2001, 06:06:46 PM »

try this trick.. buy 2 or 3 10 foot lengths of copper water pipe and the solder on splices to join them.
AND a brass water hose fitting that will fit (you want to screw this on your hose) .. take a hammer and mash
one end of a pipe a little on each side.. ( make the
hole smaller)  use a propane torch and solder the hose fitting on the other end. hook it to your hose and you
have a long jet drill.. grab a ladder ( i used my tower) turn on the water and move the pipe up and down
 it will wash its way down if you don't have loads of
LARGE rocks in your soil..small ones will wash away.
when you get that pipe down.. solder on a splice and
another pipe and go deeper.. when you get deep enough
clamp on your ground wire and backfill any hole around
the pipe with dirt. works great..  but if in doubt..
disconnect ha ha!!   73's phil
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