Hi Gene,
Some of what I say you probably won't like. There are reasons, though, why
there are no simple answers to your questions as you will see.
>I'm new to tower construction, in fact this will be
>my first tower. I'm considering 30 or 40 feet of Rohn
>25G bracketed to my house at about 15'. I'm trying
>to figure out what size antenna I could have on
>such a tower.
This is a very common problem and I believe you have come to the very best
place with your questions. It still does mean you will get the answers you
want . . .
>The Rohn manual gives antenna wind loading information
>on 25G self-supporting towers with and without ice.
>But for the bracketed information they don't have
>any ice numbers. Is there a standard formula to
>apply to non-ice numbers to get the ice-numbers?
>I'm in the Cleveland, OH area and we get freezing rain.
You have noticed already that Rohn does not give you all the info you want.
They don't give you much guidance for reading between the lines, either.
The reason is simply that they are liable for every bit of information they
publish. Less info published equals less risk for Rohn. I have not heard
of any standard formula to get from non-ice numbers to ice numbers.
>They also show a bracket elevation column with
>two values: UPPER (FT) and LOWER (FT). Does this
>mean that a single bracket must be placed between
>these values or that two separate brackets must
>be used and placed at these values. In the case of
>a 40' tower the upper value is 30' and the lower is
>15'.
Again, Rohn's info does not help much. Most bracketed towers use a single
bracket fairly low on the house, usually at the roof line of a single story
house. If the tower is any higher than about 30 feet, a set of guys would
be a good idea. For the installation you are planning, you can go about 10
feet above the top bracket without guys.
>I'm sure this will be trivial for the experienced
>tower gurus so lay the info on me.
Well, such towers have been installed hundreds of times but much of the
design is done by "gut feel" or years of observing other people's failures
and avoiding their mistakes. What you need is the book that is about to be
published by K7LXC.
The very best advice I can offer you is to consult a structural engineer to
look over your plan. This will cost you some money but it may also save
your neck or at least your wallet.
If you want to look at an article on how to do a first class job on a house
bracketed tower, look at August 1996 QST, page 35. There you will see an
excellent approach to making sure the tower will stay where it is supposed
to, thanks to K1KP and N1CQ.
I suspect you will get several comments from others that will help you.
Stan w7ni@teleport.com
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