I bought at used MFJ 1786X (240v version) from Tony G3RGQ (sadly now SK), 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 🙂
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
hola no encuentro dicho rele de vacio, podrias poner el link donde lo encontraste.
un saludo
joluvelo.vega@gmail.com
Hola, los relés se pueden encontrar en eBay y he adjuntado un enlace a continuación, solo busque el relé de vacío.
https://www.ebay.com/itm/2x-pcs-V1V-W1W-B1B-Vacuum-Relay-SPST-switch-Antenna-Tuner-RF/143494888029?hash=item2168f63a5d:g:M68AAOSwai5eFPdF
en ese enlace no envia a españa , una pena , no doy encontrado dicho rele
un saludo
https://www.ebay.es/itm/NOS-V1V-W1W-B1B-Vacuum-Relay-SPST-switch-Antenna-Tuner-RF-27V-24V-28V-/143293328833
me podrias decir si este valdri , pues los valores no son los mismos de la modificacion esta es 10A 1.5kW SPST.
Esto será perfecto, el relé manejará bien en exceso el poder de la capacidad de los bucles para transportar.
Muchas gracias por tu aportación
No hay problema, espero que todo salga bien para ti
y estes valdria cambia el ultimo valor de 10 a 8, no encuento los del articulo
K15U-1 47 PF 10 kv 8 kvar
un saludo y nuevamente gracias por tu atencion
El valor del relé no es tan importante, los condensadores se pueden encontrar aquí: https://www.ebay.com/itm/2x-K15Y-1-27pF-10kV-8-kVAr-HIGH-VOLTAGE-Doorknob-Ceramic-Capacitor -USSR-NOS- / 264530885426
Nuevamente muchas gracias
Un saludo
J Luis
73
Hello and thank you for your tutorial.
Do you know how to modify the MFJ-1786X antenna for 11 meters? I find that from 10 meters it loses a lot of efficiency.
Best regards
HI Niko,
Sorry I don’t know how to do that, however, others who read this may and can give insight on how to do it.
Greetings!
I made the modification with 3 – 50pf caps in parallel and it was right in the middle of the 40 meter band. Used some copper banding stand offs with holes in copper from the hardware store electrical or plumbing dept for the caps about 5″ long
What I did different is I skipped the magnetic relay and put a 300amp rotating DC switch inline & installed it on right side of the variable cap with hole in the back plastic with same copper strapping pieces, completing the circuit. So now I can just click / rotate the switch to get 10-30 and then 40 is the other selection. I also put a new 12vdc motor with .6 rpm reduction from e-Bay in a new mounting configuration with original plastic piece with hole that motor shaft goes through, glued, using some small pieces of plastic cutting board to stabilize & support motor to keep it in place and works well. I have the loop for vertical mounting but if on a balcony just use the switch with less wires, cleaner look when you want 40 meters. I appreciate your inspiring article.
Hi Rich,
Glad it all worked out, great idea to fit reduction gearing for fine control in getting that ‘sweet spot’
Put a 147pf cap in and a vacuum relay. I am going to use Cat 6 shielded cable to switch the 40M in. Mod works perfectly for 40M, great SWR almost 1:1. BUT (There is always one :-)) if I leave cat 6 shielded cable in and turn switch off the other bands don’t tune at all, if I remove Cat6 cable they are fine. Same if Cat5. It may behave when I have it installed correctly higher up, just bench testing at the moment with 5W. (Oh and BTW great write up….) Any thoughts?
73
Chris
M0YGH
Hi Chris,
Glad the mod worked, the cable I used to switch the relay was 2 core flex from Screwfix (927JX) also I didn’t ground the loop controller.
Hope you get it sorted.
Nothing grounded either. As an aside Those vacuum relays have a limit of some 2 to 3 Amps, the current flowing through the loop is some 50++Amps@ 100W, but not across the capacitor? (I know you don’t switch when its transmitting…. and voltage will be different across the caps because they are in Parallel ) https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx
I must admit to only using the loop for low power data mode, so never really stress tested the relay, (so far).
Hello
Allow me to introduce myself. I am F4MQS operator Niko and I recently obtained my licence in July 2025.
I have carefully read your blog, especially the section on modifying the MFJ 1786 antenna (https://www.chatteris.biz/blog/mfj-1786-magnetic-loop-modification/).
I have the same antenna and have started making the same modification, thanks to your tutorial and YouTube video.
For the moment, I don’t have the ‘Vacuum Relay 10kV 3A’, so I’m using a temporary connection with two wires and banana plugs.
I have a ‘150 PF 10 KV 75 KVAr k15Y-2 (к15у-2) HIGH VOLTAGE CERAMIC CAPACITOR’.
When I test it on 40 metres, everything works great, but, because there’s always a but, I lose the 10-metre band.
Even with the capacitor disconnected, I still lose the 10 metres. If I dismantle my entire setup, I get the 10 metres back.
Have you had the same problem? What did I do wrong?
I am asking for your help. As a young ‘HAM hobbyist’, I still have a lot to learn, and I would be really happy if you could help me make this modification.
Currently, I can operate from 40 to 160 metres with the gutter of my building, but the results are not as good as with my MFJ loop.