Satgen396 Antenna Rotators Pt1 by GM4IHJ (BID SGEN396) 26 Oct 96 The first thing which must be said about these devices is :- If you do not absolutely need one , do not use one. Use an omni directional or better a colinear antenna with an all round horizontal beam. Secondly, please do not assume that the average amateur radio rotator is a precision instrument. However much you paid for it . Higher price generally means , tougher , higher loading capacity, not increased accuracy. Ham rotators have to endure high wind loading and juddering is squalls. So they are relatively tough simple first order servo devices. One azimuth rotator at IHJ has carried a 5 XY 2m yagi and a long 21 ele 70 cm yagi for 19 years and not been touched since installation. By contrast the control box in the shack needs annual relay cleaning to avoid excess burning. The same cannot be said for the backyard beam elevation rotator, that has been replaced 3 times. Not because it is poorer manufacture, but simply because elevation motion imposes far greater unbalanced forces than simple azimuth horizontal motion. This carnage of elevation rotors takes place despite the antennas and feeders being carefully balanced about the turning shaft. Just watch an elevated antenna on a typical Scottish 60 km per hour breezy day, and you will see how harshly the rotator is treated in this mode. Being simple first order servos , the alignment error on ham rotators may at first glance seem to be enormous. Run in from clockwise versus run in from anticlockwise can easily be 5 degrees different. The shorter the run in angle the greater the error. Because first order servos do not handle static friction at all well. On a 16 inch battleship turret the servo motors have electronic control functions featuring 1st (distance/degrees), 2nd (velocity), 3rd (acceleration) and often 4th (acceleration rate) terms . Only the first of these features is present on most ham rotators. So ham rotators are inaccurate ? But do we need this accuracy ? Our antenna beam widths at the half power points can be 10 to 20 degrees , so 5 degrees off is not really a problem . Equally disturbing but less obvious most ham radio antennas in urban areas squint several degrees in electronic alignment versus apparent physical alignment. We can gain some advantage by using the type of rotor which displays antenna position on an ammeter type electrical display and uses finger switch controls to drive the antenna motor. These systems are inherently more accurate than the turn and press knob type of controller which is then left to run in until corrected by a resistance balance reset network. Though the knob and turn type are easier to use. But nowhere as far as this writer knows is there a family of amateur radio rotators with the degree or half degree accuracy which some correspondents seem to yearn for. Next weeks satgen will take this story further , discussing how to get greater accuracy when it is needed for say moon bounce , and equally important, the pros and cons of home brew build it yourself rotators , for which old motorcycle drive systems, TV dish actuator screws and aircraft propellor pitch motors seem to be popular routes.