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antenna question about dc grounding

Discussion in 'Amateur Radio Antennas' started by joespunn23, Oct 24, 2011.

  1. joespunn23

    joespunn23 Member

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    hello everyone i have been looking at purchasing a new antenna . i have been reading on differant antennas and im leaning in the direction of the maco v5/8 but thats beside the piont . i am new to the hobbie well my question is about what it means when the antenna is dc grounded for better lighting protection can someone explain this to me please thank you


     

  2. W5LZ

    W5LZ Crotchety Old Bastard

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    That about lightning protection is a sort of exaggeration. There can certainly be a direct path to ground for DC, but it will seldom be adequate for much protection against lightning. That direct connection just isn't going to be heavy enough. It can also drain off static to some extent, but not all of it. That's because 'static' can have many forms and may not be DC.
    A DC ground isn't necessarily an AC ground, or visa-versa. That depends a great deal on just how that 'grounding' is done. A coil can show a direct path for DC but no path for AC because of the inductance of that coil (frequency related).
    Does that mean that a DC grounded antenna is junk? Nope, sure doesn't! It also -contributes- to lightning protection, but don't count on that alone!
    - 'Doc
     
  3. Special Publications | Technical Publication #80

    "DC Grounded Antennas - The Myth, The Legend, The Fantasy
    This is a subject that we just had to write about. In the lightning protection business we come into contact with many people who have had both dangerous and disastrous experiences with Mother Nature. And one that has perplexed antenna users for decades is the very common damage and destruction to radio equipment when connected to a so-called "DC Grounded" antenna system.

    For many years, antenna manufacturers have touted the positive advantages of owning and operating a station with antennas whose feed systems are a direct DC short across the input terminals, and hence both sides of the coaxial feeder cable are placed at "ground" potential at the antenna site. In reality, there are no such advantages to this kind of feed system, but it is singly the most dangerous ever used from a lightning perspective.

    The reason is pretty easy to both explain and understand. Lightning bolts that streak from clouds to ground frequently hit exposed metallic structures like towers and high antennas. This is simply because the metallic nature of the object electrically shortens the striking distance between ground and sky. When a large voltage potential is reached between the two during a storm the metal antenna acts like a prod, sticking up in the air and drawing the first arc.

    Lightning wants to reach ground, and that's pretty much all it wants. And it will get what it wants in the easiest and least resistive way possible. Just about anything in the way can be easily vaporized out of the way by a good sized lightning blast. If ten different paths to ground are presented to a striking bolt (such as numerous transmission line conductors, the tower frame, etc.) then the currents will divide quite nicely between all of them, with the larger amount of current flowing in the path of least resistance and so on.

    "DC Grounded" type antennas provide a very neat dual path for those lightning currents. Some of the blast will flow down the shield of the cable to ground level earth terminal connections while the rest will simply flow down the center conductor and ravage the radio connected at the other end. Keep in mind that at the point of impact a bolt of lightning can easily deposit 50,000 volts or more respective to ground. And for an instant the voltage at the radio equipment end will be the same. By the time the balance of the surge comes to an end the equipment will have long since been toasted, probably beyond repair.

    The myth is that "DC Grounded" antennas offer good lightning protection. The legend is that antenna manufacturers have been claiming it for decades. The fantasy is that some of them still actually believe it. But it's not all hopeless. Here's how you can tell if your present antenna is one of these and what you can do about it. Disconnect the transmission line at the equipment end and measure across the center and outer conductors with a VOM on the R X 1 scale. If only a few ohms are measured then the antenna at the other end is a DC Grounded type. If you're satisfied with the performance of the antenna otherwise and wish to continue using it then you have two choices. First, disconnect the antenna whenever a storm approaches and hope you'll always be there to do it on time. Or second, install a blocking-type lightning arrestor that will shunt center conductor voltage to ground while blocking voltage from passing through the arrestor. Be sure to install the arrestor at ground level and ground the body of the device well.

    If your in the market for an antenna and wish to enjoy a bit of protection select the ones offered that use capacitor or link feed systems. Capacitor feed systems such as gamma matches are excellent feed systems and lightning protectors as well. They isolate the center conductor and force lightning into the shield."

    i wouldn't rely much on the last paragraph .........
     
