[TowerTalk] Wind Loading redux

[Kurt Andress]

EUGENE SMAR wrote:
> 
> Kurt:
> 
>      With my planned antenna arrays, I won't have appreciable mast torque
> caused by  off-center mounting of the boom on the mast.  I'll have a small
> V/UHF vertical at the top, a D40 rotatable dipole (same length on either
> side of the boom bracket) below that, and the Bencher Skyhawk (purportedly
> torque balanced by the element and boom compensator pieces.)  If and when I
> install the 2M Yagi (KLM 144-148-13LBA), it will be mounted at the center of
> its boom; there's not enough element aluminum to worry about finding the
> balance center of the boom.)
> 

Sounds like you have it covered Gene! 
Except, that the mention of element compensators smells like some old world
thought, which would be incorrect in light of the information you have
shared. So, my nature would make me want to know enough to figure out how
the element compensators on that antenna work. I have no direct knowledge
of the Bencher antenna, other than someone suggested that it was designed
with my software, by someone I don't know of, so I'm just another guy
without enough information to understand it. Maybe you can help?

>      I think what you're cautioning us about is the case where we have large
> HF Yagi arrays in which the balance point (and mounting point) of the array
> is not the center of the boom.  In these cases, wind blowing broadside to
> the boom will want to turn the longer side of the boom away from the wind,
> creating torque on the mast/rotator/tower.
> 

Yes, that's right, and it doesn't have to be a "large HF array", just a
really imbalanced one to be a problem. 
The notorious "T2X sticky brake" thing probably has more to do with this
than anything else. I have a perfectly (as good as it gets) balanced (mass
& wind load) antenna on a Ham III that never...EVER...has the braking wedge
fetch up, no matter how hard it is blowing. OTOH, I also have an antenna
that is not perfectly balanced, and it fetches up the T2X wedge about 50%
of the time in just about any wind over 15-20 mph. Call me crazy, but I
think it shouldn't happen.

We all get to decide how important any of this is to us and proceed
accordingly. My point really was that people are buying and having problems
with antennas, that are not properly balanced. 
You introduced this incarnation of the subject, I commented, because I do
not think it is all fixed, by virtue of the fact that you now have some
good reference information. I'm the first one to wish to never comment on
the subject again, three years on this frequency is probably long enough!

>      Dick's papers on aerodynamic balancing discuss how to compensate for
> such imbalances, e.g., fixing a weight onto the boom so the balance point is
> shifted to the center of the boom, or entending the shorter side of the boom
> with a piece of aluminum tubing sleeved onto the main boom, or installing
> compensators.
> 

Yes, this covers it properly, I'm glad you now have it...everyone else go
get one! 

I'll add, there is another version of the sleeved piece on the boom to
compensate. This just uses a boom that is longer than required, but
centered on the mast, that allows the element ensemble to be placed so the
element ensemble CG coincides with the boom center.
It doesn't pass muster, according to our current paradigm of "what an
antenna should look like," but it works just fine. One of my tribanders is
done this way, and it never does the "hula" when it blows (just about every
day). 
Fortunately, it was really easy to do, my neighbors don't know the
difference, and I only had to please myself when I did it. 

My old software release incorporates all that you have recently discovered,
so finding your favorite solution probably doesn't need to take several
weeks to sort out, but that can't be nearly as much fun, and that's why we
really do this.

I just wanted to point out that the other source for antenna torque is
there, as it was not covered in your latest comments on the subject. I
think there are quite a few TT subscribers that do not posess the
information you have been talking about. I apologize if you thought I was
picking on you, that really wouldn't do much for me.

>      BOTTOM LINE:  You have to do some "cypherin' " before you put up an
> antenna/tower system.  Apparently, there's more to ham radio than Ohm's Law.
> 
> GL es V=IR de

F=MC/I, and I am not responsible for "I", it exists on its own merit,
regardless of what I wish it would do! Sometimes, too much E=IR (the other
"I") causes problems with F=MC/I, and the other way around. That pretty
much defines the system design challenge.

It really must be time to talk about "True North" again. I'm sure I'm done.
TT is fortunate to have someone else step up and be willing to beat the
drum!

-- 
Have fun with it, 
Kurt

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