2024 EFRW Transformer

Completed board Completed board


  • Bare PCB
  • BNC connector
  • Toroid T94-6
  • 48" of 22AWG enamel coated wire


  • flush cutters
  • mini pliers
  • utility knife
  • ruler
  • soldering iron, set to 350C
  • spudger
  • heat safe work surface (or board holder)


bare board bare board

Winding the toroid

We will construct a 12 Turn trifilar winding.

  1. Cut the enamel coated wire into 3 equal lengths of 16". These will be your winding wires.
  2. Take one length of wire, bend it in half to create a U shape.
  3. Hook this U shape over the toroid to start your winding.
  4. Wind each half of the wire around the toroid.
    • Each pass of the wire through the toroid counts as 1 turn.
    • You want 12 turns around the toroid with each wire.
    • Be careful to not scrape off enamel coating when pulling the wire through the toroid.
    • Press the wire tightly against the toroid.
  5. Repeat the above process for each of the two remaining wires.
    • These wires should be wound between the turns of the previous wire(s).
    • Do not cross the wires over each other. Each wire should have its own separate, parallel path around the toroid.
  6. Once all wires are wound, double-check your work. You should have 12 turns per wire, for a total of 36 turns.

Testing and soldering the toroid

Wires through the back of the board Wires through the back of the board

Construction and testing:

  1. Feed the ends of the wires through A1, B1, C1, and A2, B2, C2 on the other end. DO NOT TRIM the wires yet!
  2. Remove a bit of the enamel from the ends of the wire. Where the wire is NOT in contact with the through hole it is passing through.
  3. Use an ohm meter to check that you have continuity between A1 and A2, B1 and B2, and C1 and C2.
  4. Now, carefully trim the wires, and remove enamel so that the wires will contact the through holes.
  5. Solder connections.
  6. Test continuity from BNC center to J4 - Antenna.
  7. Test continuity from BNC ground to C1 and J3 - CP.

Note: the spudger can be used to evenly space the wire around the toroid.


  1. Solder the BNC to the board.
  2. Check the transformer with a nanoVNA and 450 Ohm resistor.

transformer connected to nanoVNA transformer connected to nanoVNA