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[TowerTalk] twr selection update

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Subject: [TowerTalk] twr selection update
From: (Chad Kurszewski WE9V)
Date: Tue, 17 Jun 1997 17:54:42 -0500
>> >> Guyed Rohn 45, 70', 70MPH is only rated for 17.5 sq.ft. 
>>  >
>>  >    Actually, if you add in the 8.0 sq.ft. that Rohn has deducted for a
>>  >commercial antenna mount, you get 23.5 sq.ft. Just FYI.
>>  No, 17.5 is including the 8.0 side mount.  Rohn specifies 9.5 sq.ft. in
>>  the catalog.
>        My mistake - I didn't look it up. For your example above (70' @ 70
>MPH), the Rohn spec shows 15.9 sq.ft. Adding the 8.0 sq.ft. already netted
>out gives you a total capacity of 23.9 sq.ft.
>      The 9.5 sq.ft. figure you cite is in the rectangular box meaning that
>it is the figure for "flat member" antennas. You should more properly use the
>other figure, the one that is in the rounded box; this is the one for "round
>membered" antennas. Amateur yagis are round membered antennas.

No, that isn't correct either.  Yes, amateur yagis are round membered
antennas but their wind loading is NOT given in projected areas.

Rohn's General Note #2 states:
   (circled area) Allowable Proj. Area (sq.ft.) for round member antennas
   (square area)  Allowable Proj. Area (sq.ft.) for flat member antennas

Most antenna manufacturers do NOT give their wind loading is projected area,
but already incorporate the 0.67 shape factor for round members.

For example, the projected (length x diameter) area of a Hy-Gain 105CA
is 4.37 ft^2 for the elements and 3.98 ft^2 for the boom, which would
give you a PROJECTED area of 5.91 ft^2.  Multiplying that by the 0.67
shape factor gets you 3.95 ft^2.  The manufacturer's data I have says
3.9 ft^2.  Pretty good correlation.

Now, if you were to use Rohn's circled figures for wind load capability,
and use the manufacturer's wind load figures, you are double dipping, or
applying the 0.67 shape factor twice, which isn't correct.  (Rohn uses
a shape factor of 0.6, so their 15.9 round PROJECTED area equates to a
9.5 flat projected area equivalent.)

So, verify with your antenna manufacturer if they are using projected
areas or equivalent windloading (incorporating the 0.67 factor).
It's more than likely the latter, because the projected area of an
antenna is NOT the wind load.  After you do this, make sure you are
using the correct figure from Rohn's catalog.  If the antenna manufacturer
gives you "real" windloading, you MUST use the "Allowable proj. area for
flat member antennas", or if you wish, divide the antenna wind load by
0.67 and then you can use the "allowable proj area for round member antennas.


Chad Kurszewski, WE9V         e-mail:
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