[TowerTalk] FW: Wind survival + load ratings... vs reality.

[maflukey at gmail.com]

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 at contesting.com] On Behalf Of Jim Thomson
Sent: Tuesday, October 31, 2017 1:53 PM
To: towertalk at 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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