The formula Gene proposed is not exactly correct, nor does he precisely
state what to do with the taper.
Regarding the formula, Gene's thinking is: If you think of the wind as
hitting the tube broadside (at 90 Deg to the tube), the exposed surface area
as seen by the wind looks like a rectangle whose length is the length of the
tube and whose height is the full OUTSIDE diameter of the tube. Hence, L x
Dia. Unfortunately, it is not that simple.
The tube is a cylinder as seen by the wind, hence the angle that the wind
*hits* the tube, even if it is perpendicular to the tube, hits at 0 Deg on
the centerline and then the angle increases to 90 Deg as the wind hits the
tube away from the centerline (above and below the centerline of the tube
for a horizontal element). This assumes the wind-front is wider than the
outside diameter of the tube, which is probably a very good assumption. So,
the surface area exposed to the wind by the tube is not simply L x Dia.
It's the exposed tube length times the integral from 0 to Dia of the surface
area of each tube (Pi x Dia), where Dia is the Outside diameter of the tube.
Gene, this is why the manufacturer's wind area is less than what you
calculated, and theirs is correct. [If anyone has trouble picturing this,
let me know and I'll try to explain it in more detail.]
But, I question the correctness of adding all of these calculations up and
saying that's the *wind area*. It certainly is the 1/2 of the *surface
area* if it's done this way, but the wind cannot be simultaneously hitting
the elements at 90 Deg and the boom at 90 Deg, so it should take that into
account by specifying the *larger* of both calculations, but not the sum of
both.
Regarding the taper: each tube should be calculated separately for the
*exposed* length of the tube *only*. Remember, some tubes are *inside*
other tubes, causing the taper.
Bill, N3RR
----- Original Message -----
From: Stu Greene <wa2moe@doitnow.com>
To: EUGENE SMAR <SPELUNK.SUENO@prodigy.net>; <towertalk@contesting.com>
Sent: Sunday, June 10, 2001 2:42 AM
Subject: Re: [TowerTalk] Antenna surface area
> At 09:06 PM 6/9/01 -0400, you wrote:
>
> > Area = L X diameter for the exposed surface of each piece of aluminum,
> > simple as that.
>
>
>
> Shouldn't the calculation be L X (Diameter X pi) ? Or length times
> circumference?
>
> And this doesn't reflect tapering of the elements.
>
>
> List Sponsor: Are you thinking about installing a tower this summer? Call
us
> for information on our fabulous Trylon Titan self-supporting towers - up
to
> 96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
> www.ChampionRadio.com</A>
>
> -----
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
>
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
www.ChampionRadio.com</A>
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|