At 05:42 PM 3/25/2006, ersmar@comcast.net wrote:
>Ted:
>
> Your method of calculation would give you the outer bound for the
> area of the mast. That is, it's the worst case number, as if your mast
> were actually a flat plate 2 inches wide and ten feet long.
>
> In reality there is some reduction in this effective area figure
> because the wind blows over a cylindrical surface, resulting in the wind
> force acting on the mast at shallower and shallower angles as it blows
> around the sides of the tubing. How much this reduces the worst case
> figure of 1.6 sqft would be a guess on my part. (A few years ago we had
> a heated discussion of antenna tubing shape factor on this reflector and
> I don't intend to restart it now, but I might have already.)
This is the "what's the drag coefficient of a tubular member" issue. It
can all be boiled down to:
1) It varies (according to the air speed, the roughness of the pipe, and
the diameter of the pipe). Worse than that, the variability is highest for
these sizes and speeds.
2) It's never bigger than 2 (that is, the equivalent flat plate area is
never more than twice the length *diameter)
If you're that close to the ragged edge, where it makes a difference
whether it's 2 or 1, you've probably got bigger issues that you need to
worry about (like how the drag area of the antenna was calculated).
> Also, remember that the wind force or antenna size limitation on any
> tower is based on that force being applied at a single point on the
> tower/mast, usually within a couple of feet of the top. If you mount
> antennas more than a foot or two above the tower top, i.e., stack 'em,
> you have to further derate this maximum wind force or allowable antenna
> area due to the longer moment arm against which this wind force pushes.
Gene makes an extremely good point here.. There's a fair amount of
difference betweeen a 10 sq ft antenna right at the tower top, and 10 sq ft
at the top of a 20 ft mast.
--
For generalized planning purposes, you can kind of do the approximations:
Hmm... I guess the Rohn 25 won't hold up that 80 meter beam... we're way
out of the ballpark. or.. I'm just putting up a 2m ground plane, I guess
the drag area of the whip is small enough that even if it's wrong by a
factor of 5, I'm ok.
If you're in the "if I tape the coax to the mast, that reduces the drag
enough to get my equivalent flat plate area down to n 9.247 square feet and
the spec says a maximum of 9.25, so I guess I'm ok" category, you might
want to spend a bit more time and effort.
If you're taking this to a city planning department for a building permit,
a healthy dose of conservatism might make the difference between them
trusting your simplified calculations and them wanting you to get a P.E. to
do the analysis.
There are several truisms:
1) You'll eventually wind up with more drag area on the tower than you
originally thought you would. (i.e. buy a bit more tower than you think
you need)
2) The tower probably won't fail when you ignore #1.
3) If your antennas stay up in the stormy season, they're too small.
Jim, W6RMK
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