This note describes a problem I encountered when I replaced the actuator of my BABY loop. After replacing the actuator and ensuring that it was optimally located in its clamp, when the antenna was remounted on its rotator and connected up, imagine my surprise when nothing worked properly! The Auto-Detect mode failed to correctly identify the antenna, in fact, when it had completed its operation it returned the message 'UNKNOWN ANTENNA'. Even after I had manually selected the BABY type, the controller could still not tune the antenna, and usually any tune attempt would result in the antenna being driven into either its inner or outer electrical limit with the resultant 'MOTOR FAIL' message. I found that if I identified the actual resonant frequency of the antenna and then entered this frequency, provided that any subsequent frequency increment was small, then the controller could usually manage to tune the antenna. However, if a large frequency increment was selected, for example, from 7MHz to 14MHz, then this would result in the antenna being driven to its fully closed position, or else an error message 'WARNING: SWR>3 PRESS ANY KEY' would be displayed and no further tuning would occur. Thinking that perhaps I had a controller problem, I swapped back to the earlier model controller, but this too was unable to tune the antenna, and would also tend to drive it to one of its electrical limits. By now I had no idea of exactly where the antenna was tuned, so to identify its actual resonant frequency I set my antenna analyser to sweep from 6MHz to 30MHz and when I did so, it revealed a number of broad but spurious resonances as well as the very sharp actual resonance. Now I could see what was wrong, I had a cable fault. Indeed, if only I'd thought of putting the analyser in its Time Domain Reflectometry mode this would have immediately shown that I had a cable discontinuity and precisely where this was located from the analyser. In fact, I knew as soon as I saw the spurious resonances where this discontinuity was located. When the antenna was originally set up although it worked, it was not in an optimum location. I then relocated it, but this was now 5m further away, and so rather than 'waste' a perfectly good 20m section of RG-213 I simply added another 5m by means of a SO-239 coupler. This had worked perfectly well for several years, and indeed prior to my actuator failure it was still working. When I carefully examined the coupler the outer threads had corroded even though I'd protected the unit. As soon as I replaced the coupler and cleaned the threads of the connectors and the centre pins, everything worked as it should. The reason for the strange behaviour is easily explained. The tuning algorithm used by the controller relies on the SWR being high and constant when far removed from resonance, and when the controller detects the SWR decreasing it assumes that it is now on the initial slope of the resonance curve, and switches from the continuous drive mode into the fine tuning mode. In this mode it steps the motor with short pulses, and you can see this as the blue LED will change from being continuously illuminated to pulsing rapidly. The algorithm now searches for a SWR reversal, and from this it knows that it has just passed through the actual resonance point and will step back. If there are spurious resonances, the slope of these curves confuses the algorithm and it is then unable to locate the true resonance. The moral of this tale? Don't splice your feeder, but use a single continuous length. 73, Adrian, 5B4AIY
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