[TowerTalk] tensioning guys

[Kurt Andress]

Dick Green wrote:
> 
> I have a Loos PT-2 that I use to measure and adjust the tension of the
> pull-down cable on my U.S. Tower MA-770MDP. It works well for that.
> 
> But what about measuring tension on types of cable for which the Loos is not
> designed? My new AB-577/GRC military surplus portable mast has three sets of
> 1/8" stainless steel guys. The installation instructions simply say to "make
> sure the mast is vertical and the guys are taut". I ended up tensioning the
> guys so that there was no visible slack, there was some (but not a lot) of
> "tightness", and the mast didn't move when a guy was grabbed and shaken
> vigorously. The guy tension adjusters (called "snubbers") have a sort of
> self-limiting feature -- they're designed to be tensioned by hand and
> there's only so far you can turn them.
> 
> The mast looks quite sturdy in a 35MPH wind, and I'm sure the guys are not
> so tight as to risk their breakage or compression damage to the mast. I
> tried to make the tension as equal as possible, but it's hard to know. Does
> anyone know of any measureing devices or "rules of thumb I can use to set my
> mind at ease about this.
> 
> 73, Dick WC1M

Hi Dick,
I know we've discussed this privately in the past.

I think you are pretty close with what you have done. If the pretension
doesn't allow any mast movement when you tweak a guy, the thing is
pretty close. We just want to get the slack out of the guys so they
behave linearly under load, but not have them so tight that they create
a a ton of compression in the mast before the wind blows.
Since, we cannot use a Loos gage to set these things up, we get to use
our best judgement.

Don't worry about trying to make the guy tensions equal. The AB577 mast
has very low stiffness, it's a real noodle all by itself. Because the
mast is not stiff, it will readily move to a position of equilibrium for
each set of guys, thus creating equal guy tensions automatically. 

Just get it tuned straight (straight is far more important than
vertical) with no movement when twanging a wire and it should be ok.
That's what I did last winter and mine handled 80+ mph several times
with the TH7. Most times I was able to orient the antenna to present
minimum antenna area, but not always.

I'd recommend that you contact your antenna designer and find out which
is the lowest exposed area and do the same (not available from published
literature), just a safer M.O. 

For a tribander, the lower area stable position is usually with the boom
broadside to the wind. The lowest exposed area is usually somewhere
between 0 deg and 90 deg, approx 45 deg if the elements and boom
projected areas are equal. But, this is not always the most stable
orientation, due to torque created by the boom to mast centerline
offset.

Last winter, when my T2X clamp connection to the tower was slipping, the
TH7 always seeked a boom broadside to the wind orientation, suggesting
that it was the most stable orientation.

Got to check the AB-577 every storm to make sure the screw in anchors
are not moving. I used a different anchor and mine took a few storms to
settle in, been stable ever since.

Mine has been up for over a year, don't know if that is a temporary
installation, I think it's a semi-permanent one. There's another guy
near me with a KT34XA (no rotator) for over 2 years and his is also
still up. It would definitely scare me to have that antenna up here on
the 577 (have one laying in the antenna pile) with its severe torque
problems. The TH7 has rather good torque balance, so that was preferred
here.

I hate to offer such purely empirically based advice, but sometimes
that's all we can find. The AB-577 should be considered a high
maintenance item, meaning you have to go check it out all the time,
until you satisfy yourself that it is stable. Definitely not a
plug-and-play device, in modern rhetoric.

Other than that, it is a great toy!


-- 
73, Kurt

K7NV "That's K7 "Nevada" (ex - NI6W)

YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/

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