Monday, 3 September 2012

Building a new matching transformer

Having a recent antenna failure I decided to rebuild the matching transformer that feeds my end fed wire. This design comes from the “Emergency Amateur Radio Club” based in Honolulu ( This project creates a trifilar wound 9:1 UNUN (unbalanced to unbalanced) toroid matching transformer. This will match the high input impedance of an end fed wire into a range that most antenna tuners can handle. The designer says it should handle 100 watts of power.

The key parts are: 
  • A small plastic box (something that can be made waterproof)
  • A powdered iron toroid (EG T106-2)
  • Three 20 inch pieces of 22 gauge solid insulated copper wire (in three different colours if possible)
  • An RF panel mounted connector (BNC or S0-239)
  • You will also need the relevant screws, nuts and solder tags etc, to fit the package together.
I first drilled the box and fitted the connectors to ensure that everything would fit correctly. 

Next job was to wind the toroid. As I didn’t have access to different coloured copper wire, I instead fitted coloured sleeves to the ends of each cut length.
It is important to keep the wires in the correct order and not cross them as they are wound. They should be appear in the following order (left to right), Green, Black and Red. Wrap these round the toroid 9 times as shown in the picture below. I found that the first few windings were a little loose so I added a couple at the end and unwound the loose ones from the start. That way I still had 9 turns but a much better spread and even tightness. After winding, ensure that the wires on the last turn are in the correct order. If they aren’t then they must have twisted at some point.


It is important to get the next sections correct else you will have to start again with new wire. Holding the toroid as in the above picture, solder the “Left Black” wire to the “Right Red” wire. I found the best way was to first scrape off the insulating coating from both wires where they would touch. Then twist both wires together with a single turn. This will pull both together and make a sound mechanical joint. Solder the two wires and remove the trailing ends. 

Next you need twist the “Left Green” wire and “Right Black” wire together. First clean off the insulation before twisting together. This joint needs to be attached to the centre connection of the RF connector fitted to the box. Before cutting off the excess wire. Place the toroid into the box to see where the cut should be made.

The (Left Red) wire should be cleaned and soldered to the connector on the box that the wire antenna will be connected. The remaining (Right Green) wire should be cleaned and soldered to the outer ground connection of the RF connector.


Now inspect the solder joints for any dry joints and ensure the box is free of any solder splashes etc. I used a silicon grease spread around the lip of the box and lid before screwing the two together. In the past I have used a silicon sealant, the type used in the bathroom. If you use this type and it has the vinegar type smell, keep this away from and metal as it tends to promote corrosion.
In the next few days I hope to get up the ladder and fit the unit to my mast. I will take additional pictures and post these in the next week or so. Please ask any related question and I’ll try to assist. Further details including a kit of parts are available from the EARC website:


  1. In the second photo the color order is shown as : Black, Green, Red.
    The test describing the photo says: "They should be appear in the following order (left to right), Green, Black and Red."

  2. Well spotted. I think I just took a random picture of the wires and they are not in the same order as the text. The text is correct, the picture is not.