Hi All,
I was wondering when this subject would really surface, maybe I missed its
earlier incarnations. The many comments on this and the K Factor thread
have placed a fairly well described box around the problem.
I reluctantly enter the discussion, as the effort is likely not to lead to
anything other than an interesting discussion.
Here are some of my thoughts on the topic.
Surface area ratings for rotors has been one of my pet peeves for many
years.
The Effective Moment/ K-Factor approach is a big move in the right
direction. It, or some similar more accurate form may be all we need to get
antennas and rotator suitably matched.
We all should agree that a thorough and proper solution via physics
principles is not so straightforward and readily attainable in all quarters.
To pursue this form of solution will only lead to no change in the
situation.
What has happened as a result of our discussion about correcting antenna
area spec's. Absolutely nothing, from what I can determine. Resolving that
problem is way easier than this one. So, we need to keep this solution
simple to expect to see it in our lifetimes.
I understand from some of the comments that the K factor or Effective moment
is just the product of the turning radius and the entire antenna weight. No
confirmation on my part from any manufacturer.
I would favor a value that more accurately represents the material
distribution in the antenna.
Like, summing up the products of the element weights and their distance from
the mast connection, boom sections included.
Yes! I understand the difference between weight (Lbs) and mass (Slugs), for
the physicists. We're after a readily obtainable solution to our problem of
trashing rotators.
The method doesn't really matter much as long as:
1) All rotator manufacturers use the same method, so all antenna
manufacturers can do the same.
2) The rotator manufacturers have done their due dilligence to ensure that
the method covers the broad range of ways we can arrive at the same value.
I.E. 6 element 20 vs 4 element 40 vs 2 element 80. And, confirmed in the
proper way, that the ? factor is suitable.
After that, they should have accounted some margin for dynamic loads.
Bottom line, the rotator builders have to agree on the required antenna
system values and pass it on to the antenna manufacturers in a way that they
can execute it!
The next part of the problem.
There has been much attention given to including the "wind resistance" into
this value.
A proper antenna mechanical design, IMHO, should not create any "wind
resistance" to the rotator.
If the antenna has neutral torque balance in the wind, there won't be any
rotational loads on the rotator.
This is not always possible or easy to obtain with some of the unique
element spacings and sidemounting clearance requirements.
So, the other piece of information we need is the antenna torque at various
windspeeds.
We take the ? Factor and add to it the antenna torque to get a total.
This method will not please the purists, but it can be made to work and is
far more attainable than the other option.
The final solution is probably a ? factor plus a "not to exceed" antenna
torque value. Which might contain the folks that are shipping you guys a
bunch of "windmills."
One comment that requires a response:
> If I were the suspicious type I'd conjecture that this K-factor was
> included as an after thought (CYA) when rotors started failing and some
> way was needed to show that the buyer unknowingly exceeded some spec or
> other.
I disagree! The guys working at the commercial manufacturers are not idiots!
Anybody who can design a gearbox understands the problems! I'd bet the
gearbox designer asked "What kind of stuff are you gonna connect to this,
and what loads are expected?" The answer was probably "all sorts of things
that behave very differently. We can't get enough information to describe
it, so, why don't you just use this." Everyone on this reflector should
relate to that situation.
They probably never have enough time to get everything done, like most of us
involved in designing and building stuff for a living.
The old surface area rating was an easily obtained value based on empirical
experience with certain antennas and rotors. I'd bet that you can use the
surface area rating for rotators and antennas from one manufacturer with
good probability of success. If not, they owe you one!
The change from area to the EM/K/GD^2 Factors came from the manufacturers.
Everyone recognized a problem, they made the positive change toward a
solution. Being a part of the problem or part of the solution are different.
The market is pressuring them to design new better electrical designs, I.E.
"32 bands on a 6' boom in my attic", not to provide the mechanical
information we need. So guess where the time gets spent.
Has anybody refused to buy an antenna lately because the mechanical spec's
are suspect?
We still only have one manufacturer providing clearly defined antenna area
values.
The problem, again, is that there is not any standard, agreed upon by the
manufacturers.
Everybody does their own thing, and the rest of us have to grope to find
meaning in it all.
I'll bet very few know that while EIA was writing spec's for tower design,
they also wrote one for antenna design. It's called RS-409, last time I
checked. Not what we need, but a decent start that needs input and support.
The last guy who tried to to offer antennas compliant with this spec died on
the vine.
I'd be more than happy to include the necessary calculations for the ?
Factor, in my software that is used by many manufacturers you all buy
antennas from. But, I'm not gonna do it until there is agreement on what
numbers we're after. And, I can achieve some sort of comfort level that the
effort will achieve the desired results. I.E. everybody is on the same page,
it makes sense, and then, someone commits to turn the solution into
available information for the users in a reasonable time frame.
I'd like to add some opinions about "torque dampeners, " or the many other
things the rubber coupling gizmos have been called. This may just be a
matter of semantics.
They are shock absorbers. They do not appreciably reduce the amount of
torque applied to the rotator by the antenna system. They just spread the
load across a longer time span, reducing the peak dynamic spikes, which can
be higher than the input loads. They do a good job at this but seem to enjoy
fairly short service lives.
There are some really neat couplings for industrial applications that keep
the rubber in 100% compression instead of compression/shear. Get ahold of
some industrial coupling books. Probably need to select one that has a high
safety margin for our loads to get the long life. Maybe one of the clever
fellows around here will find a reliable solution. I haven't found the time
yet. It's probably easier to just go up a notch in rotator selection, after
the other questions are answered.
Glad to see this topic surface. There is plenty of additional stuff
involved.
I'd bet that there is more than enough empirical experience on this
reflector to back into some kind of useable solution, way before the other
guys get around to it.
Until a solution seems likely, we should all be sharing our records of
success and failure to help ourselves and others make the right guesses!
One of the problems with human nature is, no one wants to advertise their
failures.
I really appreciate the few who have stepped forward to tell us about these
things, because success documentation without an equal list of failures,
gets us nowhere in advancing the state of the art!
Let's be careful not to get too wound up over this one!
Probably going to be having the same discussion a year from now.
--
73, Kurt, K7NV
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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