[TowerTalk] wind load vs Rohn specs

[Bill Coleman AA4LR]

On 9/3/00 1:26 AM, Kurt Andress at K7NV@contesting.com wrote:


>Take one example:
>
>>40'
>>Round 18.4x1.2=22.08
>>Flat 11.0x2.0=22.0
>>
>
>This is correct!
>It says that it takes 18.4 SqFt of Flat Projected Area (FPA of your antenna
>made from cylindrical members) to create 22.08 SqFt of Effective Projected
>Area. This is where the drag coefficient is applied.

This discussion of projected area has always made by head spin. Seems to 
me it doesn't account for antenna members that could add significantly to 
the drag of the installation.

If we take a small tribander and optically project a shadow of the 
antenna vertically on the ground, then measure the area of that shadow, 
we fail to take into account say, the boom to mast plate. Since the plate 
sits vertically, it casts a small shadow, yet it presents a big area to 
the horizontal wind.

Similarly, if we optically project a shadow horizontally against a wall, 
and measure the area of that shadow, we'll take into account the boom to 
mast plate, provided we have the antenna oriented correctly. However, 
we're likely to miss either the boom or one or more of the elements, 
whose shadows overlap one another.

So, which is right? The vertical projection or the horizontal projection? 
Neither seems to be well correlated to the actual drag of the antenna.

--

Seems to me a better measure was the old "flat plate equivalent" -- which 
is the drag of the antenna expressed as equivalent to a flat plate of a 
given area. This FPA and EPA for round and square antenna members just 
leads to a lot of confusion.




Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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