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Anyone modified an MFJ 1786 Tuneable Loop to work on 40 meters?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4SC, Dec 8, 2015.

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  1. KC9ZHR

    KC9ZHR Ham Member QRZ Page

    Well just like a ladder line doublet the rf has to go somewhere provided nothing is getting warm.......just might not be the direction we want or straight into the ground.
  2. KO6WB

    KO6WB Premium Subscriber QRZ Page

    The calculations for a 36" diameter loop for 40 meters, with 1" tubing is 7.3%, if well made.
    If it where made of 1" copper tubing then the efficiency jumps up to about 15%, again, if it's well made.
    That's competitive to most mobile antennas that are often used in a home station installation.
    That figure (15%) is down about 8db below a properly sized magnetic loop. That's slightly more than an S-unit.
    The capacitance between a MFJ-1786 and that needed to obtain resonance on 40 meters with it, is about an added 125pfd.
    Rated at greater than 4Kv. Doorknob capacitors have much too low of a Q to be efficient in such an application.
    It might be better to have a vacuum variable capacitor installed to cover the entire range from 5pfd to 250pfd.
    Interfacing copper to aluminum is always a problem unless done carefully. What might work one day may be a disappointment the next.
    It corrodes that fast, at times.

    Have fun
  3. AK5B

    AK5B Ham Member QRZ Page

    True about galvanic corrosion but a thin coating of Noalox should suffice for the copper clamped to the aluminum MFJ loop.

    As for efficiency of various sized loops much depends on the area of the loop circumference, the type of capacitor, the diameter of the element, whether it is copper or aluminum and whether or not it is continuous, bolted or soldered/brazed together.

    The purported efficiency of my 1.125" polished copper loop 39" in diameter ranges from a low of 68% on 20M and rises to 97% on ten meters and it's performance that I've observed gives me no reason to doubt those figures. I took considerable pains to ensure the lowest possible ohmic losses and those efforts have rewarded me well.

    On the other hand, a poorly constructed loop of the same dimensions wouldn't fare nearly so well---the devil is really in the details of construction.

    73, Jeff
    Last edited: Dec 10, 2015
  4. AF7ON

    AF7ON Ham Member QRZ Page

    I'm sure that attention to resistive losses is very important in small loops but I just wonder if the really high computed losses are real?

    The idea that 95% of the RF energy goes to heating the antenna just seems inherently counter-intuitive and I wonder if such losses have ever been measured rather than calculated theoretically. 100 watts should heat things up enough to measure!

  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    Yes, of course, there have been studies. These antennas were used at embassies, Canadian Navy & in other installations. That would be why we have the figures for losses.

    I would think that with all of the government money invested, it would be a science & not conjecture.

  6. AF7ON

    AF7ON Ham Member QRZ Page

    The only reference I have found to actual measurements was the presentation by G3LHZ, which discusses heat loss measurements made on copper magnetic loops in an article published in Radcom in 2004. These measurements showed heat losses on 1-meter diameter loops to be around 12% at 7 MHz - a thermal efficiency of closer to 90%. That's more what I'd expect and not in line with the more pessimistic theoretical formulas.

    However, G3LHZ goes on to discuss overall efficiency, including ground losses. These may be more significant than has been typically considered. I think we have some very complex issues here!


  7. KC9ZHR

    KC9ZHR Ham Member QRZ Page

    Well practically the time someone goes to the expense & trouble of a mag loop most if not all other options have probobly been exhausted. So "it is what it is" at that point. I imagine the only real tinkering you could do outside of elevation is maybe see how the different feedpoint methods perform.
  8. AF7ON

    AF7ON Ham Member QRZ Page

    It is also of interest to the original OP - if the MFJ is closer to 90% efficient at 40 m than 5% it's a project worth considering. I'd also consider a loop for portable use if 95% of its energy doesn't go into warming up copper tubing and worms!

  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    1. The site that you posted seems to be rather subjective.
    2. The sites that I posted for your reading have facts.
    You are NOT going to get 90% efficiency out of a 1 meter aluminum loop at 40 meters. It is just not going to happen.
    The site that you posted just specifies "Thermal loss". There is quite a bit more than that. You cannot determine RF efficiency with a thermal camera.

    When many writings agree, why use a single webpage that differs?

  10. AK5B

    AK5B Ham Member QRZ Page

    Bravo, Ed!

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