In a message dated 97-03-15 21:34:32 EST, dickrts@texoma.net (Dick Weber)
writes:
> 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.
>
For a long time the time honored way of rotator selection was by using
the 'square foot' wind loading of the antennas method described above; it is
imperfect. The biggest deficiency was that two antennas with similar square
footage could put very different demands on a rotator (i.e. 3L 20M beam vs.
long boom 10M beam).
Several years ago Hy-Gain came up with a figure they call Effective
Moment that more accurately took antenna forces into account. The formula
for EM is antenna weight times turning radius. Yaesu uses the same figure
but calls it a 'K-Factor'. Emoto uses a GD2 formula that is slightly
different but is a similar measurement. This, and much more info, was
covered in my 3-part series on rotators that recently appeared in CQ Contest
magazine (reprints are available if you missed it).
BTW, the TOWER TECH MARC (Mast, Antenna and Rotator Calculator) software
program is almost ready and it calculates the Effective Moment of the
antennas under consideration and makes rotator recommendations as part of the
calculation.
73, Steve K7LXC
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