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Grounding Antenna Question

Discussion in 'Electronic Repair' started by Humbled Survivor, Jun 2, 2013.

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  1. Can I ground out my outdoor FM antenna from the pole?

    Does the negative of the coaxial cable have to be grounded?

    Which method is best for preventing lightening strikes?
     
  2. Phil Allison

    Phil Allison Guest

    "Humbled Survivor"
    ** Yep.

    The pole needs to be linked to metal plumbing with a heavy gauge copper
    wire.


    ** If it is isolated by a "balun", then yep - it a good idea too.


    ** Can I suggest you make your peace with god right now ??




    .... Phil
     
  3. Jamie

    Jamie Guest

    actually, grounding it will attract near by strikes, but what ever..

    THe emf pulse of a strike directly on a grounded mast will more than
    likely cook the front end of what ever is connected to it and maybe
    even jump around a bit.


    Jamie
     
  4. bud--

    bud-- Guest

    But the provisions of the NEC are not to protect from direct lightning
    strikes to an antenna - much more elaborate protection is required. They
    can provide protection from other surge sources, like near strikes.

    For best protection the ground from antenna line entry protectors must
    connect with a short wire to the power earthing system. You want to
    minimize the voltage between power and antenna wires.
    Not according to what I read.
    Hams with high antennas routinely protect from direct strikes to their
    antennas. The rest of us are not likely to install the protection hams
    install. But most of us do not have antennas as exposed as some ham
    antennas.
     
  5. Jamie

    Jamie Guest

    I am a HAM, so if you're getting your information from there, then you
    better read between the lines..

    Ham radio towers get hit many times in a season, they are not immune
    from it.

    NEC guide lines is there to protect people and that means the
    structures people live or work in. THey don't give a rats ass about
    antennas and devices connected to them, getting destroyed, they encourage
    it. That way the lightning will be drawn away from the structure in hopes
    that any thing conducting to ground, your antenna and mast, will steer
    it away.

    In most cases as for HAM radio, the radials are horizontal because most
    of them put their beams at the vary top. normally you extend the mast
    above the mounting axes. I add a

    The tower our 2 meter equipment is at has lightning rods at the very
    top with a beacon light, however, it does not always save the day.
    antennas mounted on the side of the tower still get a nice strong pulse.

    We use polyphaser protection devices, they seem to work the best for
    what we need.

    Jamie
     
  6. Guest

    If you get a 100,000 ampere lightning strike directly to anything
    other than a metal building, the V=IR (voltage = current x resistance)
    drop is going to raise the voltage of everything tied together many
    many volts. All you can do is try to keep everything at the same
    (elevated) voltage to avoid killing people/animals. This is done by
    tying everything to the same local internal "ground". By having a low
    impedance to earth ground using grounding rods and water pipes, you
    try to minimize the difference between the local ground and the earth
    ground. The current from a lightning strike will induce many
    thousands of volts in any conductor a few feet long within 1000 feet
    of the actual lightning strike. I have seen an arc form from a tv
    lead-in wire to a ground when lightning hit 2000 feet away. The arc
    jumped a 1" air gap, so the arc voltage was well over 1000 volts. A
    protection device on the lead-in wire would probably have kept that
    voltage under 100 volts. So grounding the antenna mast helps, but a
    protection device on the lead-in is adding suspenders to a very weak
    belt.
     
  7. bud--

    bud-- Guest

    And hams with high antennas are very likely to have good protection from
    a direct lightning strike.

    The OP is very unlikely to have antenna that is significantly exposed.
    The provisions in the NEC, which Rich referred to, are not intended to
    protect from a direct lightning strike and won't protect from a direct
    strike. NEC compliant wiring will not 'draw' the lightning away from the
    structure.
     
  8. bud--

    bud-- Guest

    The NEC *requires* a metal water service pipe (minimum 10 feet long in
    the earth) be used as an earthing electrode, just like it has since time
    began. Connection now has to be withing 5 feet of the entrance to the
    house, and meters still have to have a bond around. Because plastic
    water service pipe is becoming more common a "supplemental" electrode is
    now also required. A metal municipal water system is the best earthing
    electrode that is available at a house.
    Gas service pipe is not allowed to be used as an earthing electrode by
    the NEC. But gas pipe is grounded by branch circuits, like at gas furnace.

    CSST (corrugated stainless steel tubing) is becoming real common for
    interior gas pipe because it is so easy to use. It is easy to use
    because the wall is so thin. There have been many fires from arcing
    between the thin pipe and nearby grounded surfaces. I believe all the
    manufacturers now require the pipe be bonded to the house earthing
    system in a way that results in the gas supply pipe being an earthing
    electrode. 'Properly' bonded CSST has also caused fires. A
    recommendation by an electrical inspector is for electricians to not do
    the bonding, then they will not be named in the lawsuit.
     
