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Solid dish sanity

I have been reading a number of posts boasting the cons of solid dishes
mounted on 'fragile' Yaesu rotors.  I have also spoken with other well know
satellite ops who also feel this is a bad idea.  However, no one that I've
seen has put any numbers to this hypothesis.

It seems to me that the only way to do any real damage to the rotator is to
exceed the maximum braking torque, causing the rotator to turn and possibly
doing damage while doing so.  I may not be completely correct in this
assumption since I am not a designer of rotators.

I have a Yaesu G-5400B.  From its manual, I have the following, given

    Braking Torque Azimuth    145 ft-lbs
    Braking Torque Elevation    289 ft-lbs

>From the manuals of my 2m and 70cm antennas, I know their wind areas are:

    Cushcraft A148-20T    1.21 sq ft
    Cushcraft 738XB          1.4 sq ft.

I have a DH Satellite 36" solid aluminum dish.  The actual dia. is 34.5".
Therefore the area should be:

    ((34.5/2)^2 * 3.14) / 144 = 6.49 sq ft

In the gulf coast region of the USA it is customary for industry to use a
max. 3 second gust wind speed of 125 MPH in open areas.  Of course, I am not
rating pressure containing equipment.  Also, I am not in an open area.  My
antennas are coincident with the peak of my roof and I am surrounded by
trees on all sides except for a small slice of sky to my south/southeast.

I also have a US Towers crank-up tower which is only rated for 50 mph winds.
True, I can crank it down.  If I happen to be home.  For major events, such
as hurricanes, I will be home as my place of business dismisses
non-essential personal when hurricanes are bearing down.

For all of these reasons, I feel very comfortable using a max. wind speed of
100 MPH.  According to the American Society of Civil Engineers, this wind
speed corresponds to a wind pressure of 22 psf (psf = pounds per square
foot).  This is providing that the "Importance Factor" is equal to 1.0.

>From my very hazy recollection of college statics and free-body diagrams, I
realize that the only forces that count for this analysis are the unbalanced
forces.  Since the 2m and 70cm antennas have basically the same wind areas
and are both equal distances from my rotator (one on each side of the
rotor), these forces should cancel out.  This leaves me with a resultant of
0 ft-lbs of torque.  This still does introduce a over-turning moment at the
base of the tower but the load on the tower is not the issue

My dish will be mounted off to one side of the rotator.  From the center of
the rotator to the center of the dish will be about 1 ft.  This introduces
an eccentric load into the system.  Using the information I have given
above, the following calculation will give the torque acting on the
centerline of the rotor:

    (6.49 sq ft) * (22 psf) * 1 ft = 142.78 ft-lbs

Comparing this to the braking torque of the azimuth rotor we find that they
are nearly equal.  And it is only half of the braking torque for the
elevation rotor.  I have stated that this is the biggest dish I would put on
rotator and the calculations prove it to be true.  The problem here is that
the only available safety factor in the azimuth rotator is whatever fat
Yaesu put into their published braking torque numbers.

Other things to consider:

1.) The Yaesu G-5500 azimuth rotator has double the braking torque of the
G-5400B (for those who have G-5500's).

2.) Maximum winds in Baton Rouge during Hurricane Andrew were 100 MPH.

3.) Both my towers can be folded-over to ground level by two people in less
than 1 hour.

4.) The numbers get considerably better using a max. wind speed of 90 MPH
(18 psf wind pressure).

5.) Rotating the dish to an elevation angle of 45 degrees reduces the wind
area to approximately 4.31 sq ft.  Rotating the dish to an elevation of 90
degrees reduces the wind area to 1.07 sq ft.

I have no doubt that these three antennas are riding at the limit of my
rotator's capabilities.  However, I do not foresee the destruction of the
rotator in the near future.  I'm sure there are some holes to be poked into
my theory and in Yaesu's torque ratings.  But those seem to be the best
numbers we have to estimate the limits of our equipment.

Karl R. Sandstrom, K5MAN
AMSAT Area Coordinator
Greenwell Springs, Louisiana
k5man@arrl.net / k5man@amsat.org

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