On 11/15/18 5:53 AM, john@kk9a.com wrote:
My OptiBeam experience mimics AB7E. I had a 40m beam in Aruba where the
wind almost never stops plus I have a couple 40m beams at my NC residence.
I had no mechanical failures and the 40m elements are not saggy. I found
DF2BO to be responsive to any technical questions and a pleasure to deal
with. I choose their 40m beam because at the time it was the only one with
high-Q loading coils plus I believe that it is the only 40m beam with
detuning stubs to minimize interaction with other HF beams. I am not sure
what aluminum that OptiBeam uses, I seem to recall Tom just calling it
aircraft aluminum. I am not sure what is even available in Europe. I have
heard of mechanical issues with their 80m beams so perhaps that is was
K7NV was referring too. I think it would inaccurate to say that all
OptiBeam antennas have reliability /durability problems.
A not-insignificant problem that I face all the time at work is that
what you get today may be the same part number and look the same as what
you got 10-20 years ago, but is actually different, even though both
"meet spec"
A mfr might be getting metals from different sources than previously.
Sure, it's all labeled as 6061-T6 or whatever, but the reality is that
maybe in the past, the metal you got was significantly better than the
minimum the specification requires, and that increased capability got
"baked into" the design.
I would find it unusual for an antenna manufacturer to actually *test*
the metal properties, especially on a lot to lot basis. Usually, what
happens is that you do a design, assuming "data book" values, applying
some design margin to account for uncertainty in loads and
idiosyncracies of the design. You build the article, test it, and it
works. Then you just duplicate it.
What typically does not get done is a re-evaluation of the design
calculations, or a test to destruction of the finished article. So you
don't know (for sure) that "survives in use" was due to your
outstanding design abilities, your use of giant design margins, or the
material you built the first one out of being stronger than the data
book value.
It could also be something as minor as things like how fastener holes
are drilled and what tolerances. If the hole is slightly larger or
smaller than before (same drill bit, just a different mfr process or
maybe the material's surface hardness is different), then the clamping
from the fastener changes and the overall structure might fail in a
different and unexpected way: maybe it can slide a bit because the hole
is bigger, or the hole is undersize and the threads cut into the sheet
metal and start a crack.
We go around and around on this in the space business - it's expensive
to do low level material tests, and tests at each assembly level.
That's why flagship missions cost billions of dollars. And these days,
it may not be practical to do testing at lower levels. So you kind of
want to build it all up, and test the completed device and validate
design and components/materials at the same time. There's a certain
amount of risk - you might fail the completed device test, and then have
to go back and redesign, and most likely, do more low level testing.
With modern manufacturing, too, material property testing may not
provide any useful information: If you're doing additive manufacturing,
the physical properties of the metal powder being sintered has a pretty
tenuous connection to the properties of the finished device. Composites
have this problem too: Sure, you know the properties of the carbon or
glass, but the actual manufacturing process has a much larger effect on
the properties of the final product.
That's one advantage of machining stuff out of a solid block of
aluminum. The properties of the finished product are very predictable.
Unfortunately, milling a 80m Yagi out of a suitable monolithic billet
and then heat treating it is probably not practical for most
hams.<grin>. If someone wants to try, I'd love to come watch.
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