[TowerTalk] Tower Questions

[Bill Coleman AA4LR]

On 4/2/00 2:25 PM, Kurt Andress at K7NV@contesting.com wrote:

>
>> 1.  Is it true that the wind speed at say 100' is greater than at 50'?
>> always?
>
>The EIA-222-F and UBC 1997 spec's both produce higher wind loads (not
>necessarily wind speeds) as the height is increased. Remember that these
>spec's use a "basic wind speed" which is for the fastest mile of wind
>measured at 33 feet. That remains constant, what increases with height
>is the windload. "Is it really true?", they think so.

In aviation, we observe this frequently. The air near the ground is 
slowed by contact with the surface, while air further up can blow 
significantly faster. Typically, airspeeds increase as you go higher in 
altitude (and they also change direction slightly, due to coriolis forces 
of the spinning earth).

For tower-related heights, there can be a substantial increase in wind 
velocity from the ground to 200 feet, especially if there are buildings, 
trees, or other obstructions nearby. I used to fly out of a small field 
surrounded by trees. Nasty crosswinds would often disappear when my plane 
dropped below the tree line.

>> 2. For a free-standing tower, is the failure mode most likely at the very
>> bottom of tower?,
>
>We'd have to analyze each one to really know what is going on. 
>For these kinds of questions, ask the tower designer (usually not the
>sales guy), I'd bet he knows.

For some types of tower, foldover due to torque stresses is more likely 
(eg Rohn BX-type tower). But the answer is basically "it depends".

>Since you raised questions about antenna loads and towers, I'll take the
>opportunity to post my bi-annual reminder that the antenna area ratings
>cited by most tower designers is not necessarily the same antenna area
>that one is likely to get from the antenna folks.

Which raises my question -- how do we get these antenna area ratings and 
antenna area specifications into the same units so we can compare them?



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


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