Bottom posted comments...n2ea
Steve, K7LXC wrote:
> 10 Feet - 55 Sq Feet
> 20 Feet - 25 Sq Feet
> 30 Feet - 9 Sq Feet
> 35 Feet - 4.7 sq Feet
> This is based on the 70 MPH Winds.
> To find this chart, go to
> http://www.radiancorp.com/ROHNNET/rohnnet2004/html2004/index.html
> Click on On Line Catalog (Bottom Left of the Menu Bar)
> GT Series (Top Right)
> 25 G (Again, Top Right)
> Then Allowable Antenna Areas
I followed the above instructions and couldn't find the
Allowable
Antenna Areas anywhere. :-(
OTOH I do have a Rohn brochure entitled "Manufacturer's
Technical
Information for Bracketed or Self Supporting Towers". I don't know
the date of
it so I don't know what rev of the TIA-222 it's based on. This is
what it
says for 70 MPH:
10 feet - 42.5 sq.ft.
20 feet - 22.0 sq.ft.
30 feet - 12.0 sq.ft.
35 feet - 8.7 sq.ft.
40 feet - 5.1 sq.ft.
45 feet - 2.3 sq.ft.
I'm not sure how to reconcile the different specs but if
you're
going to err, err on the side of being conservative engineering-wise.
For the record there is another base spec for a self-
supporter that
is kind of hard to find too.
For the guy who wanted to go 45 or so feet above a
housebracket with
45G @ 70 MPH, the same pamphlet says
14 feet above the housebracket is 31.0 sq.ft.
24 feet above the housebracket is 13.8 sq.ft.
34 feet above the housebracket is 5.5 sq.ft.
-0-
My comments:
The question with which we are left is the following: Were those
specifications established to
satisfy engineering criteria, or were they established to satisfy
liability criteria, with sufficient headroom
to minimize Rohn's insurance premiums?
Suspecting the latter, with multiple overkill in the dynamic
reserve, I believe this is why we find hams
breaking the prime directive and getting away with it. And
everybody has an annecdote.
Here's mine:
I had 45G house bracketed at 6' and 16' with an n2ea custom designed
bracket. The tower was
initially 50', and later extended by a hinged section to 60'. On
top I had a T8 lpda, plus an r7000 vertical
above that, with the mast extending perhaps 1 meter above the tower
top.
Winds of 90mph recorded, with higher gusts. Rock solid.
My brackets had over 30,000 lbs of shear withstand capability, and
almost that in the extraction
mode. Considerably stronger than the rohn, but less flexible.
I suspect that the real variable in the house bracketed situation is
the house, and the engineering
that goes into the house attachment system.
I should also note that I was a scofflaw, and didn't get a permit for
the tower. So I never had to document
my calculations, or have someone certify them. But that was
Vermont, nobody much cared, and you
couldn't see the tower unless you were on the property, looking for it.
Don't do what I did; follow the prime directive!
N2EA
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