[TowerTalk] Optibeam OBLY14-5

[jimlux]

On 11/15/18 5:53 AM, john at 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.