[Fwd: Re: [TowerTalk] wind load vs Rohn specs]

[Kurt Andress]

-------- Original Message --------
Subject: Re: [TowerTalk] wind load vs Rohn specs
Date: Fri, 01 Sep 2000 20:59:08 -0700
From: Kurt Andress <K7NV@contesting.com>
To: Pete Brunet/Austin/IBM <brunet@us.ibm.com>
CC: towertalk@contesting.com
References: <OF2A9E3B73.DA09BF53-ON8625694E.000B4FC1@raleigh.ibm.com>

Pete Brunet/Austin/IBM wrote:

> >Pete, You are in the wrong code. There is no .67  To get the ROHN assumed
> EPA value you multiply their are by 1.2 for rounds and 2.0 for flats.
>
> According to the Force 12 brochure they start by knowing their antenna's
> projected area.  The generate their windload number by multiplying their
> projected area by .67 and they publish that number in their brochure.  In
> order to know what projected area they started with I would have to
> multiply their windload by 1.5.  Make sense?
>

Correct, give their area x 1.5  to your P.E.as the antenna projected
area,
he'll figure out the rest from the sepc required in your area.

The reason they do that is because most other antenna mfgrs are still
giving
antenna areas that came from the old EIA-RS222-C days. That is where the
infamous  .67 came from. It is ~ the relationship between flat members
and
long cylinders. The newer specs use the classsic aerodynamic drag
coefficients
of 2.0 for flat and 1.2 for long cylinders.

It is really time for the antenna guys to catch up with modern
practices. They
are playing marketing chicken, no body wants to be the next one to
advertise
projected areas, it is expected to hurt sales because some uninformed
buyer
will choose the other guys antenna because it has a lower area number.
Hygain did it quite a while ago, before they sold. MFJ hasn't screwed up
Roger
Cox's numbers yet, let's hopw they don't. As soon as they start
delivering
antennas again, their figures will be the only ones you can directlt use
in
selecting a suitable tower.
Remember that when you try to compare these figures from different
vendors,
you probably ought to ask what they mean before making any decisions.

Thanks to Force 12 for clarifying in their brochure what their area
values
mean. I would be useful for them to include the other area at 90 deg to
the
one given and know which was for the elements and which was for the
boom. But,
hey! It's progress.

Who wants to step up and and be understood next??


>
> If I know their projected area, can I assume that since it is a round
> member antenna I can use the round member spec?

Correct.

>
> The antenna is 30' of Rohn 25.  My zone is rated 70 mph.  The shortest
> tower I see in their manual is 40' so I will use those numbers.

Conservative, but close enough to hand off to the P.E.

>
> Concerning the coax, let's forget about it and say that our configuration
> will be close to the Rohn configuration.  Besides the antenna, what other
> top of tower items will a PE want to know about, e.g. mast and rotator?

Yep, all of it, and how far above the tower top you are going to mount
the
antenna(s), it all determines the tower loads.


--
73, Kurt, K7NV

YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/

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