  9. Guest

    Jon, Please note that I said you can "try", I didn't say you "could"
    keep the IR drop low. I agree with all your other comments.
     
  10. Phil Allison

    Phil Allison Guest

    "bud--"


    ** AFAIK - the main reason TV antennas are required to be grounded is NOT
    because of possible acts by a mighty and vengeful god.

    The simple reason is that the antenna may become live AC supply voltage and
    so make the TV set (and anything attached to it) inside the premises into a
    lethal shock hazard.

    This could be because an overhead power cable has detached in a strong wind
    or because of an automobile accident etc.

    ALTERNATIVELY - the antenna may become live due to a faulty equipment or
    wiring inside the premises and so present a lethal hazard to any poor damn
    fool who is game enough to get up on the roof for any reason.

    Grounding the damn antenna any which way you can is essential.



    ..... Phil
     
  11. Phil Allison

    Phil Allison Guest

    "bud--"

    ** Really .........

    Wot a fucking, tenth wit num skull.





    .... Phil
     
  12. bud--

    bud-- Guest

    The way it is done is all wires - power, cable, phone, antenna, dish -
    enter at about same location. Entry protectors for the signal wires
    connect with a short wire to the power earthing system. A surge
    protector at the service panel limits the voltage of power wires to the
    earthing system. In an 'event' the building "ground" can rise thousands
    of volts, but all the wires rise together.

    It is not perfect protection. During an 'event' a pad mounted A/C
    compressor may have its ground potential at the earth where it is
    located. The power system, and power wires to the compressor, can be
    referenced to the earthing system, which can thousands of volts different.

    A ham with a high antenna would have a better earthing system and the
    common reference point may not be at the power service.
    Suppose you have a power system earthed by a single ground rod. The
    antenna entrance to the house has a ground block, as required by the NEC
    and an 18 ft ground wire to the power earthing system, as required by
    the NEC. You add a ground rod at the entry protector. The NEC now wants
    a #6 or larger bond wire to the power earthing system. Also suppose the
    ground rods have a near miraculous 10 ohms resistance to earth.

    Now suppose the antenna is hit by an "average" 20,000A strike. The
    potential at the rod will be 200,000V from 'absolute' earth potential.
    In general 80% of the voltage drop will be in the first 3 feet (which
    probably results in arcing across earth at the rod). The potential of
    the antenna lead will many tens of thousands of volts from the potential
    of other wiring, referenced to the power system ground rod. There will
    be major damage. The #6 (and your #4) bond wire help, but not
    significantly. A lightning strike is a very short event and produces
    relatively high frequency current components. The inductance of the wire
    is much more important than the resistance. Inductance doesn't change
    real fast with larger wire. There will be arc-over from the antenna and
    down leads into the house anyway.

    Now suppose there is a 5,000A surge to earth on the power earthing
    system. The power system 'ground' will be 50,000V above 'absolute' earth
    potential. It will be tens of thousands of volts from the ground rod for
    the antenna. Power wires and antenna wires will be many thousands of
    volts different. That will cause damage to anything connected to both.
    Without the antenna ground rod the antenna lead would be lifted with
    other house wiring. The same thing happens with a strike to a tree in
    the yard with one rod nearer than the other.

    I am not a fan of isolated ground rods. If you really want to protect
    from a direct strike you need lightning rods.

    And suppose the antenna lead goes close to a CSST gas pipe, which is
    bonded as the manufacturer requires. There is an arc from the antenna
    lead to the CSST which melts a hole in the 0.008" wall. If you are lucky
    that just ignites the escaping gas and causes a fire. (My intent is to
    comment on CSST, not so much the antenna ground rod.)
     
  13. bud--

    bud-- Guest

    Other reasons are to earth static charges from the wind and control
    surges induced by near strikes.
     
  14. bud--

    bud-- Guest

    It is, essentially, how hams protect high antennas that are likely to
    get hit. The intersystem bonding is heavier and the earthing hams have
    is much more extensive.

    An electrical inspector in another forum has a weather station that is
    on a metal post with the post tied to the power earthing system It has
    been hit twice. Minimal damage on the first hit and with fairly minor
    added protection he had no damage on the second hit. The weather system
    has a data lead that comes down the pole.
    If you want to protect from a direct lightning strike install lightning
    rods. Lightning rod protection is more complicated than you antenna
    earthing scheme and your antenna will not substitute for a lightning
    rod. Side flashes into the house are likely. And damage is much more
    likely from power service surges and near strikes. Lightning rod systems
    are required to be bonded to the power earthing system.
     
  15. bud--

    bud-- Guest

    Wot an insightful (but typical) comment.
     
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