Not only is it that the M2 rating of 35 sq feet is meaningless, also the
rotational torque inertia may even be more important!
See my previous posting on 4/28/06 at 9:08 am. There have been no responses
to that previous concern.
k7puc
----- Original Message -----
From: "Mike Harris" <mike.harris@horizon.co.fk>
To: "Al Williams" <alwilliams@olywa.net>; "Tower Talk"
<towertalk@contesting.com>
Sent: Monday, June 12, 2006 5:27 PM
Subject: Re: [TowerTalk] Rotator Questions
> 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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