G'day,
| The M2 2800 rating of 35 sq feet is comparable to my own rating of 400
sq
| feet (on an absolute calm day!). Without a specified wind speed the
rating
| is meaningless?
An interesting point. Maybe there is someone out there who can offer
clarification on this.
I found the windloading of the Optibeam OB9-5 somewhat confusing:
Windload at 130km/h 578N / 0.72m sq / 7.8 ft sq
I received the following from Thomas at OB:
<quote>
on our web site you should find all the data which are usual, i.e. m2,
feet2 and Newton.
And we always calculate it at 130 km/h.
So the windload on the 9-5 is 578 Newton = 0.72square meters = 7.8 square
feet.
I am not doing the calculation, a friend of mine has written a program
which is doing the work.
The program checks what is the bigger wind resistance, the boom or the
elements when pointing into the wind with the maximum surface.
<unquote>
>From this I assume that the antenna at some angle to the wind has a
maximum surface area of 7.8 ft sq and at 130km/h offers a windload of 578
Newton.
Maybe someone can comment upon this interpretation as well.
My heavy duty 12 metre crank up/fold over tower has an unguyed and
extended "area of headload" rating for "tubular aerials" at 130km/h
(80mph) of 24.5 ft sq. At 100mph 14.4ft sq and 120mph 8.7ft sq. 80mph
winds would be very unusual here and higher speeds even more so, an OB9-5
on a PST61D on my tower should be pretty solid, even taking into account
the wind area of the rotator and mast.
We had a blow a couple of weeks ago that was 55kt sustained peaking 75kt
that caused a little damage around town. My 6ft sq log and rotary 20m
dipole above didn't complain. I couldn't telescope the tower if I wanted
to 'cos the wind pressure stopped the top section from lowering under
gravity. A pull down rope would seem useful in extreme circumstances.
Regards,
Mike VP8NO
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