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Thread: Question about lightning ground.

  1. #1

    Default Question about lightning ground.

    I'm going to be setting up my antenna mast this weekend, weather willing. I settled on a 40 foot aluminum mast, 30 feet or so of which will be above the second floor roof level.

    My question is, can I just ground the base of the mast, or should I ground up at the antenna itself?

    My plan is to use 8 gauge copper wire and go straight to an 8 foot grounding stake. Should that be sufficient?

  2. #2

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    Provided that there aren't any large non-conductive sections, then just the base will do.

  3. #3

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    It would be a good idea to invest in a lightning arrestor for the coax line as well.

    Not sure the antenna you plan to use but I will use my Sirio 827 as an example.... On my antenna there is a loading coil at the base. Although the tip of the antenna is at DC ground due to this coil, the turns of the coil are so tight (2 inch diameter maybe) that if lightning were to strike it, the magnetic field caused by the current of the strike would blow the coil apart. The instant that happens, the rest of the juice will follow the coax indoors and into your radio.

  4. #4

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    "Lightning protectors" are really misnamed. If your mast gets hit by lightning, then that's it, it's "Game over, man!". Their proper name is surge or overvoltage protectors.
    Ground your mast by all means, that should reduce the risk of static build up on the mast during a storm, but that is it. In the event of a storm being overhead, or very close by, your best bet is to unplug every electronic device in the house from both the mains supply and any aerials. If you can't put the coax plugs outside, then I have heard people put them into glass jars.
    I use this website: blitzortung.org to see how far away the strikes are. If it's within a few km of home, we unplug everything.

  5. #5

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    I've never had the misfortune of testing the theory, but I agree with OH8GAD. I've always had the idea that most of the power would still jump straight to ground and what went into the coax would be a mere fraction of the energy, but yea, perhaps its false hope. I unplug as well.

    lightningmaps.org is the site I use. It shows the propagation of the sound wave too, kinda cool.

    EDIT: the website above, upon further research, gathers some of its information from the site OH8GAD listed.
    Last edited by brandon lind; Sun 2nd Dec 2018 at 18:26.

  6. #6

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    I lost an Icom 756 Pro III and an alpha amp to lightning...I did unplug the antenna coax but forgot to unplug the power... surge through the main power line got them both... along with a few other things in the house...grounding well will help, but not prevent the issue.

  7. #7

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    What exactly is a glass jar supposed to do with lightning? Become shard material when it explodes?

  8. #8

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    glass is supposed to be an insulator. Years ago I brought all my coax runs to one board on the back of my bench. I used thru the wall coax connectors and drilled holes in jar lids and put them on the connectors... in storms I would drop the jumpers from my rigs and screw jars onto the lids....never had the misfortune of seeing if they helped or not...an effort I am glad never paid off....or failed.

  9. #9
    K7KBN's Avatar
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    Glass IS an insulator. So is air.

    So seeing lightning bolts hundreds or thousands of feet long through the air, searching for "ground" or a point close enough to "ground" so it can arc across doesn't change the properties of glass. If lightning strikes the far end of that coax you put in that jar, it'll find ground, and all that energy will be dissipated as light, heat, sound and molten glass.
    73
    Pat K7KBN
    Semper ubi sub ubi.

  10. #10

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    I've always wondered about the glass jar thing myself, it makes no sense using a glass jar to me either.

    Anyone with a strong knowledge of electricity should know that electricity follows the path of least resistance. That might be from 1) a coax center conductor through a lightning arrestor to ground, 2) from the center conductor to a coax shield (that was grounded) making a half inch jump (assuming a radio don't complete the path), or 3) from an un-grounded coax shield (that is routed nowhere near ground, stupidly), out from the jar that one thinks is a "safety precaution", and to the grounded outlet screw as it crosses the bed where you sleep. The glass jar only adds a few meaningless inches of air if the energy from an un-grounded coax shield were to seek dissipation at the nearest grounding point.

