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[TowerTalk] Re: Standarizing Antenna Windload

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Subject: [TowerTalk] Re: Standarizing Antenna Windload
From: (Kurt Andress)
Date: Sat, 28 Aug 1999 11:42:42 -0700
Hi Stan,

Wow, almost got a headache!

Here's my view...
Trying to match low numbers with low numbers and high with high doesn't
work. The area information needs to be accompanied by the knowledge of
the members shape. Round antenna members go with with round tower areas
and flat with flat.
But the value we take to the tower party is the projected area, along
with the knowledge of the shape that accompanies the area.

I think it would be very difficult for most antenna designers to give us
equivalent flat area values for round membered antennas.
Each flat equivalent value for a round membered antenna would need to be
specific for the tower design spec. And we really don't need it.
I think it would make life more confusing for everyone.

Most amateur antennas are made from round stock. Those that do use
rectangular sections, seem to use them for the boom. It seems to be easy
to find out what a particular antenna has in this regard.

But, to remove all doubt the values could be presented this way:

Elements - Projected Area = XX.X SqFt - Round
Boom - Projected Area = XX.X SqFt - Flat

Now, if someone builds us an antenna with mixed rectangular and round
members in either boom or element axes, we need to have his head
examined, and get a big discount to cover getting some experienced help.
The antenna load on the tower would have to be determined using the
tower design spec.

Another thing to keep in mind:

The direct application of antenna areas to towers can have limited
When the antenna configuration gets away from simple, we start to lose
the ability to do this.

For a single antenna at the top of a tower, the projected area (for an
all round member antenna) used with the Rohn round value (in the oval
outline) works fine.
Or all flat in one major axis, with round in the other.
Each antenna value needs to <= the appropriate tower value to be ok.

As soon as we start stacking antennas, things can get difficult.

If we stack all round membered or all flat membered antennas on the
tower below the top, we can compare the sum of the antenna areas at each
possible configuration of azimuths, to the appropriate Rohn value.

If we have mixed round and flat members along the major axes, it gets
confusing pretty fast, but can be figured out. Has to be done for each
combination of directions.

If we are stacking on a mast above the tower, we need to do the tower
analysis over for the intended configuration, because it is a different
load case than the one Rohn presents. 10 SqFt at tower top + 5 SqFt 5'
above tower + 5 SqFt 10' above tower is not the same as 20 SqFt at tower
top, it is more severe.

There are lots of other permutations and issues not discussed here.

Not trying to give people a headache here, just some perspective.

Let's see if we can first get to the point where we can use the values
to make the simple evaluations. After that we will be armed with the
correct info to do the more difficult ones.

Your last comments bring up a good point. 

The user needs to find out what the area rating for the tower refers to.
Round or flat membered antennas. I suspect most amateur tower mfgr's
mean round membered antennas, as that is the most prevalent
Asking the question will remove all doubt.

73, Kurt, K7NV

Stan Griffiths wrote:

> Rohn actually uses two wind load figures on their guying diagram pages.  One 
> is for
> "flat membered loads"  (the lower of the two numbers) and the other is for 
> "round
> loads" (the higher of the two numbers).
> Looking at it this way, there are two numbers for every antenna and two 
> numbers for
> every tower.  If you are using the high number for the tower, you should also 
> use the
> high number for the antennnas you are putting on it.  If you are using the 
> low number
> for the tower, you can use the low number for the antennas you are putting on 
> it.
> we get into trouble is when we use the high number for the tower and the low 
> number
> the antennas . . .
> So if you are given only ONE number for the antennas, which number is it?  
> High or
> I will bet it is the LOW one since that is the number that makes the antenna 
> seem like
> smaller load on the tower and will, therefore, sell the most antennas.  So, 
> when
> matching these antennas to a tower for checking the tower windload, you had 
> better use
> the low number provided by Rohn.
> This is why it is so critically important to know how the antenna manufacture 
> has
> determined the windload of his antennas.
> I am not really aware if tower makers other than Rohn give one number or two. 
>  From
> I have read from Kurt, I would guess they supply only one number and that 
> would
> be the low one, or the most conservative one, which is the appropriate one to 
> use with
> the numbers most antenna makers provide (the low one).
> Stan

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