MFJ 1786 Magnetic Loop Modification


I bought at used MFJ 1786X (240v version) from Tony G3RGQ, I have been after a magnetic loop for quite a while, so when the opportunity presented itself to buy one at a reasonable price, I went for it.

The 1786 covers from 30m through to 10m, looking on Youtube was a modification to increase coverage into the 40m band.

After looking at the video it was off to eBay:

  • K15U-1 47pF 10kV 8kvar Capacitors including delivery from Ukraine £13.57.
  • Vacuum Relay 10kV 3A including delivery from Poland £23.53.

Opening the case and looking at the butterfly capacitor, I located a way to securely mount the capacitors and relay.


I had some left over copper sheet, so I cut a 25mm wide strip 100mm long and used this as the capacitor linking strap and mounting brackets, making sure I had a solid connection to one side off the butterfly capacitor.


The relay was mounted to the opposite side of the butterfly capacitor using a similar strap to that on the capacitors. To avoid straining the glass relay switch connection, I used some RG213 braid to link the capacitors connection to the relay, I deliberately didn’t secure the base of the relay to the fibreglass insulating sheet, this is to make the relay ‘floating’ in an attempt to reduce the chances of relay breakage.


I installed a ‘flywheel’ diode across the relay coil (coil voltage  27VDC) to protect the operating switch in the shack.

Once the loop case was refitted, the modification was tested and works fine, many thanks to the Youtube author for sharing his practical experience.

Update 1 June 18

I bought a offset pole and 9 x 9 bracket to mount the magloop on, the existing 6 x 6 bracket was too undersized and slightly in the wrong position, the new bracket was fixed with M10 sheild anchor rawlbolts.

The tricky bit was getting it mounted due to it being top heavy compounded by a cranked pole, the solution was to use a pully arrangment from the mast to take the weight to help get me up the ladder and also position the pole to put the ‘U’ bolts in 🙂

loop up

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One thought on “MFJ 1786 Magnetic Loop Modification”

  1. Have you seen a video and it looks good, I’m going to apply that modification to my mfj-1786.
    I am also looking for vacuum relay and the capacitors.

    Greetings Dick Meijer from East part Holland place Almelo

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