[TowerTalk] Antenna & Tower Wind Load Ratings

[Kurt Andress]

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 at earthlink.net>
To:towertalk at contesting.com
Subject: Re: [TowerTalk] (no subject)
Message-ID:<587f8125-a530-995c-fad2-e680a9515641 at 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)