Re: [TowerTalk] t2x, c31xr and ef240x thread 09-01-99
Hi All,
Fun stuff eh? To bad this thread hasn't carried a more descriptive
subject heading. Wonder if anyone will find it later with the search
engine?
Time out!
I think this basket contains mixed fruit!
Everyone who commented that there is another more accurate way to
determine the polar moment of inertia for an antenna, and subsequently
what the real torque loads are, is right. That is an apple.
Those who commented (like K7LXC) on the correct way to determine the
"effective moment", as defined by Hygain are also correct. That is an
orange.
N7CL is correct about a whole different problem needing attention,
antenna torque caused when the wind blows. We'll call that one a lemon,
cuz it usually makes eveyones face look funny when it comes up.
Taking them one at a time:
The Apple:
Figuring out how much torque can be developed by rotating an antenna is
way more complex than has been discussed, and is why it isn't usually
done for our situations. Everyone is correct in suggesting that the mass
distribution about the mast and the distances to the distributed masses
is what really helps determine the potential for torque development is
correct. That determines what is often refered to as the polar moment of
inertia.
There is more to arriving at "the answer".
When the antenna is at rest or is turning at a constant speed there is
essentially no torque load presented to the rotator (just friction).
When the system starts or stops rotating is when the largest loads
appear.
To figure this out we need to know the dv/dt relationship, which is the
acceleration or deceleration. We can't just get this from the rotator
mfgr. It is dependent on the polar moment of inertia of whatever the
rotator is trying to accelerate/decelerate.
So, for the any given rotator, the dv/dt value changes with every unique
antenna selection or configuration. To get the dv/dt we have to look at
the mechanical advantage back through the gearbox to the electric motor
and its horsepower value. Anyone got a headache yet?
Every rotator is going to accelerate/decelerate a different
configuration at a different rate. Without the acceleration, we can't
figure out how much load is on the rotator.
So, since doing it "the right way" is kind of in the too hard box, some
folks tried another approach....
The Orange:
Hygain and Yaesu came up with a method of establishing a torque value
(be careful here) that could be determined by anyone with always
available information. Hence, the "Effective Moment" (Hygain) and
"K-Factor" (Yaesu) numbers we've seen.
Value = Weight x turning radius.
A meaningless value in rigorous engineering circles, but fairly valuable
if taken in its proper context. It provides an empirically based method
of ballparking antennas and rotators that comes entirely from the
experiences of the rotator manufacturers. What the value means is that
mfgr's experience with failures tells him that a rotator will provide
useful service life if the antenna value does not exceed the rotator
value.
I thought Roger Cox (ex-Hygain) did a really good job of explaining this
in his post to TT quite some time ago.
So, the orange is not an apple. It was never meant to be one!
The Lemon:
Ok, now the real fun starts.
We've been discussing how it works before the wind starts blowing, what
happens when it does? Sometimes little, sometimes lots!
Some antennas, due to their physical configuration produce rather
spectacular torque loads.
>From my experience with analyzing Hygain antenna models, I know that the
Hygain antennas exhibit good torque balance under windload. Yaesu
doesn't build HF antennas (to my knowledge), but I'd bet a paycheck they
don't expect the wind on the antenna to produce additional torque loads.
So, I'd guess that the "effective moment" and "K-factors" do not account
for loads created by this problem. Because the values are empirically
derived, they, via the process, probably do include some of this. But,
no one has any idea how much.
Here's a little example from my '99 Dayton Antenna Forum presentation
that might help explain why we ought to pay some attention to the lemon.
Rotator - T2X
Antenna KLM 20 Meter Big Sticker - 5 EL 20 M Yagi - Early version
Note: This antenna was chosen as a good example of what one might do
(and several have done) with a midsized monobander with aysemetric
element spacing. The challenge with them is achieving CG and torque
balance at the same time.
Ok, we read in the rotator manual that the "effective moment" is 3400
Ft-Lbs.
We look at the antenna manual and see that it weighs 65 Lbs and the
turning radius is 28 Ft. 65 x 28 = 1820 Ft-Lbs "effective moment" for
the antenna. No Problem, right? Almost a 2:1 safety margin there. We're
good to go! Let's put this stuff up!
Now, for some unexplained reason, it rains and we can't just put it up,
we get really bored and make a model of the antenna using YagiStress and
find out that at 62 Mph, where elements start yielding, the antenna,
when broadside to the wind, is generating 4720 In-Lbs of torque. We
think gee, that sounds like a lot, and go back and look at the rotator
manual to find that the stall torque for the T2X is 1000 In-Lbs. That
might not be too good?
So, we run the antenna at a several lower windspeeds and find that the
antenna and rotator reach equilibrium at 30 Mph where the antenna torque
equals the rotator stall torque.
So, as long as the wind stays below 30 Mph, our T2X will make the
antenna turn from all resting azimuths.
Now, we may decide to select this option for our pocketbook and
situation, but I venture a guess that the rotator does not like being
operated at anywhere close to its stall limit, and it will probably not
last very long when asked to do so.
The next rotator on the list that will handle this thing at 50 mph is
the Orion 2800 with 3500 In-lb of rotating capacity (antenna is
generating 2777 In-Lbs). At 62 Mph, the antenna needs an HDR300A @ 5000
In-Lbs (antenna @ 4270 In-Lbs). Big Boy rotators were not available to
be included in the rotator list at that time, but those folks like to
talk about SqFt ratings, and I just don't understand that figure. I
think everyone has so far agreed that 75 Lbs and 10 SqFt is worse than
50 Lbs and 10 SqFt.
One of the interesting things found in the presentation was that when
the antenna configuration gets corrected, the thing can be turned with a
Ham IV.
The lemon is sometimes a bitter fruit, but it can make the difference
between buying a HDR300 or a Ham IV.
Everyone gets to figure out which solution is best for them. I just want
them to do it from an informed position.
Everyone have lots of fun and put up really great stuff that stays up.
--
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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