> The interesting question is why they're requiring the rebar
> to be bonded to your UFER ground. That's a pain, because it
> means the cage has to be welded. Much easier to do the 20
> feet of copper wire in the concrete thing.
I don't know the exact language of the requirement yet, so it's not clear
they will require a welded rebar cage. According to the Polyphaser book,
wire-tied rebar has been shown to be a very effective conductor because
"there are a lot of junctions in parallel cinched tightly (IEEE Seminar
Notes 1970.)" The book goes on to say that "the use of large amounts of
copper wire coiled in the base of the tower has been shown to cause flaking
of the concrete and could, over time, also cause corrosion of the rebar."
> You'd only need a form at the top, right? (so that you have a
> surface that
> is level to smooth the concrete to) You don't care if the
> excavator makes
> the hole a bit bigger, and you just wind up buying a few more
> yards of
> concrete to fill the hole. There's nothing requiring the
> base to be a
> perfect square.
Yeah, that's the way it turned out on my first tower. But the excavator had
a really hard time digging to hole to spec (3'x3'x6') and I ended up with a
trapezoidal hole 7 feet deep and about 4' x 4' at the top and maybe 5' x 6'
at the bottom. It took the better part of a truckload of cement to fill it.
Luckily, the contractor bid a fixed fee and had to eat the cost of the extra
concrete (this was done as part of a very expensive home remodeling, so he
wasn't going to argue.) I ended up with a fabulous pier, but I don't think
I'll find another contractor like that one!
> Might be too late, but perhaps your ground radials shouldn't
> be in the
> plans to be approved? You're going to install them later,
> after all the
> structural work is done.
Not necessarily. I had planned to do it while the concrete cures, which
takes close to a month.
> You could point out to the electrical guy that those radials
> are purely for
> RF grounding, and don't form part of the electrical or
> lightning protection
> grounding system.
Not so. These three radials are designed strictly for lightning protection,
according to recommendations from various sources, incuding Polyphaser.
That's why I'm using 1/0 conductor and placing 8' ground rods every 12 feet.
In addition to being tied to the tower and rebar, they'll be tied to the
utility box next to the tower. Inside will be a grounding panel to which
lightning suppressors will be mounted. Of course, I'll tie this ground
system into any radial system I put down for vertical antennas hanging off
the tower.
> However, bear in mind
> that you'll
> probably have some number of feet of steel guy attached to the anchor
> before going to the synthetic.
Yes, but I'll be using a fairly short length of steel wire as I'm not
concerned about vandalism or brush fires at the tower location.
> Or, you can bury it less deep and cover it with concrete, if
> that's an issue.
200+ feet of concrete?
> yes, the electrical code DOES address low voltage cabling AND
> also coax and
> antennas. Go to the Mike Holt website (http://www.mikeholt.com/) and
> download his lowvoltage handbook (it's free). It covers most
> of the code
> issues around low voltage control, coax, phone, etc. along
> with grounding.
Thanks! This is very helpful.
73, Dick WC1M
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