  4. Kamikaze

    Kamikaze Active Member

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    I wouldn't rely on any of it.
     
  5. then what would your recommendation be ?
     
  6. Kamikaze

    Kamikaze Active Member

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    Antennas which are DC grounded and have multiple sharp metal pieces are proven to dissipate pre strike charges.

    Less chance of getting hit in the first place.

    What happens during a strike is a different matter. Low inductance bonding of all equipment and grounds is the name of the game there.
     
  7. how is that unique to dc grounded antennas ?
     
  8. Kamikaze

    Kamikaze Active Member

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    I don't understand the question.
     
  9. you implied that DC grounded antennas have more sharp metal points than non DC grounded antennas . i don't see any similar antennas having more sharp metal points than another . a tapped coil doesn't add more sharp metal points than a gamma matched antenna . a tapped ring will ad only 2 or 3 at the antennas feedpoint .
     
  10. Kamikaze

    Kamikaze Active Member

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    Maybe I should have written it differently.

    DC grounding does not make antennas have sharp points.

    The combination of DC grounding AND sharp points on antennas is known to drain pre-strike charges.

    Better?
     
  11. thanks for clarifying :D

    something im curious about ....... ive seen these advertised .....

    [​IMG]

    ""Porcupine" static charge dissipater. Provides a large number of sharp points which discharge charge buildup before strike potential is reached, similar to the old fashioned lightning rod. These devices are controversial, but sworn by, and used throughout the private and commercial community in greater numbers every year. They are found on buildings, towers, bridges, fuel and gas storage tanks, computer installations, communications equipment, etc., all over the world. Stainless steel bristles in a diecast copper alloy mounting terminal, instructions."

    The WireMan - Grounding and Switching Accessories

    but FWIU sharp points on antennas attract noise/static that's herd in the receive . also FWIU sharp points can also attract lightning because it focuses what ever charge is present to a concentrated point (exp. lightning rods have long sharp points on their tops) . i also thought it was the earth ground systems that drain static off the antenna . so apparently my understanding isn't too great about this .... but if someone could explain what's really going on i'd appreciate it
     
  12. W5LZ

    W5LZ Crotchety Old Bastard

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    Lightning is a result of opposite charges trying to 'equalize' themselves. Lightning doesn't just go from sky to ground, but from ground to sky also. Depends on where the stronger 'charge' is. If you can gradually equalize those charges instead of all at once, then there's no lightning. that's what those 'brushes' mounted on metal objects attempt to do. They dissipate charges 'gradually' (drain them), give lesser 'sized' charges a way out of where they are (those sharp points). That would certainly lessen the chances of a lightning strike. But they aren't fool proof, they don't always work, or drain a charge quick enough. So, there's an accumulation of 'charge' which just happens to be 'attractive' to an oppositely charged sky born clump of stuff, and bang, lightning strike. that's just one scenario, there are a bunch of circumstances where the same basic thing happens but in a different manner or direction.
    The whole idea is to make the current from a strike, or near strike, go somewhere other than to the things connected to the antenna/tower, to divert it, or make the path to those things you don't want 'hit' to appear less 'attractive' than someplace else.
    - 'Doc
     
  13. so should those brushes be at the top of the antenna ? its feed-point ? the base of the mast ? in multiple places or all ?
    if they go on top of the antenna do they add length electrically to the antenna like a bunch of little top hat elements ?
     
  14. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Booty, those devices do indeed work but there are better units than shown above. They cost a lot more too. They are meant to be mounted on the antenna support structure and not the antenna itself. In the case of a tower, one or two at the top and another one or two in the middle of a really tall tower. They work by allowing the built up charge to bleed off into the air before it builds up enough to cause a strike.If the charge still builds up enough to cause a strike the strike will be attracted to the porcupine device itself and be shunted to the tower and then to ground with minimal damage ( note I did not say NO damage) to the antenna or connected equipment provided the ground system is installed properly.

    I know you are on eham now and then but not sure about QRZ but ask Glen, K9STH, about them. He has had great success with them on his towers located on the highest point in Richardson Texas.
     
  15. thanks , ill check out qrz ;)

    so would those things be effective on a modest 20 or 30 ft of mast ?
    it looks like they would be very easy to make with with scraps of old solid house wire .
     

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