In a message dated 4/6/2007 3:24:30 P.M. Pacific Daylight Time,
towertalk-request@contesting.com writes:
> I would like to put up a C31XR tribander with a Mag240N 2 element 40 meter
yagi above it. The total tower windload is:
> C31XR: 10.7 SF
Mag240N: 6.8 SF
16 ft Mast: 1.6 SF for 2 inch or 2.4 SF for 3 inch
Rotator: 1 SF (approx)
> Total load: 20 to 21 SF
> Still deciding on height, but the desire is for either 80/87 feet or 90/97
feet. In either case in a 100 MPH environment, this load exceeds the
capacity of 45G and 55G.
My calculations show that 55G will take about 23 sq.ft. @ 100 MPH.
Sounds okay to me.
> A light just lit up in my head. What if I rotate the 240N by 90 degrees?
Now
the cross section in parallel with the tribander elements is that of the
boom rather than 6.8 SF. ie) Assume average 3 inch boom by 24 feet =
3/12*24*0.67 = 4 SF.
Well, IMO you're not buying anything by offsetting the 40M. It has not
been proven conclusively that element into or boom into the wind matters.
Besides your wind will probably be shifting around somewhat as it blows
exerting
more windload on off-axis antenna orientation.
> I don't know the actual boom diameter, so it may be slightly smaller.
Since
I am right on the edge, the reduction in 2.8 SF is a big deal to me. Is this
really legit, or am I tempting the wind gods?
I wouldn't worry about it - you're still within the manufacturer's
specs. Rohn tower owners are notorious for WAY overloading them with very
little
repercussions. Keep your homeowner's insurance paid up and go for it.
Cheers,
Steve K7LXC
TOWER TECH
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