In a message dated 98-11-13 02:39:18 EST, Swilsonac6@aol.com writes:
> I am about to put up my first "store bought" tower and intend to do it "by
> the
> book" with permit and inspections. I have decided to put up a 72' crank-up
> tower. My immediate plans are to mount my Force12 C-3E tribander (5.9 sq.
> ft
> area) and 13 element 2M beam (1.8 sq. ft area). My county of Marin, CA is
a
> 75 mph wind zone.
>
> After talking with a few locals about their installations I had decided on
a
> US Tower model TX 472 rated at 18 sq. ft max allowable antenna area @
50mph.
> Problem is that this rating drops to 7.3 sq. ft at 70 mph and I calculate
> about 5 sq. ft at 75 mph. Upgrading to a TX 572 rated at 30 sq. ft at 50
> mph
> solves the problem but at twice the expense and seems like overkill for
what
> seems to be common practice for supporting a small tribander and a few VHF
> goodies. Obviously with the approach of a winter storm which roll in here
> from the Gulf of Alaska with some forewarning, thoughts would drift towards
> cranking down the crank up. Still even if I could get a permit through
> these figures are a worry.
>
First of all, Alameda County is a 70 MPH windspeed zone - the lowest
they give.
Second, the towers are rated a for the square footage spec at 60 MPH even
though the literature says 50 MPH. That may move your derating curve up enough
to make it work.
If you're really worried about it, don't crank the tower all the way up.
Even retracted a couple of feet increases the wind load capacity.
I've got a couple of crank-up rating articles that cover these topics
that you may want to read. One is by Roger Cox, WB0DGF, chief engineer at Hy-
Gain and the other is by Bruce Horn, WA7BNM. They're a buck each from Tower
Tech, Box 572, Woodinville, WA, 98072.
Since you're getting a permit, that means that you'll be getting the PE
stamped drawings from the manufacturer. Take a look at them and either make
your own calculations based on actual use or have an engineering type buddy
run through them with you. It may be just the psychological help you need
since I suspect that the tower will handle your proposed system just fine.
Cheers, Steve K7LXC
US Tower approved installer
Tower Tech
Champion Radio Products
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