Hi Dana,
The answer to the question about whether you need to multifly your Force 12
numbers by 1.5 or not is contained in th following information:
First, you must remember that both antenna makers and tower makers are
trying to make their product look as good as possible to potential buyers.
Anything an antenna maker can do to make his wind load look as small as
possible will be to his benefit in his advertising literature. Also,
anything a tower maker can do to make his tower look as strong as possible
will be to his benefit in HIS advertising literature.
It has been pointed out that round members (most antnnas) have less wind
resistance than flat members by a factor of about 2/3. This concept and
figure is well accepted among structural engineers who are the experts in
this field.
Antenna manufacturers, in order that their antennas will appear to have less
wind loading in their literature, usually take this fact into consideration
when specifiying the square footage of their antennas, ie: if you add the
areas of all the elements (or the boom) that will be seen by the wind when
blowing directly at them, and take 2/3 of this number, you will have the
lowest figure for antenna area for that antenna. (Of course it will be a
higher number if you don't take 2/3 of it . . . )
Tower makers, on the other hand, wanting to make their towers appear as
strong as possible, will calculate the acceptable load on their towers for a
flat plate at the top with its maximum exposed area catching the most wind
force. They will then figure how much MORE area they can safely take if the
load were a round (tubular) member rather than a flat plate and the original
calculation for a flat plate can be multiplied by 1.5 (same as dividing by
2/3). This is the figure the tower maker will generally publish in HIS
literature. (Rohn gives you BOTH numbers.)
The problem is that in any over-all tower/antenna design, you can only use
this 2/3 (antenna maker) or 1.5 (tower maker) calculation ONCE . . . either
by the antenna maker OR by the tower maker . . . but NOT by both of them.
So in order to know if the antenna maker's square footage number for applied
load compares properly with the tower makers square footage number allowed
load, you have to know if either of them used the 2/3 or 1.5 figure in thier
calculations. If they BOTH used it, then you have to take it out of one of
the numbers before you can compare the antenna load to the tower's
acceptable load. Since neither tower makers nor antenna makers generally
TELL you if they used the 2/3 or 1.5 factor in their calculations, you have
to ASK them their calculation methods. There are acceptions to this. Rohn
is one, since they DO tell you and I have heard there are antenna makers who
will also share this information with you. An extremely common mistake is
to take Rohn's allowable load figure for round members (1.5 times the
allowable figure for flat members) and use it to determine how much antenna
load you can put on it using the antenna maker's load ALSO figured for round
members (2/3 of flat members). This results in a dangerously overloaded
tower.
Stan
w7ni@easystreet.com
----- Original Message -----
From: "Dana Roode" <K6NR@ARRL.net>
To: "Stan & Patricia Griffiths" <w7ni@easystreet.com>
Cc: "Towertalk" <towertalk@contesting.com>
Sent: Sunday, February 09, 2003 10:06 AM
Subject: Re: [TowerTalk] Calculating Antenna Wind Load
> Stan,
>
> Thanks, the emergence of a new method may explain why Force-12 does things
> the way they do. The square root of the sum of the squares method also
> seems to be what ex-W6QHS uses in his "Physical Design of Yagi Antennas"
> book as well, but I may have misunderstood.
>
> For my C3, the 2 numbers are very close (5.7 vs 6.0), so this isn't really
> an issue for me. The notion that I might need to multiply Force-12
numbers
> by 1.5 or use the total boom + element projected areas against my US Tower
> 12.3 sq ft wind load limit was what concerned me. I believe this is
false.
>
> Dana Roode
>
>
> Stan & Patricia Griffiths wrote:
> > Hi Dana,
> >
> > It has been pointed out to me several times that the method I used to
> > calculate antenna wind area in my original article (published in about
1985)
> > has been replaced by a newer method. The newer method used either the
> > element OR the boom area and not the square root of the sum of the
squares
> > of those two figures. K5IU can explain this much better than I can.
> >
> > Stan
> > w7ni@easystreet.com
> >
> > ----- Original Message -----
> > From: "Dana Roode" <K6NR@arrl.net>
> > To: "TowerTalk" <towertalk@contesting.com>
> > Sent: Saturday, February 08, 2003 6:41 PM
> > Subject: [TowerTalk] Calculating Antenna Wind Load
> >
> >
> >
> >>TowerTalk Folk,
> >>
> >>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.
> >>
> >> Dana
|