[TowerTalk] Antenna surface area

[Hank Lonbeg]

Time Out everyone:

All this talk about integrating and double integrals has given me a head
ache. As defined in the Building Code I know the most about ( the UBC), the
surface area exposed to the wind is the plane projected area normal to the
wind direction under consideration. For a flat plate of W width and L length
with the W dimension normal to the wind the projected area is the width W
times the length L. It is really  that simple. A cylindrical member of
diameter D and Length L has a projected area of L X D. Any modifiers to the
projected area are used to describe the effects of shape and size(the
Reynolds number or Froude number) to the air mass moving over the particular
element. At the normal range  of element sizes and distances between
elements it is safe to assume that the elements do not shade each other and
that the total projected area is the sum of the elements and any connnection
items that have an normal area in the same direction. The same is true for
wind normal to the boom which might have a shape factor coefficient because
it is of large enough diameter. (The UBC uses 1.0 for elements less than or
equal to 2" in diameter and 0.8 for cylindrical elements large than 2" in
diameter.)

This method is used not only for towers but is used for buildings and other
structures to determine their design loads. You must remember that all these
loads are approximations of the real world and are not exact numbers. Use
the most significant digit analysis and realize that any wind load numbers
you come up with are really only significant to the nearest 5 or 10 pounds
anyway.

What the manufactures need to do is to publish the antenna areas as the
projected area to the nearest 0.5 Square foot and be done with it. It really
makes no sense for them to publish wind load information in Pounds Force at
some wind velocity because they normally don't publish the height and method
of calculation and other assumptions for that load. It is meaningless
without the additional information.

In reality considering a triangular open trussed type tower system, unless
it is extremely short, the wind load on the tower is the largest force in
the system, not the wind load on the antenna. The absolute antenna load is
most important for the selection of the mast materials and size and of
course the load capacity of the rotor.

73 and happy calculating


Hank Lonberg P.E. / KR7X








-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Tod - Minnesota
Sent: Sunday, June 10, 2001 8:33 AM
To: towertalk@contesting.com
Subject: RE: [TowerTalk] Antenna surface area


Wasn't the original purpose of this discussion to allow someone to put
together a tower and antenna system that would neither blow over in the wind
nor be 'crushed' under its own over-designed weight?

If we use equations that calculate forces and torques that are high (two
times or even three times high), aren't we simply adding a safety factor to
the overall design? It is really hard to work DX when you spend all of your
free time calculating wind loading. At some point I would think we would
have a sufficiently good set of equations and models to allow quick, safe
values to be determined and then we can get on with the assembly, hoisting
and maybe even use of the antenna system.


Tod, KØTO



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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>

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