On 6/13/17 10:19 AM, JVarney wrote:
You are correct Kurt. I've had great fun reverse engineering old towers
with little historical documentation and stamping them under 222-G.
The same approach can be used on antennas as well.
In an ideal world with unlimited budgets, ham installations would be
designed the same way as cell tower installs -- where the antennas
are considered structural elements and the tower/antenna system is
designed as one united structure.
The TIA-222-G standard uses the phrase "structural antennas." So yes,
ideally a yagi manufacturer would supply a matrix listing its
performance under various heights, wind zones, terrain, environment,
seismic loads, ice, etc., along with the Effective Projected Area
under each of those conditions. There will and should be some
combinations where the manufacturer puts "not recommended."
I'm not aware of any that do that now.
I hate to say it but sooner or later a ham is going to run into this
when they apply for a permit. A sharp-eyed city or county reviewer
who does a strict reading of 222-G can insist on a stamped plan for
the _antenna_. I'll have sympathy for the ham when that happens!
For windloads, I don't think it's all that hard. What we need to do is
come up with a sort of cookbook for Yagis. They're structurally simple.
Maybe an updated version of YagiStress or something might a place to
start.
The most tedious part, in my experience, has been getting all the
dimensions into some form suitable for computation.
After that it's applying the formulae from TIA-222G/H/whatever.
I'll see if I can get a copy of the standard at work to look at. From
the ppt slides I linked to earlier, it looks pretty straightforward, but
there were some definitions and things that weren't given there. And it
was cut and pasted and the resolution wasn't so hot.
For seismic loads.. to a first order, the antenna is going to be a small
mass compared to the tower. Maybe it's reasonable to model it as a
point mass. Yeah, the elements will flex, and unfortunately, their
first mode frequency is probably right in the seismic energy band, so
there is some potential amplification. Again, maybe there's a bounding
case that could be used.
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