On 10/31/17 11:58 PM, Kurt Andress wrote:
Message: 5
Date: Tue, 31 Oct 2017 11:53:01 -0700
From: "Jim Thomson"<jim.thom@telus.net>
To:<towertalk@contesting.com>
Subject: [TowerTalk] Wind survival + load ratings... vs reality.
Message-ID: <1A1BAFDA3D7F4F67B3F5A6C6AAE8716D@JimPC>
Content-Type: text/plain; charset="utf-8"
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 $9935.95
I have also stuffed several other yagis, like 40m, and 20, and
multibanders etc through the software. Eye opener, but not as bad as
the 80m yagis above. I tried Mosley, Hy-gain, old telrexs, KLM, and
anything else I could get exact dimensions for.
Back in the day, ant makers could get away quoting BS gain and FB
numbers...... until software came along. They are still doing it,
but with BS wind load ratings, and max wind survival ratings. The
mechanical software is readily available, so why isnt anybody holding
them accountable ?
In a lot of cases, hams are being sold a... bill of goods.
Jim VE7RF
------------------------------
Message: 6
Date: Tue, 31 Oct 2017 14:53:39 -0500
From:<maflukey@gmail.com>
To:<towertalk@contesting.com>
Subject: [TowerTalk] FW: Wind survival + load ratings... vs
reality.
Message-ID:<00c101d35281$f34fd2f0$d9ef78d0$@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Hi Jim,you wrote...
-----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
W3JK, who uses my software, put me onto this post...
Now you guys are catching up with me, from the work I did in the 1980's
to spend about 8 years creating YagiStress, and getting it verified by
one of my P.E. colleagues with $18k software, Yagistress is within ~ 1%
(or rounding errors) with the pro finite element linear analyzing
engines. Paul Sergi, NO8D (Summit Racing & DX Engineering) and his
people bought my software and decoded it and made their own version,
that was bench marked against the work I had done...I cannot speak for
the voracity of what they have done!
What you're seeing Jim is what I have seen for around 30 years, and I
have made comments on this reflector many times about that, but they
were greatly ignored! I got run off this platform by too many other
jungle knowledge experts that want to rule the roost with their ever
present emperical expertise! So, that's why I no longer devote much of
my time to this venue....it is frought with way more "Jungle Knowledge"
than engineering expertise!
Have fun out there imagining how you wish it would be, but not how it is!
73, Kurt Andress, K7NV, author of the YagiStress software...and tower
service provider
P.S. You should simply throw away the EIA/TIA 222-C spec, it is now
about 28 years old and does no longer apply!
I would say that perhaps we're at about the same place where we were
when amateur runnable NEC models started to be practical - and folks
found that the gain in the model was *substantially* different from the
gain in the ad - leading to ARRL banning claims of gain in QST without a
published model or test data.
Overall, hams are probably more comfortable running and believing an
electrical model than a mechanical one (more time to become familiar,
etc.? or just because the ham tests ask you about electrical stuff, but
don't ask you about mechanical stuff)
There's another pervasive factor - probability
For electrical performance, the ionosphere is the wild card in most ham
experience - Put up your new model ABC antenna during a sunspot peak and
it works gangbusters - the vast majority of ham experience is empirical
with an N of 1: "I put up this antenna and did that with it"
For mechanical performance, the wild card is the weather - how many hams
have towers that are overloaded according to the code, but survive,
essentially because they've been lucky. So performance becomes anecdote
- I put up a ABC antenna, and they reported 70 mi/hr gusts in my city,
so my antenna survives 70 (whatever the wind speed actually was at your
antenna site)
Another factor in mechanical designs is "hidden safety margin" -
typically in a design, you don't claim the actual expected yield
strength, you design for a bit lower (or design for a bit higher loads)
- that accounts for material properties variation, variation in
structure strength from the assembly process, etc.
So you might have a design which "officially" calculates out at 70
mi/hr, but which actually survive 100, sometimes, on a lucky day, with
the wind from the right direction. That 100 is what gets claimed as the
survivability.
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