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.
Mike Weir, VE9KK, is a regular contributor to and writes from New Brunswick, Canada. Contact him at [email protected].

2 Responses to “Tweaking the MFJ 1788 mag loop”

  • Dave K8RDG/4:

    Hello Mike,

    I realize I’m over 3 years late in commenting, but I came across your posting regarding the MFJ-1788 mag loop antenna. I, too, have the 1788 and like you, I don’t want to mess with anything for fear of making it worse. I have it configured horizontally on my condo 3rd floor balcony. (See my K8RDG/4 QRZ page for photo.) I only work FT8 or FT4 at 25 watts collecting grid squares. In this configuration I have the following SWR readings for what it’s worth to anyone:
    40M 2.0:1 3.5W Ref.
    30M 2.0:1 3.0W Ref.
    20M 1.2:1 1.0W Ref.
    17M 3.0:1 6.5W Ref.
    15M 2.9:1 4.5W Ref.
    While QSOs are VERY challenging and often frustrating, I’ve been able to work several countries in Europe, as well as European Russia, South Africa, and even Australia, mostly at night on 40 and 30 meters. A lot has to do with propagation, of course, and the other stations’ antennas, but even at these less than ideal SWR readings and challenging conditions, the antenna does seem to perform and is a good compromise for a balcony situation. (Being so close to the Atlantic Ocean may help some too.)

  • Mike VE9KK:

    Dave no problem at all as they say better late than never. I visited your QRZ page and first off I want to say the warm sunny pictures look great! Right now we are in the middle of a snow storm. The average storm this year has been in around 30-60cm for each storm.
    I am no longer using the MFJ 1788 actually it has been sold. I have since retired and we have moved to the Canadian Maritime. I now have the room for an End-fed antenna that is working just great. As for the loop antenna it is ideal for the digi modes. It really never has to be returned other than when you change bands. I did get the hang of the turning and could do it fairly fast.
    For a balcony it’s a great antenna and gets you on the air and fills the void of a ham radio fix! Great to hear from you Dave and thanks for the comment.

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