>Greetings, fellow towerophiles --
>
> In the course of a tower construction project involving Rohn 45G, I
>made a discovery that I think has gone un-noticed by amateur tower owners and
>builders. In their catalog, Rohn provides drawings for 70, 90 and 110 MPH
>along with different tower heights. At the top of each sub-drawing is a
>little rounded box that has the "allowable projected area" in square feet.
> The round member number on the left is the one that everyone uses for
>planning and construction. In the general notes with each drawing, Rohn
>builds in (deducts) an amount equal to a couple of feedlines; one 1/2 inch
>and one 7/8 inch cable for 45G. So far so good.
>
> Upon further reading of the notes, it states that "tower designs include
>three side arms, symmetrically placed having a total effective projected area
>equal to 8.0 square feet". For commercial antenna mounting, this is normal
>and Rohn has allowed for it. In the case of amateur installations, the side
>arms are not used. In other words, Rohn has deducted 8.0 square feet already
>from the tower loading figure and the number that is on each drawing is the
>NET tower load. For amateur loading, you can ADD the 8.0 square feet onto
>the posted figure to come up with a more appropriate wind load number. This
>same situation applies to 55G as well.
>
> One note of caution; Rohn states that the side arms are "symmetrically
>placed" meaning the load is distributed over the whole tower, not just on the
>top. While it may not be appropriate to add the whole 8.0 square feet to the
>top of the tower, it does show that there is additional tower capacity
>available that is not obvious from the number in the box. Maybe you do learn
>something everyday.
>
> Comments?
>
>73, Steve K7LXC
>
> TOWER TECH -- professional tower supplies and services for amateurs
My only comment is that you are right, I never noticed that before.
Try this one: As you have noticed, there are TWO boxes with square foot
numbers in them. One for ROUND member antennas like Yagis and one for flat
member antennas. I notice the flat number is about 2/3 of the round number.
This all makes sense, but raises an interesting question. When figuring the
wind area of an antenna, it is common practice, is it not, to take the
projected (shadow) area and multiply it times 2/3 if you use round members.
If you then use Rohn's "round member" number, aren't you using the "2/3
factor" twice when you should use it only once to determine wind load?
Stan w7ni@teleport.com
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