George Fremin III wrote:
>
> Hi,
>
> I made a post the other day and based on several responses
> it appears I was not clear - so let me try again.
>
> Here is the orginal post with comments..
>
> > I was looking through my rohn book last night and I noticed
> > a section on TVRO dish mounting.
> >
> > There are drawings showing the use of 25g/45g/55g towers
> > for supporting TVRO type dishes.
> >
> > I find these pages very interesting.
> >
> > It has this heading:
> > #25 guyed tvro towers
> > A.N.S.I. 70 mph basic wind, Exposure B, Designed to support maximum
> > load of 6' diameter dish 48.9 sq.ft maximum effective projected
> > area.
>
> Note the 48.9 sq.ft. projected area.
>
> >
> > There are drawings that look much like the guyed tower drawings
> > in the front of the book in the 25g section. But these wind
> > load figures seem to exceed the ones in the front by a huge
> > amount.
>
> What I was really getting at is:
>
> Why do these towers - that do not appear to be much diffrent
> in desing than the towers in the "guyed tower section" of the
> Rohn book have wind load figures that appear to be MUCH
> larger?
>
> >
> > I dont know what "Exposure B" means - maybe someone on here
> > can tell me.
>
> I got several answers to what "Exposure B" means - but no
> comments on the wind loads.
>
> What I really want know about are the windload figures.
>
> Thanks
>
Hi George,
Good question, and it certainly appears that something is different!
I'm sure there is a good answer, although I don't have all of it right
now.
Here are some thoughts.
The two different tower configuration drawings use different design
specifications.
Drawing C870484 R4, 25G 40' - 190', 70 mph uses the ANSI/EIA-222-E spec.
Drawing C850496 R3, 25G TVRO, 70 mph, Exposure "B", uses the A.N.S.I.
A58.1 spec.
I don't have the A58.1 spec, so I don't know what it is doing.
The first obvious difference is that the TVRO drawing states that the
"Max Effective Projected Area = 48.9 SqFt". The wording can be
misleading, but what they are talking about is the "Effective Area" of
the dish. This value cannot be directly compared to the "Projected"
areas in the Standard Tower configuration drawing.
This is just like taking an antenna manufacturers area figure and
applying it to a tower, without knowing what it means.
If we calculate the projected area of the 6' Dia dish we get 28.27 SqFt.
If we wanted to try to compare projected area figures, we would use
28.27 in the TVRO scenario against the standard tower values. But,
that's still not a safe bet.
Different spec's treat components of the system differently, they use
different drag coefficients, local terrain factors, and different wind
pressures. Dishes are treated separately from round and flat membered
antennas in 222-E.
The tower configurations are different. Different # of guys, guy sizes,
guy placements, anchor locations, and antenna locations.
The TVRO probably has only one feed line, the Standard Tower config has
one per tower face.
If we want to make comparisons, we need to work our way thru the spec's
to arrive at the loads applied to the tower and examine the differences
in the configurations under the loads. What appears to be similar, at
first blush, may not be.
It is very likely that the commercial users of Rohn towers for TVRO, use
the A58.1 spec and the designs satisfy it. Rohn wouldn't have presented
them if they didn't.
There is probably someone around that knows more about the design spec
used for the TVRO drawing. Till then it remains an interesting
curiosity, that will likely make sense with all the information.
--
73, Kurt, K7NV
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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