This article discusses the replacement of the actuator of a 'BABY' Ciro-Mazzoni magnetic loop antenna. Whilst the antennas are very reliable, occasionally an actuator will fail and require replacement. In order to ensure that the replacement results in a fully working antenna, a number of points need to be considered. The first is that the position of the actuator must be such that when it is driven to either its inner or outer electrical limit that this occurs before reaching the hard mechanical limit. Inside the actuator are two normally closed limit switches. If the actuator is driven to either of its limits one of these switches will open placing a reverse-biased diode in series with the armature and blocking the flow of current. To exit the limit it is only necessary to reverse the polarity of the drive signal, whereupon armature current will flow in the reverse direction, and the actuator will return to its normal operation. If the actuator were to encounter a hard mechanical limit this would stall the motor, and then the only thing limiting the current would be the armature resistance itself plus any cable resistance. The very large current that would flow in this case would almost certainly damage the motor driver integrated circuit, and so must be avoided at all costs. Equally, as well as avoiding hard mechanical limits, it is necessary to ensure that at the inner limit the resonant frequency of the antenna is well below the nominal low frequency limit of 6,600kHz, and at the outer electrical limit its resonant frequency is above the nominal upper limit of 29,800kHz. In addition, as the resonant frequencies are somewhat temperature sensitive this too has an effect and must be taken into account. The initial replacement instructions required you to accurately measure the distance from the edge of the clamp to the end of the actuator housing, but this did not always result in a satisfactory position. The latest method involves closing the capacitor to a position that is 15mm from its fully closed position and then tightening the clamp. The actuator must be at its inner electrical limit when this is done, and it is normally supplied in this position. It is most important not to rotate the actuator rod! In my case I made a simple jig out of a piece of wood with two large diameter countersunk headed screws driven in until the flat portion of the screw heads were protruding exactly 15mm, and then by holding the capacitor closed on this jig, the clamp screws could be tightened securing the actuator in its optimum position. With the antenna then mounted temporarily in a Black & Decker 'Workmate' table I measured the resonant frequency at the inner electrical limit and found it to be 6,484kHz. With a bench 24V current limited power supply I drove the antenna to its outer electrical limit and the resonant frequency measured again, and this time it was 30,314kHz. As these frequencies were well outside the nominal 6,600 - 29,800 kHz tuning range the actuator was correctly located. The armature current when driving the actuator with 24V was between 350mA and 400mA. With the actuator positioned away from either electrical limit the armature resistance was about 3 to 5 ohms, depending on where the armature had stopped. The resistance can be expected to vary somewhat depending on where the brushes land up on the armature segments when the motor stops. When correctly positioned, the Mk-II controller can easily determine both the 'sense' of the motor connection, as well as the type of antenna when using the Auto-Detect mode. However, see a later post of mine where I ran into some quite unexpected trouble! Adrian, 5B4AIY
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