I am just trying to catch up with you folks on some of this
traffic......we are out of control here making things and planning
projects and trips........
Here is how I have seen it for the past few decades, and still how I see
it today.....I am trying to really simplify this so it can be followed
by most readers that really need to understand this......
There are basically two types of "Antenna Area" values to be considered.....
1) The most basic and easiest one to determine is its "Projected Area"
which is simply the sum of the lengths and widths of its combined
members. This produces a profile of all members that are exposed to the
wind, when the wind is normal (perpendicular) to the antenna
members center axes. This number is always the same for every antenna,
and every Yagi antenna has two, and only two Areas to be considered,
one is when the wind is normal to the sum of its exposed elements, and
the other is when the wind is normal to the boom members. Whichever one
is the greatest is the maximum one each antenna has. This was clarified
by Dick Weber, K5IU, P.E., in Communications
Quarterly, 1993. It informed us that what everyone was doing to
determine antenna loads on towers was being done wrong,
according to existing conventional knowledge about how airflow
over things in the air stream actually behave! We learned that
everything we were doing was wrong, and that the wind flow
over any and every kind of member in the wind stream produces
resultant load vectors that are always only normal (perpendicular) to
the center line axes of those members in the air stream at all the
points on them, as they become deflected (bent) by the wind loads. This
becomes a very difficult non-linear problem to solve with any kind of
mathematics with any kind of the most expensive software available
today! I have discussed this with some really qualified colleagues that
run the really expensive software.......they just say "we need another
level of software we don't have" to be able to do that......think about
this......as each inch of an antenna element is deflected down wind, the
actual forces on it are reduced because it becomes inclined to the
wind......therefore the forces normal to it (the ones that are trying to
bend it) are reduced.
So, since we can't really do that, we go back to the EIA/TIA spec and
find what they tell us to do with the linear engineering mathematics
they know we can do!
2) There are established "Drag Coefficients" for the shapes of things in
the air stream. They are defined in the EIA standards for us to use. As
Jim Lux points out, they are dependent upon the Reynolds numbers for the
sizes & shapes of things at various wind speeds! I went and studied the
EIA 222-G criteria for this, and came to the conclusion that everything
we might be doing with Amateur Antennas will fall into sub-critical
regimes and that we will be ok using the standard Drag Coefficients of
1.2 for cylinders, and 2.0 for flat plates or rectangular things. These
become the "Effective" or Effective Projected Areas" as the nomenclature
has evolved, its all the same thing "Projected Area X Drag Coefficient!"
As You & Jim Varney presented, there are so many other things to be
considered, but this is Towertalk....where most of the readers have not
spent several decades trying to figure out what is really going on.......
So, the intent of my original post was not to dig this deep into this
stuff, but to simply remind everyone that tower designers and antenna
designers do not share the same skill sets and are not at all on the
same page, so their ratings can not be correlated by anyone that is not
a real engineer that spends the time to unravel it, and come up with his
own understanding of how it really is.....
I don't want to bash my friends/clients at some of the antenna
companies, because I understand exactly how hard it is for them to even
remain in business to deal with this market! It is a thankless endeavour!
I happen to think that it is rather unfortunate, for an established
industry, like this, to remain so blatantly disfunctional after so many
decades.......I will continue to do what I do for my clients to get
their things as right as possible.......which consumes almost all of my
available bandwidth, which explains why I rarely show up here on TT....
YMMV, as they say, and so on......
73, Kurt, K7NV
Message: 5
Date: Wed, 14 Jun 2017 07:01:04 -0700
From: jimlux<jimlux@earthlink.net>
To:towertalk@contesting.com
Subject: Re: [TowerTalk] (no subject)
Message-ID:<587f8125-a530-995c-fad2-e680a9515641@earthlink.net>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 6/13/17 11:23 PM, Kurt Andress wrote:
I am going to make an attempt to present some information and history
that might make this conundrum more understandable for those that follow
this Reflector......
Don't shoot the messenger, I'm rooting for everyone that has a stake in
these matters! If I didn't, I would not have said a word!
So, in summary, sort of:
Tower mfrs cite a "X square feet" as opposed to a "X lbs load"
The X could be either the actual projected area (length x diameter) or
some "effective area" (a number that you could plug into F = rho*V^2*A)
Antenna manufacturers gave either actual projected area, or some
"effective area" as well.
So nobody really knows whether the tower or antenna is calculating for
the coefficient of drag.
And, of course, the Cd approximations in 222C are wrong (not only are
the actual numbers off, but they don't allow for change in Cd with
respect to Reynolds number regimes)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|