Steve, How high is the tower in question? Is it set into a concrete base?. I think that the NEC has something to say about the Lightning Protection for structures such as these. What I would do is this: Get 4 Ground Rods and drive them at at least 6' apart and bond them with bare copper of at least #4. From there, take 4 wires from the tower to each Ground Rod. This could be over-kill for your particular application , but it is better to be safe than sorry. Also, any antenna that is on top of the tower should be fed via a Gaseous Surge Diverter, which should also be Grounded to the tower Grounding system. The Impedance of the outside Grounding system needs to be a lot lower than any Ground impedance on the inside of the building that that antenna feeds. It needs to be that way to keep Lightning currents out of the building. Oh and by the way Steve, it also has to be bonded to the Building Grounding system too, so that if there is a strike, the whole Grounding system is at the same potential during the current distribution of the energy. Hope this helps.
Let's face it, these days if you're not young, you're old - Red Green
#53667 - 07/03/0512:31 PMRe: Grounding Antenna Support Structure
(a) Electrodes of pipe or conduit shall not be smaller than metric designator 21 (trade size 3/4) and, where of iron or steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection.
That said, I agree a copper or copper clad rod is a better choice. The radio towers I have inspected had Ufers on the shelter building, a ground ring and radials out to driven rods on the corners.
#53669 - 07/03/0504:55 PMRe: Grounding Antenna Support Structure
The "tower" will be approx. 20' tall and the tallest antenna is approx. 10.5'.
The support structure will be set in a concrete base that is 4' deep x 4' long x 2' wide. The engineer will tell me if I need to rebar it (which I think it will need to be). It will also have a support tied into the rafters of the roof for stability at approx. 8' height.
I will be running LMR 400 coax cable to the antennas, with each having a lightning suppressor.
I have some leftover EMT, so was wondering if I could use that, but as resqcapt19 points out, it has to be "pipe" and not "tubing". I knew I had read "soething" about using 3/4", and was thinking EMT was ok, but couldn't remember where I saw it (the little grey cells can only do so much at one time).
I was planning on driving two grounding rods, but not four. Perhaps splitting the difference and going for three would be ok.
It will also be tied in to the house ground system.
Thanks again for all the responses and help.
#53672 - 07/04/0501:06 PMRe: Grounding Antenna Support Structure
The radials are really to establish a better ground plane from what I understand but they also give you a great grounding system. If you do end up with rebar in the concrete you should bond that too. Every little bit helps. You could simply bond it to the "J" bolts you set in the concrete to mount the tower. If you end up with a total of 20' in the base, tied together, it is technically a Ufer (concrete encased electrode. Around here they are always building something so spare rebar and J bolts end up in the dumpster all the time. I can send you some J bolts if you want. Rebar would kill us with shipping tho ;-)
#53673 - 07/04/0502:09 PMRe: Grounding Antenna Support Structure
I just can't imagine EMT surviving the effort necessary to pound it into the ground. Depending on local conditions, it may take only a few years for EMT to completely rust away in the first 6" or so underground- and wrapping it in tape seems self-defeating!