[TowerTalk] (no subject)

[Kurt Andress]

I very much appreciate and respect the contributions of Jim Varney & Jim 
Lux, who are both practicing engineers and their comments clearly show 
that they understand all the nuances of the EIA/TIA 222 communications 
tower and antenna design specs! And Gerald , K5GW......
Also, thanks to KK9A & K9YC, who have been friends and/or clients for 
their comments.

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!

Way back in the late '80's I was trying to figure out how to properly 
mechanically design Yagi antennas for Amateur use. Doing it long hand 
with a calculator or spreadsheets was pretty painful! So, I acquired the 
EIA/TIA 222 Rec C spec to find out how the Pro's were doing it. I 
started writing code on my Commodore 64 in their basic programming 
language (very limited). I worked on that for several years to see if I 
could create a user friendly tool for figuring out everything one might 
need to know about what Yagi Antennas really needed to be designed 
correctly and not fail, as so many had, but do them according to the 
industry accepted design standard......

By the early 90's I had a real computer and a much better programming 
language, so I transported the previous work I had done into a real Yagi 
Antenna design program...... I found several errors in methodology and 
results and corrected them, and released YagiStress in 1992, after one 
of my good P.E. friends ran some of my models on his $18k Ansys Finite 
Element Analysis software and reported that the results of my software 
were within 1% (just rounding errors) of what his software was finding 
with linear analyses (that cost me some beer) ;-) Non-linear analyses 
are much more accurate, but most of the EIA/TIA standard methods are 
fashioned to make them useable for those that can only do linear 
analyses......

Since then, I have acquired data and modeled many amateur radio yagi 
antennas, either all on my own for clients of my tower service business, 
or as a consultant to several amateur antenna suppliers. Along the way, 
I have followed the changes in the EIA/TIA design standard to try to 
stay current with their changes. I might not be current with every 
nuance and detail that Jim Lux & Jim Varney are, but that is not what is 
at the heart of the problem I have identified and commented on in my 
previous post to this reflector.

So, here is my explanation of the problem I see it.......
Back in the day (circa~ 27 yrs ago) We had EIA/TIA 222 Rev C.
For determining loads on towers & antennas, it used the Projected Area 
of everything on it, to be multiplied by what was then called a Drag 
Coefficient, which was determined by the shape of the tower/antenna 
member in the wind, a flat member (like a flat bar) would have a Drag 
Coefficient of 1.0 times its Projected Area (which is simply its length 
x width). A cylindrical member would be 2/3 of its Projected Area.
So, back then, every antenna manufacturer chose to cite the lowest 
antenna area they could calculate with the spec, to make their antennas 
appear to be small. They had no idea what the tower designers were 
thinking antenna areas were......and the tower designers were thinking 
they knew what they were doing, but nobody in the tower design business, 
and those in the amateur antenna business ever got together to 
coordinate their design protocols, they still never do!

EIA/TIA 222 Rev D & E were pretty uneventful,
Revisions F & G were huge changes in the methodology to determine the 
loads on things! The EIA committee realized the previous rev's were 
using drag coefficients that were conflicting with everything that is 
known as reliable drag coefficients for flat or round things in the 
wind, so they changed them to be correct with established science. I 
confirmed these changes with college aerodynamics textbooks....
So, now the projected area of everything needs to be multiplied by it's 
correct drag coefficient. established by longstanding aerodynamic 
principles.
The drag coefficient for a round tube is 1.2 times its projected area, 
and a flat thing is 2.0 times it's projected area!


So, at the end of the day, your tower designer has applied all these 
things to his tower design, and expects that the antenna areas will be 
defined as simple Projected Areas (the sum of lengths times widths), 
because in his software he is applying the current EIA/TIA 222 drag 
coefficients, of the members to that area to determine the loads on his 
tower.
I had to go into their many pages of analysis to find out what they were 
doing...! I asked them straight up what their antenna area rating 
figures meant on their official stamped drawings, because they are not 
defined? , they could not tell me!.....
When someone says XX.X Square Ft, what the heck does that mean?  It is a 
very important number, and most tower designers and antenna builders 
cannot tell me what it means!

Your antenna builders, on the other hand are still using the 27 year old 
222-Rev C effective area calculation for his antenna (which is 2/3 of 
its projected area, not the simple projected areas. So most of my 
observations indicate that one  needs to multiply an amateur antenna 
builders area by around 1.5 (drag coefficients of 1.0/.666) to get to 
what the current towers are designed for expecting antenna projected 
areas....
This does hold true for all antennas I have analyzed, some of them...I 
can't possibly figure out how they ever came up with their numbers? And 
it is important to state that I have not analyzed every antenna out 
there, because some of the builders are protecting their assumed 
precious IP so that no one can understand what they are building. In 
mechanical engineering, there is little that is new about these things, 
they are either capable of surviving what they need to survive, or they 
are not! I have installed and serviced most of them that did, or didn't. 
But that is a different matter....

I have made trips to, and sat down in the offices of some lead tower 
designers at some well known notable tower vendors and shown them my 
analyses and discussed the problem with them.......

The good news for everyone that is that getting this all wrong is what 
the safety margins demanded by the EIA-222 spec, for ~30 year service 
life towers, and the additional margins chosen by each tower 
manufacturer, are covering everyone's ass, while things are not being 
properly done.

I consider my job to be to show up every once in a while, and tell you 
what I think I know...what you do with that is entirely up to you. 
Challenge any of it as you may......I rarely say little, until I think 
it needs to be said....
Sleep well and enjoy knowing what you are currently doing is probably 
really wrong and eating up most of the safety margins the industry 
standard and your tower builder provide for your towers, ;-)

So maybe I'm the only one that is really bothered by it being done all 
wrong for as long as I have been following it......none of that is 
important to anyone but me.....see you next decade to remind you this is 
still a recurrent and unresolved problem.....