Page 1 of 2 12 LastLast
Results 1 to 10 of 12

Thread: Ham Radio Towers & NEC Bonding Requirements.

  1. #1
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,829

    Ham Radio Towers & NEC Bonding Requirements.

    Pretty sure I know thee right opinion, but want to hear your views. Hopefully I can describe the application clearly to minimize confusion.

    First what I am NOT TALKING about is a tower or Antenna Mast located on or next to a building. I know what the Bonding requirements are in that application as I have made a career out of that on the commercial side. Ham Radio operators have a lot of misconceptions about bonding and what they call a RF Ground is outside the scope of NEC but there is a Gray Line. So here goes.

    Ham Radio Operators operating on the lower frequencies of 14 Mhz and below operate be on what is called a 1/4 wave vertical antenna, and the antenna is the Tower itself and can be anywhere from 33-feet to 200 feet in height depending on the frequency of operation. Lower the frequency, taller the tower. The Towers will be located some distance from the building roughly as far as the tower is tall.

    The Tower itself is the radiator and physically isolated from the Tower Base mounted on Ceramic Isolators. In other words it is electrically Floating. With this type of installation, requires an RF Ground to operate against. The RF Ground is constructed or copper wire radials, and the radials are as long as the tower is high. Thus why the tower is located away form the building. There will be as few as 8 radials up to 180 radials. The radials may be bare or insulated, laying on top of dirt or trenched in a few inches below the surface. A fairly typical installation uses #14 AWG THHN.

    OK the transmitter is located in the building some distance away, and there is a COAX that runs from the Transmitter out to the Tower. Center Conductor connected to the Tower, and the shield connected to the Radials. Hopefully I painted that picture in your minds correctly.

    So now the question. Is there any requirement from the NEC to bond the RF Ground Radials to the Building Ground Electrode System?

    I say NO there is not, but physically it is bonded unintentionally via the Coax Shield.

    However I would say best practice is to run a 4 AWG Bonding conductor from the building GES to where the Radial join together at the tower so as to shunt as much lightning current off the Coax to earth as possible.

    Now here is what is really scary and most Hams have been told to do. They sink a Ground Rod or two where the Coax enters and fail to bond them to the GES. They are under the impression th eRF Ground must be isolated from the AC Service ground. They bond the ADU to their isolated rods thinking this protects them which is false and extremely dangerous. They fail to realize their Antenna Coax Shield makes the bond through their Radio > Power Supply > AC Power Cord inviting lightning in and travel through their house wiring. I digress, lets stick with the one question on RF Ground Radials.

    FWIW Commercial AM Radio Broadcasters use this method.

    THX
    Last edited by dereckbc; 05-03-18 at 04:08 PM.

  2. #2
    Quote Originally Posted by dereckbc View Post

    However I would say best practice is to run a 4 AWG Bonding conductor from the building GES to where the Radial join together at the tower so as to shunt as much lightning current off the Coax to earth as possible.

    Now here is what is really scary and most Hams have been told to do. They sink a Ground Rod or two where the Coax enters and fail to bond them to the GES. They are under the impression the RF Ground must be isolated from the AC Service ground. They bond the ADU to their isolated rods thinking this protects them which is false and extremely dangerous. They fail to realize their Antenna Coax Shield makes the bond through their Radio > Power Supply > AC Power Cord inviting lightning in and travel through their house wiring. I digress, lets stick with the one question on RF Ground Radials.

    FWIW Commercial AM Radio Broadcasters use this method.

    THX
    OK

    First: I have dealt with AM broadcast towers and commercial AM station installations for the last 45 years.

    That being said: Any quality AM broadcast tower (commercial station) will have a copper STRAP..not a wire, a 3 or 4"wide STRAP running from the radial wire junction point at the base of the tower to the building and tied at some point to the AC ground.

    This 'main' strap continues inside, directly to a specified ground point inside the transmitter cabinet and additional grounding straps branch out from there: to each studio, to every equipment rack, etc.

    In addition to the additional lightning protection, this strap keeps everything at both RF and AC ground potential.

    The reason for strap, not wire, is that strap has a much lower impedance at RF frequencies.

    To NOT do this at a commercial-power-level AM installation ( 1000 watts and up) is to turn every semi-conductor junction in the building into a crystal radio.

    So the short answer is yes, tie them together.

    That's my opinion, but bear in mind that this advice is worth EXACTLY what you have paid for it........

  3. #3
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    23,513
    Quote Originally Posted by Gary Glaenzer View Post
    To NOT do this at a commercial-power-level AM installation ( 1000 watts and up) is to turn every semi-conductor junction in the building into a crystal radio.
    I used to build those.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #4
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,829
    Quote Originally Posted by Gary Glaenzer View Post
    OK

    Any quality AM broadcast tower (commercial station) will have a copper STRAP..not a wire, a 3 or 4"wide STRAP running from the radial wire junction point at the base of the tower to the building and tied at some point to the AC ground.

    So the short answer is yes, tie them together.
    Gary I agree from a design POV that is good practice. But the question is there any NEC code requirement to bond the RF Radials to the GES? FYI Hams operate up to 1500 watts.

    Again I say NO because it is NOT a Required Electrode or Required to be Bonded. As Mike would say in his PV Grounding Video on pedestal mounted arrays could be bonded with CAT-V wire.

