I believe Phil has it backwards. Rohn specifies two alternative areas -- a
"flat member" area (in the rectangular box) and a "round member" area (in
the curved box). As you might expect, the "round member" numbers (for
things like cylindrical Yagi elements) are higher than the "flat member"
numbers (for things like satellite dishes and microwave reflectors).
However, the maximum allowable antenna square footage may or may not be
what's found in those boxes. If you look at the fine print associated with
the Rohn 45 drawings (Note 4 in my book's drawings), you will see that the
published allowable antenna square footage is _after_ allowance for three
side arms totalling 8 square foot and symmetrically placed, one on each face
of the tower. It is also after an allowance for two round cables per face.
Drawing C821662 (rev 4 in my book) shows the side arm detail, and notes that
the sidearm itself is designed for an allowable antenna loading of 90 pounds
lateral thrust. This would correspond, in 30 psf ratings for instance, to 3
square foot of antenna on EACH side arm assembly. (Whether a particular
tower installation could handle this is a separate design issue, however.)
Thus, in the absence of those side-arms, and _with appropriate
re-engineering by a qualified PE_, Rohn 45 can undoubtedly handle more Yagi
antenna square footage at the top than the little rounded boxes would
I strongly recommend obtaining a copy of the Rohn catalog (hard copy or
download the online file) and reading all the fine print as well as the
supporting drawings referenced by the Rohn 45 drawings.
Also, in response to one of W2WG's other questions, there are _three_
separate Rohn 45 drawings, corresponding to three different wind load
ratings. Each individual user will have to determine the appropriate rating
to use -- often from their county zoning or from a PE in their area. So
whatever Rohn drawing W2WG is looking at already has a wind load assumption
built into it.
> -----Original Message-----
> From: Phil Camera
> Round member antennas are sat dishes. Yagis, etc are Flat
> member antennas. You should see those rating also. Phil KB9CRY
> -------------- Original message --------------
> > Looking at the guying sheets for Rohn 45, 70 mph wind, and
> 70' tower,
> > I think I see the load capability is about 15 sq ft for
> "round member"
> > antennas. That seems like a pretty low capability, though all my
> > previous experience has been with self supporting towers.
> Am I reading
> > the sheet correctly? What approximate wind load can such an
> > installation support if I am reading the Rohn drawing in error?
> > Bob W2WG
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list