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*Subject*: [amsat-bb] Solid dish sanity*From*: "Karl Sandstrom" <k5man@xxxxxxx>*Date*: Fri, 15 Aug 2003 19:25:56 -0500

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 information: 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 here. 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 my 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 ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

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