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. K4SC

    K4SC Ham Member QRZ Page

    I recently purchases a new MFJ 1786 loop which covers 10 to 30 MHz. I think I would have been better served getting the 1788 model which 7 to 21 MHz.

    As I paid a lot of money for what I have, and MFJ does not provide schematics, I thought I would investigate what changes are necessary in this forum. My gut tells me it is a matter of more capacitance, which should be obtainable by paralleling a fixed capacitor across the variable motor-driven capacitor. If this is the solution, I recognize it will need to be a high voltage capacitor.

    Any one have a 1788 they could measure the maximum and minimum capacitance of the variable? I can do that with mine and we could swap data.
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    You would be better served by building another loop. Circumferance of the loop is what gives frequency range. Capacitor range can contribute to more range, but efficiency will be very low.

  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I only have the same loop you do and it works surprisingly well on 30m, which is the bottom end of its range. I only use it portable/camping so it's stored in the garage where it usually sits. However I think I recall the cap is 250 pF per section, differential. Never measured it, but in the old days they published a parts list.

    If that's the case, the 40m version might have 500 pF per section, but I really don't know.

    30m has less 'competition' usually so I'm satisfied with that being the low end for my little loop. The 40m loop isn't any bigger in diameter, but I'm sure has other changes. The cap does indeed have to handle high voltages, even at 100W it's likely well over a kV. If you want to experiment, I'd try using RF rated 5kV or 7.5kV doorknob caps.
  4. K4SAV

    K4SAV Ham Member QRZ Page

    Since the resonant frequency is inversely proportional to the square root of C, to go from 10.1 MHz to 7 MHz you would need to double the capacitance.

    I doubt that door knob caps would work for this. You will have to look at the kVA ratings for the caps, not just the max voltage and max current ratings. Since the caps will be delivering reactive power they are much more limited than they would be if they were operating in a circuit where they look like a very low impedance (plate coupling in an amp for example). A mag loop is going to have a lot of current plus high voltage, a worse case for reactive power.

    An example: A typical 4 ft diameter mag loop made from 0.875 diameter copper pipe, on 7 MHz with 100 watts should have about 4300 volts RMS across the cap and a current of 30 amps RMS. That's a reactive power of 129 kVA. An HT57 series doorknob cap (a big cap 1.9 x 1.2 inches) is rated for 35 kVA. Obviously the loop needs to be bigger, or run a lot less power if you are going to use doorknob caps.

    A lot of people run caps well past their limits in a circuit similar to this and get away with it for a while.

    Jerry, K4SAV
  5. PD7MAA

    PD7MAA Ham Member QRZ Page

    You can switch aprox. 250cm RG-213 (one side open ,up to 5000 volts RMS) as a capacitor over the build-in capacitor giving a 250 pF extra value. This works up to 100 Watts but efficiency on 40m will be about 5 % due to the small diameter of the loop. Still better than nothing...

    Have fun

    73 John
    Last edited: Dec 8, 2015
  6. AK5B

    AK5B Ham Member QRZ Page

    Another suggestion or two---add a fixed vacuum capacitor (much less expensive than a vacuum variable capacitor) to get the capacitance you'll need PLUS add some soft copper tubing to the MFJ loop. There's a video on You Tube of a VK ham who experimented with his 1786 like that---he used stainless "jubilee clips" (hose clamps) to bind two extra loops made of small diameter HVAC tubing so his loop would be more efficient as well as tune a bit lower---and it works FB. If you simply search magnetic loop or MFJ mag loop on You Tube you will run across the video sooner or later. It seems dead simple and effective way to go about what you want to do---but avoid doorknob caps as they are far less efficient than a vacuum cap besides being less able in the kVa department.

    73, Jeff
  7. K4SC

    K4SC Ham Member QRZ Page

    Thank you for all the responses. I had not considered the lowered efficiency, but after doing a bit of research on the MFJ loops I did find many comments stating the 1786 10-30 MHz was much preferred over the 1788 7-21 MHz because of the differences in efficiencies. I did see the YouTube video from down under and that is an intriguing idea which I may try.
  8. AF7ON

    AF7ON Ham Member QRZ Page

    I am somewhat skeptical that the efficiency of loop antennas is as low as some of the theoretical formulas suggest - I suspect they are quite a bit better than the 5% figures often quoted.
  9. PD7MAA

    PD7MAA Ham Member QRZ Page

    At 10m a 80 cm dia. loop has aprox. 90 % efficiency.
    At 40m however the same loop has only 5 % efficiency.
    The fun on ssb with this loop realy starts at 20m. with aprox. 30 % efficiency.

    73' John
  10. WA4SIX

    WA4SIX Ham Member QRZ Page

    You may suspect, but the formulas have most likely been proven out over the decades to be within a small +- factor.

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