On 8/18/99 17:22, Nat Davis at ndavis@mail.vt.edu wrote:
>My question, at this point, is pretty simple. Where can I go to learn more
>about tower installation issues?
The Rohn Catalog is the definitive resource.
>Is there a "how-to" book somewhere that
>talks about the stuff in layman's terms?
There was an article in QST a couple of years ago about a well-engineered
bracketed tower installation.
>I borrowed my friend's Rohn manual
>but am really looking more for "text" than "tables!" I need to answer
>practical questions like:
If you want absolute answers -- you'll need to hire a PE.
>-- My house is wood frame construction. Will attaching the tower bracket to
>the studs in the exterior wall frame or to the eve overhang be "strong
>enough." (My area in the mountains of SW VA is a 70 MPH county.)
Most likely not. When they built your house, I'm sure they didn't
reinforce it with the tower in mind. You'll need to add reinforcing to
distribute the tower-induced loads to the house frame. You'll also need
to do better than just lag bolting the bracket to the studs. Carriage
bolts, at the least, into very strong wood members.
>-- Is the 15'ish of tower above the bracket really "safe" without guys if I
>put a small tribander on it? How much antenna could I use (loading wise)
>since 30' height is not in the Rohn specs -- just take the 40' bracketed
>specs as being very conservative for 30 feet??
You'll note the the specs are pretty similar for all the bracketed Rohn
installations if you look at the amount of tower above the top bracket. A
small (5 sq feet) tribander will do nicely.
You'll also note that all the Rohn bracketed installations call for TWO
brackets. Although with only 15 feet between the base and your top
bracket, it may not be worth it.
You might also look at the freestanding specs as well. Rohn 25 will
almost hold a small antenna even without bracketing at 30 feet. Note that
the base requirements are different.
>-- Or even more simple: is the performance of a beam at 30' worth the
>effort and $$ (compared to my dipole in a tree at a similar height)?
YES!!!
I had an A3S at 35 feet before I moved. Sure, it isn't as good as a
monobander at 100 feet, but it is a heck of a lot better than a crummy
dipole! You might want to get enough mast to get to 35 feet. That's a 1/2
wave on 20m.
Granted, any tribander will be more effective if you get it up higher
than 30 feet.
I have a theory about antenna installations. When you first go from
single-element antennas to a tribander, you get a great leap in
performance. No matter what you do from there, you never again get the
same leap in performance. Not scientific, perhaps, but it seems to be
true.
Good luck.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
-- Archibald Asparagus, VeggieTales
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|