This is a brief tutorial on building a 9 to 1 UNUN ( unbalanced to unbalanced transformer ) Which can be used with a random length wire antenna. It is a very simple antenna that is easy to use. The 9 to 1 UNUN is also DC grounded, which can help to reduce interference and background noise.
As you can see from the above diagram, the 9 to 1 UNUN requires a trifilar windings ( 3 ). These are carefully wound around a ferrite toroid.
Here is our toroid and windings. I have used a T200-2 ferrite toroid, which is comfortably good for HF use at up to 100 watt. I mainly operate below 100 Watt. If you require more. You can get a larger toroid such as the T300-2.
Above is a close up shot of the same thing. I have used brown, green/yellow and blue wires to demonstrate the three different windings. I have placed cable clips between each set of three. to keep them apart.
The above image shows the connections. The end of the brown wire, which is Winding 1, is connected and soldered to the start of Winding 2.( Green/yellow wire ) The end of the green/yellow wire, which is Winding 2, is connected to the start of the blue wire. This is then taken out as a fly lead. It will eventually connect to the Radio equipment.
Now we have our windings and connections. We are left with three fly wires. The first is from the start of Winding 1, the brown wire. The second is from the start of Winding 3, which is the blue wire. The third is from the end of Winding 3, the blue wire.
Now we are mounting it in a box. I have chosen to use large screws and wing nuts. This will make is easy to connect wire antenna elements. The end of Winding 3, the blue wire. Is soldered to the Ground screw. You will need to use some flux. to make sure the solder sticks to the screw. If you don’t. It may fail. The ground screw is taken out of the side.
In the above image we can see the Start of Winding 1, the brown wire. Connected to the antenna screw. Again, flux should be used to ensure the wire is well connected. We can also see the SO239 plug with its connections. The blue wire from the end of winding 3 and the wire from the start of winding 3.
I have used silicone sealant to waterproof the screw terminals and hold the transformer in place.
The above image shows the ground screw and SO239 waterproofed with silicone sealant. The box is IP56 rated. so the whole thing can live outside if required.
The above image is the completed unit. If you use silicone sealant make sure you give it time to dry properly before closing the box.