This is long and only suited for those interested in the discussion of
making sense out of the exercise of trying to match antenna and tower
specifications.
Others wrote about bracketed towers, etc:
Michael Tope wrote something that sheds some light on the problem:
>
> I have a spreadsheet here for the 34' 25G tower that I had
> in Florida. My tower held a CC A3 (4.7 sqft) and was bracketed
> at around 8'. If I use this model and change the windload to 2.5
> sqft (per the proposed installation specs), the overturning moment
> reaches Rohn's maximum (6700 ft*lb) with at a windspeed of around
> 100mph. The model assumes that the wind load is given in a flat plate
> equivalent projected area (Cd = 1) and assumes the wind speed/pressure
> relationship given in Leeson (P=.004*v^2).
>
> Seems like a pretty conservative installation.
>
> Mike, W4EF...........
Hi Mike and any interested parties,
What you have done is ok, since my new Rohn book says that the bracketed
towers are still spec'd to EIA RS 222-C, which agrees with the Leeson
information. The freestanding towers are spec'd to 222-E, with a note
about calculating flat member antenna areas via 222-C, which appears
odd, but remains relevant as the difference between flat members and
round members is fairly close in the new spec, but the rest of the math
for determining dynamic wind pressure is different.
The current EIA spec is 222-F. The E & F revisions contained a
substantial change in methods for determining dynamic wind pressure.
Guess they haven't gotten around to updating the bracketed tower spec's
yet.
The bracketed tower page in my relatively new book from Champion Radio
says the spec is circa 1988, revision E was in 1991, and revision F was
1996. At low heights the dynamic wind pressure differences are not
great, probably why Rohn didn't upgrade the spec's.
BE CAREFUL!!!!
I'm pretty sure there is some hair in the butter regarding the antenna
areas we are using for these exercises.
Can anyone tell me what the 4.7 SqFt of area for the A3 really means?
I'm sure most of us don't know! I have to make educated guesses, based
on historical, behind the scenes, input. Every once in a while I get an
antenna model from a YS user that helps me figure it out.
You correctly stated that the Rohn spec and your spreadsheet assumed
that the antenna area was a projected area.
I'm very sure that most antenna manufacturers values are effective
areas, according to one of a variety of possible methods.
For many years, the standard in the industry was EIA RS 222-C or RS 409
(for amateur antennas). In these spec's one would take the projected
area and multiply it by .67 to get the effective area. If the A3 antenna
area you cited is really a 222-C effective area (we don't know cuz they
don't say, but I'm pretty sure it is), we need to multiply the published
area by 1.5 to get the projected area needed in the Rohn spec. That can
have a rather substantial effect on the tower rating.
To date I have only seen projected antenna areas from one vendor,
Hygain. All others are undefined and due to the history, suspect. Not
that they're not out there trying to do the best job they can. We just
need to know where the values came from so we can use them!
My recommendation, for anyone who is trying to figure out how to mate
antennas with towers, is to do one simple thing. Call the antenna vendor
and get a definition of what the antenna area means. When the discussion
gets too confusing (got to get past the spec reading person, who answers
the phone) just ask for the projected area (it can be determined
regardless of the spec used), that's what you need to look at your
tower.
There is a whole other area for definition in this regard. Those who
attended my Dayton presentation, learned that the method of antenna area
calculation has been flawed for many years (credit for this goes to
K5IU) and there is a new way to do it.
So, here we are not knowing how the antenna area was calculated, nor
whether it represents a projected or effective area.
We have little or no chance of making sense out of a single undefined
number, that we are all too happy to go off and assume we think we know
what it means!
Manufacturers are loath to make the change to the projected area we need
because it is a bigger number and will make them look bad if the other
guys don't also make the change. I told all of them, that use my
software, to publish both numbers and be clear in defining them so users
could sort out the differences.
Everyone is too busy, and nobody wants to take the lead, except Hygain,
which just changed hands. Hope someone at MFJ will figure this out and
stay with it.
There are several popular manufacturers out there, that I have
absolutely no idea, what they are doing. I only get an occasional idea
from a YS users models that tells me we are not on the same page. Maybe
they are using that new math!
I think there are a bunch of people out there taking antenna areas that
are too low (by approx 1/3, who knows) and applying them to their
towers, thinking all is well when it is not!
This was discussed in this forum many months ago. Some things remain
constant. Change is slow and painful!
When there are as many buyers asking for projected areas, determined in
the proper fashion, as there are those asking for the new 9 band bizzos,
something will change.
We can have a lot of fun talking about this, but nothing good will come
of it till the right people in the right places become a part of the
discussion.
Just wanted everyone to know that "an antenna area is NOT just an
antenna area!"
There is only one way it gets dealt with here, either I build my own
model and analyze it so I can understand it, or I get a definition that
I can understand from the designer. The rest is just folklore, and
appropriately ignored!!
See you in 6 months when some post will provide the opportunity for a
reminder!
--
73, Kurt, K7NV
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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