Satgen 398 Antenna Rotators Pt3 by GM4IHJ (BID SGEN398) 9 Nov 96 Having suggested in previous Satgens that Amsat users are less than happy with modern commercial antenna rotators. What else is available? Typical home brew antenna Azimuth drives are being built from :- Motor Cycle shaft drive assemblies Motor Cycle chain drives (with an essential chain tightening idler) Automobile starter gear wheels and drive motor assemblies and best of all if available - Worm gear drive assemblies which do not " walk back " in strong winds, as many gearwheel drives do. TV dish screw drives are favourite for elevation drives. Typical position indicators are being made from :- For elevation - TV positioner units, with memory and digireadout ( please note that TV screw drives are non linear ) or Precision potentiometers with pendulum drives, to voltmeter. For Azimuth indication - multiturn potentiometers can be used but modern pulse systems ( Gray code or relative count ) are favoured Most system users stress the need for easy to use alignment points at say training or elevation end stop check points. Suitable off system calibration methods employ - the Pole star ( direct bearing and reciprocal) or nearby buildings whose wall alignments can often be read off from large scale local maps. Equally useful in many cases are distinct horizon features which checked against local maps can give both azimuth and elevation check points. Magnetic compass bearings are not advised , where 1 or 2 degree accuracy or better is required. Mechanical group alignment of arrays of antennas is sometimes possible against measured painted aiming spots on adjacent buildings. While electronic checks of both mechanical alignment and radio squint are best done against discrete radio noise targets such as Cassiopeia A, Cygnus A, Crab Nebula, Virgo A, and Sagittarius A. As reported earlier some antenna rotators are safer than others in high winds. It is a wise precaution to include a sturdy mechanical locking device in the position where the antenna is parked when not in use. Equally important hard stops should be included at end of travel points to avoid antenna spaghetti in hurricane winds. At IHJ antennas are parked to present minimum surface area to the west south west, the direction from which prevailing winds come. In the case of VHF/UHF yagis this means parking on bearing 248 degrees. But in the case of the 10m and 6m beams , where antenna elements have greater surface area than the antenna boom , the antennas are parked boom towards 338 degrees. These being locations arrived at by trial and error ( not least on the night of 5/6 November recently when winds on this exposed location on a ridge high above the Forth Clyde valley, reached 150 kms per hour, briefly, and tore the roof off a school in near by Glasgow. Next weeks satgen will continue this discussion of antenna rotators versus the weather, and if space permits , report at least one hilarious exotic antenna control solution which sank without trace.