Antenna Grounding

[andy]

I am curious as to how large a conductor is required to ground an antenna mast. I have heard some people say that the purpose is to simply drain static electricity away, this helping prevent a lightning strike. Under this school of thought, a relatively small conductor could be used. Others (including myself) feel that a large conductor, maybe 2 or 4 gauge, would be better. I am installing some rooftop weather stations on some area schools, and I want to be sure I ground these unit right.
What are some general guidelines here?
Thanks,
Andy

 

[dereckbc]

Re: Antenna Grounding

Andy lets cover some minimums.

1. Antenna masts and metal supporting structures are required to have a minimum 10 AWG copper grounding conductor. Read NEC 810 for guidelines.

2. I advise you use a antenna discharge unit (ADU), locate it where the cable enters the building. Ground the ADU with a minimum 10 AWG copper conductor. Again guidelines are in 810. Particularly 810.21.

NOTE: You can use larger cable than 10 AWG. Just keep in mind if the length exceeds a few feet, a larger cable is of really no value since the impedance at high frequencies is virtually the same for all sizes.

Exact details are hard to give you since I do not know how the building is constructed, or exactly where the antenna will be located in reference to a grounding electrode. The biggest mistake people make is to drive a ground rod electrode for the antenna system and not bond it to the service electrode. This is asking for trouble. Read and understand 810.21 very carefully so you understand it, and it will help you pick a convenient location to place the mast. If you are really lucky the building will be made have structural steel that you can use as the electrode.

Good Luck

Dereck

[ March 16, 2003, 11:10 PM: Message edited by: dereckbc ]

 

[mikeackley]

Re: Antenna Grounding

Andy:

Are these truly radio antenna masts? That is to say, do these rooftop weather stations actually utilize a radio receiver and/or transmitter? Or are these ?weather stations? simply a collection of temp., rain, humidity, barometer, windspeed, etc. sensors mounted on a metal supporting structure, each of which in turn is hard wired to a display of some sort. I ask the question only because it is my understanding that 810 pertains to Radio and Television equipment - antennas and supporting structures or masts.

 

[hurk27]

Re: Antenna Grounding

Andy

Keep in mind that the NEC is a bare minimum and in some cases needs to be increased to what the site requires. As lightning protection is a trade all in its self. and many states even require a separate licensing for it. I did allot of studies when I was in school and I was living in central Florida, Lightning capital of the us and second only to Darwin Australia. With that said it take's allot of know how to install the correct system, and it can become a legal issue if it is found that it was done wrong and caused allot of damage. And it is this reason that many states do require a licenses to do this type of work and even your liability insurance might not cover you in the event of a strike that causes damage. but there are some good sites to get the info from and it will give you a good idea of what's involved.

Some good sites on the subject:

http://lightning-protection-institute.com/lightning-checklist.htm

http://www.lightningsafety.com/

http://www.lightningstorm.com/
And this one does have a fourm

 

[andy]

Re: Antenna Grounding

Thanks to all of you that have replied! Yes, these are not really radio or TV masts, they are only holding weather instruments. I had been using #4 or #2 for grounding the masts directly to a ground rod. The instrument manufacturer provided a "factory-made" cable *** 'y to go between the instrument cluster and the computer inside the building. This cable itself has NO provision for grounding. Maybe I'd better check with their tech folks for a recommendation on grounding this. Also, they made no mention of bonding to the buildings service electrode (seems like a good idea to me!)
Thanks,
Andy

 

[dereckbc]

Re: Antenna Grounding

Andy, if you ran a ground conductor to a ground rod that is not bonded to the service electrode, you run the risk of burning up the weather instruments, computer, and a fire.

 

[mikeackley]

Re: Antenna Grounding

Andy:
I think you are on the right track and agree with dereckbc. Running #2 or #4 wire bonding the mast to building service electrode sounds like a good idea to me as well. However, since this is not a NEC issue, checking with the tech folks and getting something in writing is a good idea. This is a school..... probably can't be too carefull.

 

[jxofaltrds]

Re: Antenna Grounding

Mike

I do believe this is a NEC issue and that 810 applies.

Mike P.