Satgen 397 Antenna Rotators Pt2 by GM4IHJ (BID SGEN397) 2 Nov 96 Part 1 (satgen396) discussed the gap between Radio Amateur expectations , and the hard facts of ham radio rotators which are designed to be robust, but only moderately accurate. How can this gap be closed ? Indeed is it necessary to close it. If you are using light weight broad beam antennas , you really should be happy with the current crop of rotators. If however, you are using large antennas or stacked multiple antennas you have a problem. Your antennas are narrow beam and you need rotator accuracy , and in the case of elevation rotators, greater robustness. So look no further than the group of radio amateurs who are engaged in EME ( Moonbounce) communications. This problem area of rotator design is one they must conquer if they are to get their weak signal echoes back from a very distant Moon. Considering the much abused elevation rotator first. The TV industry comes to our aid here with its polar mounted dish antenna systems turning about a single axis , tilted to point at the Pole star or its Southern hemisphere equivalent. This system requires motion in only one dimension to swing a dish accurately from one end of the Geostationary satellite arc to the other. EMErs cannot get by with unidirectional motion but they can adapt the very powerful DC motor driven screw jack and digital indicators ( available from TV hardware suppliers), to drive their elevation motion. For a better understanding of this elevation screw technique, Internet users should look at moon bouncer GM4JJJs web pages at :- http://www.braeside.demon.co.uk/eme.html Where they will find a description with photographs of the electro mechanical assembly including digital readout of elevation, and, very important , how it is calibrated. All round azimuth motion is difficult to drive with a screw. So one has to go back to the more expensive robust commercial rotators, and find a way to overcome their inherent pointing inaccuracy, or, build your own. For the commercial route the turn knob and press type of rotator control which resets via a resistance balacing network is not really worth having. The finger tip switch control with ammeter read out is better but even that is not going to give you anything like 1 degree accuracy ( something which becomes more and more necessary as we go higher in frequency ). At this point many radio amateur start from scratch and build their own rotator using say - a reversible geared 12v motor ( about 160:1 reduction), driving a large diameter gear wheel through a heavy duty worm. using proper industrial type shaft bearings( not plastic inserts) , with a home made or commercial encoder ( TV type again ?) or maybe using pseudo binary Eg Grey code . Radio amateurs who have graduated via the military will be familiar with both stepper and synchro devices for azimuth reporting. Surplus stores may have everything from ancient M type steppers and magslips to modern synchros. Again EMErs have already opened up these paths and there is lots of useful discussion on MOON-NET@VM.STLAWU.EDU at this present time - More on the all important system calibration in Pt3 .