This is a CLASSIC post from work I did 6 years ago with my friends at Len Long’s Tuesday Night Workshop. Len W1ZTL my eternal thanks for the space and comraderie you’ve provided in your home all these years!

A magnetic loop is an attractive antenna for a small space and a terrible one to tune. The bandwidth is razor-thin, which is exactly why it’s quiet — and exactly why you spend the evening with one hand out over the knob of the vacuum variable capacitor, nudging it a hair at a time and constantly zooming past the low SWR point!

So a few of us took it on as a Tuesday-night workshop project: let an Arduino do the work.

The idea is not clever, which is the best kind of idea. A stepper motor turns the tuning capacitor. A second motor rotates the loop to null out an interferer. An Arduino sits in the middle taking commands, and that’s the whole plot. The interesting part was never the code — it was the mechanics. A tuning cap wants to be turned gently and repeatably, and a stepper coupled through a bit of backlash is neither until you make it so.

What I learned, in the order the loop taught me:

  • Backlash is the enemy of repeatability. The same step count has to land on the same capacitance every time, or “tuned” becomes a moving target. An anti-backlash coupling earned its keep.
  • Slow is fast. Ramping the stepper down near resonance beats slamming it and overshooting. You feel it dial in.
  • A hard end-stop saves a soft capacitor. Ask me how I know.

It’s not finished — I want a proper resonance-seeking routine so it finds the dip on its own instead of being told where to look. But last Tuesday it tuned a band-change in a few seconds without a human hand on the cap, and the room went appreciatively quiet. That’ll do for a first light.