Mag Loop

Posted on Feb 7, 2014

Electronic equipment is not `cheap` and can be damaged in a number of ways, by electro-static discharge, by lightning and by transmitting withno antenna attached, or into an antenna with a high SWR. The controller described herein attempts to facilitate the safe and automatic tuning of a mag loop antenna by reducing the output power to an acceptable level via ALC control so the RF output

Mag Loop
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circuitry will not be damaged when the line SWR is high For several years now I have been experimenting with Magnetic Loop Antennasat this QTH. In the beginning, I tried octagonal 3/4" copper loops with homebrew trombone` capacitors wrapped in Teflon tape and driven by a surplus screwdriver motor. While they worked OK, they were a real nuisance to peak after QSY`ing. After this, I built a similar loop that used an MFJ butterfly capacitor, driven by the very same motor that MFJ uses in their magloops. The motor was controlled by a microprocessor and an SWR bridge designed to automatically sense resonance. This semi-automated approach worked better, but experienced difficulty hitting the sweet spot` as the analog motor controlled by a M/P generated PWM signal would often drift past resonance, and the controller would keep hunting back and forth over it. Operation was not always consistent. Finally, at the suggestion of Byron WA8LCZ I used a stepper motor in conjunction with the MFJ butterfly capacitor and programmed the controller to automatically hunt for resonance. The microprocessor driven controller when activated will perform the following steps: If desired, the capacitor can be rotated manually, at either slow or fast speed. Resonance will be noted by themarked increase in background noise. The current design works best with butterfly capacitors as it`s very difficult for the controller to determine in which direction...

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