I suppose guys don't generally give the win ratings much thought - until you
have a "wind event" which brings a bunch of your stuff to the ground.
That was certainly the case here - we had a tornado pass off to the north
and it whipped my stuff into quite the mess. Until that day I had never
thought the local wind would be a problem. Lesson learned; now I also check
the mechanical design out before putting something up. After the hurricanes
of this year, a lot more guys are likely going to be considering the wind
side of the spec a lot more than in the past.
I almost put up a Telrex beam thinking it was heavy duty but after finding a
few comments on the beam's tendency to toss elements, I put the design in YM
and found a 56 MPH "simple spec" rating which would be a joke here - a
routine summer storm wind for my area is gusts of 65 MPH. That all assumes
6061 - which may or may not be the case for that ancient beastie. "Heavy
duty" has proven to just be "heavy" at least from what I've figured out in
this one case.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: maflukey@gmail.com
Sent: Tuesday, October 31, 2017 2:53 PM
To: towertalk@contesting.com
Subject: [TowerTalk] FW: Wind survival + load ratings... vs reality.
Hi Jim,
Yes I have run finite element stress analysis models of some different
antenna designs - mainly to evaluate modifications or to confirm wind loads.
As you note, some can really leave you scratching your head just how certain
ratings were arrived at....
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Thomson
Sent: Tuesday, October 31, 2017 1:53 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Wind survival + load ratings... vs reality.
Has anybody tried stuffing yagi manufactures ele dimensions into software
like Yagi stress..... and or Yagi max ????
I have been doing just that on a bunch of them...and in several cases, Im
not impressed with the results.
And Im using the correct dimensions for exposed length tubing, and correct
OD and wall thickness, and correct
yield strength. Im using both the ... no spec..aka wind tunnel
spec.......and also the old C spec.
Some of these yagis that are rated at ... 100 mph are actually only good
for a paltry 64 mph...and that’s with NO ice,
such is the case with the M2 80m yagis. Their 3 el 80m yagi uses C
specs for wind area. They rate it at 32 sq ft.
Its actually 48 square foot of projected area. Their combo truss + LL
does nothing for ice loading, and nothing for
horizontal deflection. The LL reduces some ele sag, thats it. Both YS
+ YM spit out 64 mph using no spec..and
both spit out 69 mph, using C spec...and that’s with NO ice.
Toss just .25 inch of ice into the mix, and it becomes 48 mph using no
spec....and 52 mph using C spec.
The optibeam 80m yagi doesnt fare much better. Good for 72 mph, using C
spec....and less using no spec....and that’s with NO ice.
I also tried the JK antennas 3 el 80m yagi in YM + YS. Using no spec,
it comes in at 103 mph. Using C spec, its good for 107 mph.
Now that’s a helluva big difference between m2s 64 mph...and the JK’s
103 mph. Considering the M2 is not cheap at $9935.95
I have also stuffed several other yagis, like 40m, and 20, and multibanders
etc through the software. Eye opener, but not as bad as
the 80m yagis above. I tried Mosley, Hy-gain, old telrexs, KLM, and
anything else I could get exact dimensions for.
Back in the day, ant makers could get away quoting BS gain and FB
numbers...... until software came along. They are still doing it,
but with BS wind load ratings, and max wind survival ratings. The
mechanical software is readily available, so why isnt anybody holding them
accountable ?
In a lot of cases, hams are being sold a... bill of goods.
Jim VE7RF
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