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[TowerTalk] Antenna & Tower Wind Load Ratings

To: jimlux@earthlink.net
Subject: [TowerTalk] Antenna & Tower Wind Load Ratings
From: Kurt Andress <andresskurt@gmail.com>
Reply-to: kurt@k7nv.com
Date: Wed, 14 Jun 2017 22:33:33 -0700
List-post: <mailto:towertalk@contesting.com>
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)

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