[TowerTalk] Stacking Antennas and Windloads

[K7LXC at aol.com]

In a message dated 4/6/2007 3:24:30 P.M. Pacific Daylight Time,  
towertalk-request at 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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