Want to homebrew a loop tuner box?

Discussion in 'Radio Circuits, Repair & Performance' started by K3RW, May 11, 2016.

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  1. K3RW

    K3RW Ham Member QRZ Page

    I've not seen anyone try, but it would seem to be a worthy project to homebrew a loop tuner box.

    The MFJ-936B and its brethern may be fine units, but it's main drawback is the capacitor range in pF and its power handling capability. A wider tuning vacuum-variable capacitor is one way around this. But they aren't cheap. I already have the main $$ item--a Soviet surplus vacuum variable capacitor 5-100pF with a 25kV rating (twenty five!). So in some ways I'm halfway there.

    Putting it in a box gives me the ability to use a turns counter and be able to make SWR adjustments on the box instead of on the rig. I could also dial it in and put it in the yard, and run higher wattages without having RF exposure issues.

    Looking at the MFJ box, its a bit unclear to me why there appear to be two air-variables (not sure what the smaller one is). One is for TUNING and the other is MATCHING. After that, it has a wattmeter, SWR meter, and some nice touches to it. A frequent complaint about it is the location of the coax input. Well, can we do better?


    I'm game for building one if someone could explain what would be involved in terms of that second air-variable. I'm assuming its not an inductor since I don't see the roller on it--but its a bit hard to tell. Supposedly the loop antenna is the inductor, so for matching purposes, I'm not sure what I am looking at. A lot of guys just adjust the SWR on their rig, tune for their point of interest, and never fool with these things.

    Overall the cost wouldn't be a lot less than buying one with a warranty, etc, but it would have decent coverage, lower losses, and QRO capability. Buy a higher pF rated VVC and you can run any band you want, even 160, if your budget allows it.
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    Why not build a proper loop & use the capacitor in the regular configuration?
    With https://sites.google.com/site/lofturj/to-automatically-tune-a-magnetic-loop-antenna
    You would spend about the same amount in parts & have something that:
    Controls 2 loops
    Auto tunes using modern CAT & similar signals from modern rigs
    Saves on walking outside to tune
    Allows a wide tuning range.

    I found that I have most parts already in stock here, but have a 10X12 shed full of parts.

  3. K3RW

    K3RW Ham Member QRZ Page

    Wow--that thing is cool. It looks really complicated to build though! The MFJ box looks like it has only a few components in it.

    One reason I'd like to homebrew something like the MFJ box is because I could make it more capable, and 'potentially' save a bit too. Minus the automated part, that's what I already have--a high voltage vacuum variable capacitor in a decent loop configuration. But for 40m, my highly efficient loop is huge, and it isn't self-supporting because the capacitor is so large and heavy. This is where turning the capacitor sideways and mounting it in the enclosure have an advantage. That, and not needing to find something to hang it on or bring a mast out into the field all the time.

    It would be a bit fun to have the MFJ box setup, but with much more capability, for portable. Ha, if I could find a broken one I could always swap parts! Properly done, mine would have a larger wattage handling capability, better matching potential, and a better safety margin. And then be able to stand it off and feed it with longer coax for blasting away. People complain about the coax input being close to the loop terminals--well, I could just redesign it to be on a different side of the enclosure.

    Problem is, without a schematic, all I have is a coax feed into a box with a VVC, and maybe throwing in a SWR meter. But the MFJ box has another tuning capacitor (or inductor?) and I'm not sure how that goes.

    It seems most people are trying to automate it, rather than what I'm describing. Minus QRO, I could do this on a table top--as well as stand it off and blast it away in the yard, and not need a mast either way.
  4. K3RW

    K3RW Ham Member QRZ Page

    I'm wondering if the reason I see what I think are two capacitors for the MFJ box--is that one is a coarse tuning cap, and the other is a fine one, placed in parallel.

    Here's another reason not to get a MFJ loop tuner box:


    I found this QRP loop tuner schematic. Now this looks promising. Maybe the VVC will bring it out of the QRP range.


    A slightly modified one with a current meter is here:

  5. AB8KG

    AB8KG Ham Member QRZ Page

    0D5733B6-1D91-48D0-BC01-15DBB2D2452A.png Here’s what found...
    K3RW likes this.

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