Sunday, December 23, 2018

Tweaking the MFJ 1788 mag loop

The slow adjustment resistor 
In my last post regarding the SWR on my MFJ 1788 mag loop I was suggesting that I wanted to get my SWR below 1.6 and I did get it too 1.4. The reason I want as low SWR as possible is I have better  band width before I have to retune the loop. One reader of my blog Paul VA3ZC also lives in a condo and has an MFJ mag loop as well. He gave me some great great suggestions, one being to go into the loop's control box
(the box used in the shack to tune the loop) and by adjust a certain resistor I could slow the  tuning down and maybe fine tune the loop to a lower SWR.  He also suggested reshaping the coupling loop that is inside the mag loop. I decided to give the resistor a go first. It's much easier for me to open the control box and adjust a variable resistor than to open the loop up and start adjusting the coupling loop and put the loop cover on and give it a go.
What was left of battery holders
I was able to adjust the resistor to the point were the tuning light was flashing about 3-4 times per second as Paul had informed me that his loop had been adjusted to. Once done I gave the tuning process a go and it turned out the lowest SWR I could get was 1.7! I tried moving the loop all over the balcony and it made no difference. I found at a slower speed the SWR would jump very fast from 2.0 to 1.7 and bang back up beyond 1.7. It's not that Pauls suggestion was incorrect but these loops are very funny bunnies. I did take a picture of where the variable resistor was before I began the adventure and I decided to put it back to that adjustment. Low and behold I was back to the 1. 4 SWR. If I want  my next adventure will be to adjust the coupling loop inside the antenna housing BUT I just don't want to get myself in a "I wish I just had let things well enough alone" situation. I did find when I opened up the mag loop control box the battery hold turned very brittle and was falling to peaces as I handled it. The control box can run off batteries if need be but not this one until I replace the holders. With the Christmas season on us I will once again have to ask my readers to hold on with regards to the Sun SDR2 Pro SDR radio update.


  1. Morning Mike,
    Sorry to hear that the SWR situation isn't improved. I may have led you astray. I didn't mean to imply that adjusting R26 could impact the SWR just that I find it easier to stop the motor at SWR minimum.

    Reshaping the coupling loop will definietly affect SWR and your idea of taking a picture before hand is a good idea.

    Which meter are you using to read the SWR? The cotroller, an external tuner or the radio? The meter on the controller is notoriously inaccurate. I find it varies widely with the meter on my Yaesu FTDX 1200 and my Vectronics tuner.

    Oh the joys of small transmitting loops.
    Good luck and Best wishes for the Holiday Season.
    Paul VA3ZC

    1. Good morning Paul and very nice to hear from you. I think over the holidays I may give the reshaping of the coupling loop a go. The SWR meter I use is on my Elecraft KX3. I used to have an LP-100 and for some odd reason sold it. I agree the meter on the control box is not reliable at all.
      All the best to you and your family over the holidays.