Automatic Mag Loop Antenna Tuner for KX3

This page refers to my article published in Nov/Dec 2015 QEX. Any updates will be posted here.

Supporting Material:



  1. For best results perform the BAD_MATCH calibration while the loop is tuned near its upper frequency limit.

    This is because the output from the internal RF source falls slowly with frequency, so you want the Tuner to use the lowest value while calibrating for the worst-case match condition.
  2. To avoid having to disconnect the internal 5 V supply to the Arduino while using it with a USB cable, make a "development" version of a spare USB cable as follows:
    1. Make a short slit anywhere in the jacket lengthwise;
    2. Locate the red wire;
    3. Cut the red wire;
    4. Close the slit with a few turns of tape.
    While using a USB cable modified in this way, the internal 5 V connection can be left in place because the 5 V supply line in the cable is now severed. This cable is handy for other Arduino applcations as well.
  3. If the MFJ has sat idle for several days, it can help to run it from one end to the other a time or two. Hold Down until the Limit LED comes on, then hold Up similarly then Down again. Now click Tune and it might work much better.
  4. I also find my loop can become mistuned when the temperature changes more than a few degrees or it is getting buffeted by the wind. In these conditions, I recheck SWR frequently and force a Tune as required.

73, Elwood Downey, WBØOEW