    FWIW I would like for someone to say it is required and point me to the specific code requirement. I do not think it exist and completely out of NEC scope.

  5. #5
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    16,931
    810.15 Grounding. Masts and metal structures supporting
    antennas shall be grounded in accordance with 810.21, unless
    the antenna and its related supporting mast or structure are
    within a zone of protection defined by a 46 m (150 ft) radius
    rolling sphere.
    810.21 Bonding Conductors and Grounding Electrode
    Conductors — Receiving Stations. Bonding conductors and
    grounding electrode conductors shall comply with 810.21(A)
    through 810.21(K).
    ...
    (F) Electrode. The bonding conductor or grounding electrode
    conductor shall be connected as required in
    810.21(F)(1) through 810.21(F)(3).
    (1) In Buildings or Structures with an Intersystem Bonding
    Termination. If the building or structure served has an intersystem
    bonding termination as required by 250.94, the bonding
    conductor shall be connected to the intersystem bonding
    termination.
    I conclude from this that the metal mast and supporting structure of the antenna are required to be bonded to the GES regardless of whether it is being used as a radiator or not.
    Bob

  6. #6
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,829
    Quote Originally Posted by petersonra View Post
    I conclude from this that the metal mast and supporting structure of the antenna are required to be bonded to the GES regardless of whether it is being used as a radiator or not.
    Bob I wish I could run with that, and why I question it. The tower is a hot line conductor. Now you could make an argument the tower must be enclosed to prevent accidental contact like you see on every AM broadcast tower.

  7. #7
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    16,931
    Quote Originally Posted by dereckbc View Post
    Bob I wish I could run with that, and why I question it. The tower is a hot line conductor. Now you could make an argument the tower must be enclosed to prevent accidental contact like you see on every AM broadcast tower.
    I don't see how you can get past the clear language of the code.

    That does not mean that people don't just ignore it.
    Bob

  8. #8
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,829
    Quote Originally Posted by petersonra View Post
    I don't see how you can get past the clear language of the code.

    That does not mean that people don't just ignore it.
    Bob I understand your point and welcome your reply. However my question has nothing to do with the Tower. Strictly limited to the RF Ground Radials.

    However let me add a counter point. Just about every commercial AM Radio Tower is not grounded, they are isolated electrically and the only electrical connection to the tower is the center conductor either through a Capacitor, or direct solid connection. If you were to bond the tower to ground would short out the transmitter. Additionally ever commercial Shortwave, government, and military towers used as antennas is done this way. Essentially you should apply the same logic to L1 and L2 in a breaker panel and bond them to ground. Not going to work out.

    Think about a mobile whip antenna on a car or truck. It is not bonded to the chassis. If you were to touch the whip with the Transmitter keyed, you will receive a nasty RF burn. Touch an commercial AM broadcast tower, and you will get a nasty burn. The higher above the ground you touch, the more sever the burn and electrocution. FWIW OSHA requires Mast Radiators with a voltage greater than 42 volts 4 meters off the ground are required to be fenced in.

    Now back on track, the question is the RF Ground Radials, forget about the tower.

    EDIT NOTE:

    Bob or anyone else who cares to know FWIW AM Towers use a Choke at the base that connects between Tower and Ground to bleed static off. At RF frequencies and lightning is virtually a open circuit. However at DC and 60 Hz is a dead short circuit. For Lightning protection a Spark Gap is constructed.
    Last edited by dereckbc; 05-03-18 at 10:18 PM.

  9. #9
    Join Date
    Sep 2007
    Location
    MP89.5, Mason City Subdivision
    Posts
    209
    A typical AM broadcast tower's radials are connected to a ground ring buried around the base of the tower. There are usually ground rods also driven at the base and bonded to the ring and the lightning arc gap. Hopefully the hams design their insulated towers similarly.

    If the rods are there, you have to look at 810.21(j).

    Name:  bond.jpg
Views: 109
Size:  98.2 KB

    If the rods aren't at the tower base, could a buried ground ring the radials attach to qualify as a required electrode? I would probably lean that way and run the bond wire.

    As mentioned, copper strap is a better bond from an RF standpoint.

    Another choke comment...Chokes are also used on many hot AM towers to isolate the RF signal from the tower lighting conductors. They're mounted at the base of the tower in series with each conductor going up the tower (including the grounded conductor).
    Gregg

    I'm just here for the pictures!

  10. #10
    Join Date
    Apr 2006
    Location
    Springfield, MA, USA
    Posts
    3,261
    Quote Originally Posted by petersonra View Post
    810.15 Grounding. Masts and metal structures supporting
    antennas shall be grounded in accordance with 810.21, unless
    the antenna and its related supporting mast or structure are
    within a zone of protection defined by a 46 m (150 ft) radius
    rolling sphere.
    I don't see how you can get past the clear language of the code.
    IMHO the grounding requirement applies to support structures, but not to the antenna itself. If the entire tower is the antenna....?

    On the other hand, there are 'folded monopole' antenna designs which would work with a grounded tower.

    Back to the OP: one might argue that the antenna is a separate structure, which removes the requirement to bond the electrodes together. IMHO this would be a very poor design to not bond...but if you think of the building and the antenna as two different structures on the same property then I don't think the bonding is mandated.

    -Jon

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •