Stan Griffiths wrote:
>
> Hi Kurt,
>
> I just wanted to say thanks for emphasizing the need for standardizing antenna
> windload specifications. This is a poorly understood but extremely important
> part
> of the picture, ...
Snip....
> So maybe if we keep talking about this where others can "overhear" the
> conversation, a standard spec will finally emerge and we can actually get
> agreement between tower and antenna makers.
>
> Stan w7ni@teleport.com
Hi Stan & any interested TT readers,
Yep, it is quite a mess indeed!
This is a replay of a previous discussion. Those that were around last
time may want to move on to the next post.
This is potentially serious stuff, but hasn't found much of a following
yet.
It is a case where one needs to be particularly careful of just blindly
"doing what the manufacturer says" thing. Most of the the tower folks
and
the antenna folks are NOT on the same page!
There are definitely some folks out there at risk of having 50% more
antenna area on their towers than they think!
I recently helped one of our regular TT contributors fish into the abyss
and get the information he needed to realize that the antenna area he
had needed to be multiplied by 1.5 so he could evaluate it
with his tower.
There are others who are likely to be under their tower limits, but
think they are maxed out.
Technically, the solution is so simple, it would make one nauseous to
think the problem still exists!
It goes something like this, most specs used to design towers require
the antenna areas to be presented for processing within the spec as
projected areas.
The antenna manufacturers provide a mix of undefined effective areas
that are more or less than the true projected areas. There's an old way
and a new way to define the antenna areas, but nobody says which one
they are using (maybe they do, but I can't find it on their websites).
Some use the new method, some use the old, because they don't define the
type of area or the method, we have no clue what the numbers mean, nor
how to manipulate them to get what we need to apply it to a tower!
Honorable mention should go to Roger Cox at Hygain, who figured this out
and did the right thing before they passed the lot onto MFJ. Let's hope
they continue the good work!
If one would like to see what useful values look like, go to the Hygain
site and check it out. There is even a fairly concise description of the
subject available.
Roger presented three area values. Element and boom projected areas, the
one's we need, and another effective area value that is reasonably
described. The projected areas are used by those who want to mate
antennas and towers, the effective area is used in the marketing wars
and to relate these antennas to Hygain rotators in an empirical manner.
If someone wants to use an antenna area value (other than Hygain) to
apply to a tower, he/she must first understand a bit about the various
methods of deriving these figures, then call the designer and demand an
explanation of how the values were derived (I.E. which method). Then he
has a number that he understands, and can effectively manipulate to
apply to his tower and it's design spec!
Here are the questions to ask:
1) Is the value a projected area or an effective area?
2) Is the value just for the elements or just the boom, if so, what is
the other value (this will help answer the "which way do I point it when
the wind blows" question)? Or is it a combined elements/boom area.
3) Which spec or method was used to obtain the value?
With the answers one can figure it out.
I don't think it can be much more simply presented (maybe too simple).
Also, don't see the problem getting fixed at the antenna manufacturers,
The
tower guys aren't doing anything wrong. At least they tell us which spec
they have used to design the tower. So we know where we are headed when
we actually want to decide where to put the thing up.
The solution? Readers choice!
We can either bury our heads in the sand, or take matters into our own
hands.
One is painless and meaningless, the other can be just like work. Some
buyers get my software and do it themselves, others build their own
spreadsheets or use pencil and paper to figure it out.
See ya in 6 months when Stan reminds us it is time to remember...
--
73, Kurt
K7NV "That's K7 "Nevada" (ex - NI6W)
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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