----------
From: K7LXC@aol.com[SMTP:K7LXC@aol.com]
Sent: Tuesday, March 18, 1997 1:56 AM
To: dickrts@texoma.net; TOWERTALK@contesting.com
Cc: TEDMAD@worldnet.att.net
Subject: Rotator ratings
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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One last note on this topic. If the wind loading or balancing programs use the
method where the force on a tube at an angle to the wind is in line with the
wind direction the method is in error. This misconception crept into ham radio
about 30 years ago. For the past 100 years structural engineers, mechanical
engineers, and aeronuatical engineers have used the "cross flow" principle to
find the wind loads on members at an angle to the wind. This concept was proven
experimentally as far back as 100 years ago. The cross flow principle says that
the force on a tube is perpendicular to the axis of the tube. The magnitude
varies with the angle of the tube relative to the wind. There will be a
component of this force in line with the wind which is called the "Drag Force."
There is another component that is at 90 degrees to the wind which is called
the "Cross Wnd Force." When a tube is perpendicular to the wind the value of
the "Cross Wind Force is zero. When the tube is at 45 degrees the force
component in line with the wind (Drag Force) has the same magnitude as the
force perpendiccular to the wind (CrossWind force).
For anyone intereseted in seeing the derivation of this concept and a
comaprison to test data taken in a wind tunnel look in "Fluid-Dynamica Drag"
(page 3-11) by Hoerner. You'll probably have to check it out of a university
that has an engineering section. Or you can look in "Mechanical Engineering in
Radar and Communications" (p 162-165) by C.J. Richards. If you don't want to
read this stuff there's a very easy experiment you can do. Get a 2 ft length
of tube or plastic pipe. With your arm out the window of a car going about
30-40 MPH, rotate the tube relative to the wind. When the tube is strainght up
and down you'll feel only a force in line with the wind. Now rotate the tube in
the wind. If you rotate the top into the wind you'll feel an upward force in
addition to the force in line with the wind. The upward force is perdedicular
to the wind and is the "Cross Wind Force" which is the vertical component of
the force that is perpendicular to the tube's axis. Or, rotate the top of the
tube reward. You'll now feel a downward force in addition to the in line force.
Again this is the "Cross Wind force."
If the cross flow principle was not true ships with rudders wouldn't
turn. (They would only slow down as the rudder is moved in he water.) Also you
wouldn't be able to stick your hand out a car window and make your hand soar up
and down as the palm is angled into the wind. In addition all structures,
buildings, towers, and bridges that have been designed to handle winds have
been designed incorrectly by professinal engineers for the past 100 years.
If anyone would like other references on this topic let me know.
Dick, K5IU
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