On Wed, 18 Aug 1999 17:22:07 -0400 Nat Davis <ndavis@mail.vt.edu> writes:
>
>A "newbie question:"
>
>My friend is in the process of upgrading his antenna system with a
>new
>"big" beam and rotor. He has suggested I take 3 sections of Rohn 25G
>off
>his hands and thus, put up a "real" antenna at my QTH (I'm "dipole
>bound"
>at the moment). The idea has some appeal to me... The problem is that
>I
>have no experience in towers...
>
>My friend suggested I bracket the tower against my house. The bracket
>would
>be between 11 and 15 feet up (based on where I locate the tower).
>That
>would leave about 15-20' of tower above the bracket.
>
>My question, at this point, is pretty simple. Where can I go to learn
>more
>about tower installation issues? Is there a "how-to" book somewhere
>that
>talks about the stuff in layman's terms? I borrowed my friend's Rohn
>manual
>but am really looking more for "text" than "tables!"
The Rohn manual is about the best there is. There are
so many variations in tower configurations that it always
seems to come down to a table of some sort.
This reflector is a good way to get the tables interpreted
into text for a particular installation.
>I need to answer
>practical questions like:
>-- 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."
Take a look at the "table" on Rohn Dwg A871302. The supporting
structure must support a minimum horizontal force of 815 pounds.
Try to envision 8 of your buddies pulling 100# each on a rope
attached to the bracket. The wall frame studs are usually good enough
with stout lag screws. If you use the eve overhang, it should be
reinforced with some L-brackets screwed into the nearest rafters, or
whatever those wood things are called that stick out sideways.
(My area in the mountains of SW VA is a 70 MPH county.)
>-- 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??
I would use unsupported height above the top bracket as the
limiting factor.
Take a look at the same Rohn table. For the 50' case, the top
bracket is at 36', so the tower sticks up 14' above the top
bracket. Rohn recommends 14.6 square feet of antennas maximum.
I'm sure you would be ok with the small tribander you are thinking
of in this configuration. In the 90' example in the Rohn table,
the tower sticks up 24' above the top bracket, and only 6.8 sq.
feet of antenna is allowed.
>-- 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)?
>Going
>higher than that would not endear me to my neighbors!
The gain of the beam, even at 30', and the ability to point it
in the desired direction, will make the difference of night and
day on both your transmitted and received signal.
I think your project would definitely be very worthwhile, and give
you a lot of Db's per dollar.
Dave Hachadorian, K6LL
San Diego, CA
K6LL@juno.com
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