As a tower novice, I have been looking into wind load calculations, to see
what I can really afford to put on my US Tower TX-455 (rated at 12.3 sq ft
of wind load at 70mph). I had been using the antenna vendor's specified
wind load numbers, but wondered why a 6 element Force-12 6 meter beam was
rated at 2.0 sq ft where as 5 element Cushcraft beam was rated at 2.9 sq ft.
Better wind-load design on the Force-12 might explain some of it, but
something didn't seem to add up.
I posted some questions about this on the Force-12 reflector and was told
that Force-12 uses an "effective" area calculation rather than a "projected"
area calculation. Sure enough, the Force-12 brochure has the following
explanation of their wind load calculation:
"WIND LOAD is the worst case wind resistance for the antenna. Using the
latest structural analysis, the wind load is either the total element wind
load OR the boom wind load, whichever is the larger resistance to the wind.
Most beams have more element than boom wind load. The figure specified is
the effective area, which is the projected area of the elements or boom,
multiplied by 2/3 for a cylindrical surface."
It was suggested that I needed to multiply the Force-12 number by 1.5 to get
a number that would be appropriate to compare against the Rohn (or in my
case US-Tower) "projected" wind load maximum.
I did my own calculations on my C-3E yagi, adding up each separate element
section's wind load computed by multiplying the diameter times the length.
I also read an article by W7NI (January 1992 NCJ & ARRL Yagi Antenna
Classics) that said you compute the maximum wind load on an antenna as the
square root of the sum of the squares of the boom area and the element area.
Calcs are multiplied by 2/3 due to the round shape of the elements.
My calculations were:
Total Element Projected Area times 2/3 = 5.67 sq ft
Total Boom Projected Area times 2/3 = 2.0 sq ft
SqRoot of sum of squares times 2/3 = 6.0 sq ft
The Force-12 catalog has the C3E wind load rating at 5.8, but their manual
says "5.9 square feet max at 21 degrees from boom center". So, I have 6
potential C3E wind load numbers: 5.67 (total of elements), 7.67 (total of
elements plus boom), 6.0 (square root of sum of squares), 5.8 (catalog), 8.7
(1.5 times the catalog) or 5.9 (instructional manual).
Question - which one to use against my US-Tower 12.3 number? The 6.0 sq ft
number makes the most sense to me - seems like the real MAXIMUM wind load
that can be on the antenna, slightly off center from directly into the
elements. Wind doesn't blow simultaneously directly into the boom and
directly into the elements (the 7.67 or 8.7 numbers).
I also computed the wind load for the rest of the antennas I'm currently
looking at. The results, listing vendor rating, calculation using square
root of sum of squares, and calculation of total of boom plus element
projected area were:
Rated Squares Total
Cushcraft D3W 0.9 1.75 1.75
Cushcraft 3 element 6m 1.8 1.1 1.5
Cushcraft 5 element 6m 2.9 1.9 2.6
Now maybe I made some errors in my calculations, I'll have to double check
them. Perhaps the vendors use a more sophisticated technique to determine
wind load numbers.