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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