[TowerTalk] "K" factor gets sillier

Kurt Andress K7NV@contesting.com
Sat, 22 Jul 2000 13:46:29 -0700

Pete Smith wrote:
> I just received a new Yaesu G-800SA rotator (the A denotes the six-wire
> cable apparently required by CE standards).
> Imagine my amusement to discover that Yaesu now defines the K factor to
> mean the product of the antenna weight and turning radius, PLUS the weight
> of the mast (or a share of the mast in the case of multiple antennas) times
> the turning radius!
> Then just to gild the lily, they recommend that you not exceed 60 percent
> of the allowable K factor for a given rotator, and they caution that even
> if you use a thrust bearing, you still have to count the mast weight in
> your calculation!
> This K factor thing gets further and further from physical reality, and
> looks more and more like CYA/marketing of bigger rotators!
> 73, Pete Smith N4ZR

Hi Pete,

Well, we've certainly discussed this one before.
I enjoy a different view of this new revelation. Thanks to Pete for
passing the information on!

As background for readers new to the subject, the Yaesu "K" Factor and
the Hygain "Effective Moment" are empirically derived values. They are
not a rotator breaking value or any other such exact thing. They are a
convenient value, that can be readily determined by any rotator buyer to
match a rotator with the antennae they want it to turn. Each value has
been determined by the rotator manufacturer and is purported to
represent their experience with user failure reports.

I think the spec change might just be a real good thing! 
It tells us that the manufacturer has effectively derated the rotators
by adding mast mass, and furthermore telling us not to exceed 60% of the
"K" value with the antennae attached to it.

I seem to recall reports here of these rotators biting the dust in the
last few years. 

IF those reporting failures were folks that were adhering to the
previous "K" values, we could say that the old "K" values were
incorrect. I.E. If everyone did what they were supposed to do, and the
"K"'s were correct, we would have heard very little about failures.

I think the change is a recalibration of the "K" value based on a larger
database. That is what emperical development is all about. 

The potentially valuable empirical database, however, is being
negatively affected by some malpractice.

>From what I've seen, there is generally enough information to do a
Weight x Turning Radius calc on the things that have been reported
failed and process that information to arrive at a new value for the
performance for the rotators. 

There is, however, very little information on the wind induced torque
created by the antennas involved. I know of a couple of antenna
manufacturers that publish torque values for their antennas, there may
be others I don't know about.

All antennas do not behave the same in the wind. 
I've seen some popular antennas that have quite spectacular torque. Used
one of them in my Dayton '99 presentation. That particular 5 El 20 meter
beam choked the living "you know what" out of a Tailtwister at about 30
Mph (EIA-222-C) basic wind speed, probably hundreds of them still out
there trashing rotators. 
There are others that are quite nicely balanced.

The "K' factors probably reflect this problem, which is good. But the
whole thing would shift in a positive direction if people would quit
building radically unbalanced antennas. 

It is real easy to figure out what you've got. If the mast is mounted at
the center of the boom, or is mounted off center and has a compensating
device attached to the short side of the boom, you are ok. If the mast
is attached off center and there is no compensation, you're likely to be
abusing the rotator.
The obvious solution is a beefier rotator, but it doesn't need to be
that way.

I think the change you reported is good for Yaesu rotator buyers, it
probably represents a reasonable assesment of how it is, and should
result in fewer failures from that rotator family. 

Have a good weekend!

73, Kurt, K7NV

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