Mac,
I understand your line of work is a patent attorney
but it easily could have been one as a mechanical engineer.
Your description of antenna mechanical design is outstanding.
As far as designing to a yield point, Rohn did that with the
fold over tower and from experience after selling a few did
some retroactive strengthening ( I calculated it out myself
when my one failed )
On antennas I have for years resorted to the use of fishing
poles covered with aluminum foil. With respect to cost and
survivability it easily beats the traditional aluminum designs
which are rapidly getting beyond the amateurs reach.
Do the math, fishing poles cost $1 a foot
for a $800 commercial antenna you get how many feet of aluminum that
you have to lift to the tower top !
Regards
Art






"J. McLaughlin" wrote in message ...
Dear Jim:
Note the careful wording that I used.
One person's 100 mph wind is not what an engineer considers 100 mph
wind. If one considers the average density of air at sea level (kg/m^3)
and assumes that it is impacting a flat plate while traveling at a
steady speed (m/s) one will estimate a resultant pressure (newtons/m^2),
which is proportional to the square of the speed. If one then assumes a
discounting factor for a round element with respect to the just
mentioned flat plate and multiplies the distributed element area times
the factor times the pressure one will produce a distribution of force
along the element. Making assumptions about the mechanical properties
of the element, one can calculate an estimate of when the element will
be loaded to the yield point somewhere along the element.
The steady-wind-speed-to-yield (suggested by the above scheme) is
significantly higher than that suggested by good engineering practice.
Good engineering practice applies safety factors to the steps just
described. One of two major safety factors is the use of a higher
safety factor than one in the pressure calculation. This safety factor
takes into account the fact that real world wind is not steady ( it also
takes into account the increased likelihood of faster wind on taller
antennas). The second major safety factor has to do with the strength
of the element material. It is bad engineering to take material to
theoretical yield. This is especially inappropriate with antenna
elements that are able to flex in wind gusts.

It might be true that an antenna element that has been very
carefully assembled from selected materials could be placed in a wind
tunnel, have the wind speed slowly increased to a laminar 100 mph, and
have the element just have a permanent bend.

Note that I have said that the LPDAs are competently designed - both
electromagnetically and mechanically. They are good value. However,
the mechanical ratings, as is common with most antennas sold to radio
amateurs, are optimistic.
The standard of care for the mechanical design of non-amateur
antennas includes the use of safety factors.
It is reasonable to expect that most radio amateurs will effect this
class of antenna in an urban area to a height of no more than about 70
feet. Under such conditions, the probability of damaging winds is
small. Ice, in the North country, is the most likely agent that will
kill an antenna. However, antennas of this class that are placed in
clear, rural sites at serious heights will be much more likely to fail
from wind than their counterparts in town.

What I would like to see is a standard to rate amateur antennas in
terms of pressure. 73 Mac N8TT
--
J. Mc Laughlin - Michigan USA

"Jim" wrote in message
om...
The Tennadyne LPDA was developed in Colorado to withstand 100 MPH
winds, snow and ice. They offer T-6, T-8, T-10 and T-12 element models
from 12' to 30 booms.
They have been in TX about 5 years now. Prices include UPS Ground
service in lower 48 states. (The KMA is basically a T-8.)

GL,
Jim, K4SQR