    Safety to radio and safety to life are two different things, but I see a glass jar doing nothing in either case.

    Most of what is said about lightning is either based on speculation, or misunderstood experience. I've seen lightning strikes do crazy things, but "staying put" in a 3" glass jar is surely not one of them!

    K7KBN, I agree 100%.
    Last edited by brandon lind; Wed 12th Dec 2018 at 02:12.

  11. #11

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    An old battle scarred veteran of UK electric smiled at me when I asked him about grounds for lightning protection. He spent most of his life working for the BBC on new installs and in later life shifted to working for the Government on emergency service towers and installations. He asked me what I wanted the ground for? I was a bit confused. He then explained that what was needed in many cases was a way to prevent static build up on the tower, so grounding it with very light cable would do that nicely. This lowers the noise figures in the electronics and removes lots of fizzy interference. For systems where additional protection was required, then flash-over plasma devices in the coax feeds did quite nicely to protect the equipment. Protection against an actual strike was often futile. Destruction of the input stages of the equipment, and often the entire RF cabin was common, and often, the static protection links acted like fuses, breaking their bond to the tower, but unfortunately doing it after the kit had gone pop. REAL antenna bonds require considerable cross section for the current being dumped. His opinion was that full protection against a direct strike was impossible, and even protection from a local strike was often just not cost effective to do. The best practical process was simply to bond to stop static build up - which on an insulated tower could be many thousands of volts - many KV in some cases. Bleeding this off is very sensible and as it's a slow build up, light grounding works pretty well as the current is quite low during the charge phase of a storm. The ability to touch the antenna connector without getting a static shock is good for the equipment. His Government practice was to use the gas filled arrestors in all feeder cables LDF4-50 type and above. He really didn't know how well they worked, but they used them automatically on essential services. The danger, he pointed out was in using any antenna design that did not have a DC connection to the radiating elements - as in a ľ wave dipole or discone design. These will charge up and need sorting!

  12. #12

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    I agree that nothing will do any good if you get a direct or actually very near strike....none the less to make myself feel better....
    my tower is grounded and I use 1" wide braided wire for my grounding/bonding straps.... and each of my coax runs have gas filled arrestors in line...have not been tested yet...
    oh, and everything is unplugged when a strong storm is moving in my area.... I lost one radio and amp to lightning years ago and have no desire to have it happen again...
    I did do the glass jar thing in my shack at a previous location years ago, but as I said that was never tested....glass does make a good insulator, but not much works well at the levels of lightning.

  13. #13

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    My vertical Sirio 827 is on a heavy gauge 20' steel tower that has three 8' ground rods tied to the tower with braided ground strap. At the top of the tower I have an old heavy duty jumper cable end with the cable attached to the tower leg. The cable is just short enough that when a storm approaches, I can attach the cable to the vertical element above the matching section with as little bend as possible to bypass the coax connection point completely. Never had to test it, but I like to THINK that provides a significant amount of protection.

  14. #14

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    I'm very wary of removing the cables because I saw some pictures of the damage a disconnected lightning conductor caused a civic building when a section had been inadvertently removed and the flashover from the end of it to the nearest grounded thing (from memory, a radiator) destroyed the room!

    I guess that practically, if you get hit, then the equipment is toast anyway, so it's maybe a sensible move for near misses but perhaps bad for a general rule (or good, depending on what you hope it solves). Lighting is just bad anywhere near people and kit, I guess.

  15. #15

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    July 10, 1998 my 50 ft Rohn 25 tower was struck, at the time I was living alone and not home. It vaporized a Diamond 10ft antenna, $3,500 of ham gear and $2,000 of appliances. Not fun but very thankful a fire was not started.

    The tower has always had two 8í ground rods and four 60í buried #6 copper wire radials and hasnít been struck before or since. When itís your time its your time.

    73,
    Ken

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