reference: Thomas E. DeHoff, WA7MAL reques for CDE rotor info
In response to the question about the rating of your CDE rotator I suggest you
read the article in Communications Quarterly that is in two parts. (Summer 94
and Winter 95) It covers in detail the aerodynamic balancing of Yagi's. The
information in the article will allow you to figure out how much rotor torque
it takes to turn an antenna in a wind and the holding capacity required for the
brake.
Back to your question .... Some antennas are reasonably aerodynamically
balanced and some are terrible. ( The difference is mainly due to the boom. The
articles cover this in detail.) To the point, the area of an antenna is not an
indication of the amount of torque it takes to turn it or hold it. It is mainly
a function of the asymmetry of the boom relative to the mast. For example, a
highly unbalanced antenna like the KLM 20M Big Sticker can be brought into
reasonable balance by adding a compensating plate on the boom. In this case the
antenna "area" actually increases slightly, but the torque to turn/brake
decreases significantly.
Why are rotors rated in "square feet?" Well, no one ever sat down to figure it
out using principles used by structural, mechanical, and aeronautical engineers
that have been known for over 100 years. My guess is someone starting using
this method and everyone else followed suit. This has resulted in a real mess
and is somewhat comical. For example, Hygain rates its Tailtwister at 1000
in-lbs of torque and says its good for 20 sqft of antenna. ( By the way they
don't say anything about how well balanced the antennas should be.) In
addition, Hygain rates its HDR300 at 5000 in-lbs of torque and says it good for
up to 25 sqft of antenna. It doesn't make much sense that a rotor with 5X
torque can only handle 25% more antenna. Hygain is not the only company with
meaningless ratings. These companies should only advertise their turning torque
ratings and their brake holding ratings.
Hopefully someone will have the ratings for your rotor. Knowing the rating and
using the information in the above articles you can get maximum utilization of
your rotor.
73 and good luck,
Dick Weber, PE
K5IU
----------
From: Kenneth D. Grimm[SMTP:grimm@alison.sbc.edu]
Sent: Saturday, March 15, 1997 7:24 AM
To: TOWERTALK@contesting.com
Cc: THOMAS E. DE HOFF
Subject: Re: [TowerTalk] CDE ANTENNA ROTOR
THOMAS E. DE HOFF wrote:
>
> TO ANY AMATEUR OPERATOR
> I AM TRING TO LOCATED INFO ON THE CDE HAM-M ROTOR
> WHAT INO I NEED IS THE SURFACE AREA THAT THE ROTOR WAS RATED AT FOR
> HANDLING WIND LOADS------------I WOULD APPRECIATE ANY INFO OR JOURNAL
> REFERENCE THAT HAS THIS INFORMATION---------
> TNX TOM WA7MAL-----
You ask an interesting question, Tom. One that is rather difficult to
answer, I'm afraid. The spec. sheet put out by Cornell-Dubilier, the
original manufacturer of the HAM series of rotators, does not mention
surface area or wind load. The two items of interest are that it would
"support 1,000 lbs. of dead weight" and the brake was rated at "3,500
lbs. of torque from wind pressure." The Ham IV carried a rating of 15
sq. ft. of wind load when mounted inside a tower. This rotor was
essentially the same as the HAM-M, but with a couple of significant
improvements. The ring gear and brake were improved and made of harder
material. By inerence you would need to derate the HAM-IV figures for
the HAM-M. Norm's Rotor Service has done a good job of compiling and
interpolating this kind of information and he suggests a rating of 10
sq. ft. for the old HAM-M. This sounds very reasonable to me. You can
check out his comments on the old rotators on his web site at
http://www.tiac.net/users/shiacawn/rotors/
73,
Ken K4XL
grimm@alison.sbc.edu
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