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Project: 9:1 UnUn Transformer

Project: 9:1 UnUn Transformer

I really enjoy doing homebrew and DIY projects around the shack and I recently found a 9:1 UnUn kit on Amazon for pretty cheap. So I bought it and adventured into my first 9:1 UnUn transformer build. In short, I am very impressed with how easy this was to build and how effectively it matches the 450 Ohm impedance found at the end of a proper length random wire antenna.

I followed a guide I found via a quick Google search which is VERY good at explaining the science and how the transformer works.

The kit came with a long length of magnetic wire (thin copper wire covered with a lacquer based insulator), toroid,
SO-239, a box, and some electrical connectors.

The first thing I did was cut the wire into 3 equal portions and removing the lacquer on only one end. Then I used a drill to straighten the wire. If you don’t know how to straighten wire with a drill, there are plenty of guides on Google. I taped the 3 lengths of wire together so they stayed flat together and didn’t cross. Keeping the wires from crossing is very important. Then I wrapped the wire 9 times through the toroid, ensuring the wires didn’t get crossed anywhere and the wraps were evenly spaced out. Try using a zip tie on your first wrap if you find keeping everything straight is a challenge.

After getting 9 wraps, cut the excess wire off, but set aside the wire you cut off. Remove the lacquer from the end of the wire and verify you have electrical conductivity from end to end with a voltohm meter. The extra length of wire we set aside will be used to connect the shell of the SO-259 to the ground terminal.

After wrapping the wire, you need to connect the 3 wires to the appropriate things. There are 3 wires with 2 ends. I’ll refer to the wires by number and the ends by letters a and b.

Wire #End AEnd B
1SO-239 shell and GND terminalSO-239 center
2SO-239 centerWire 3 End A
3Wire 2 End BANT terminal
9:1 Transformer wiring connections

I recommend dry fitting everything and then soldering after finding a good arrangement. When soldering into the
SO-239 center, be sure your soldering iron is hot. It is counter-intuitive but a hot soldering iron will prevent the dielectric from melting. Once you have everything soldered up, close up the box and attach your antennas and counterpoise if you choose to use one.

Remember the transformer should be as high off the ground as you can get it. The highest amount of RF will come from the first 10% of your antenna so you want the transformer and first chunk of wire as high as you can get them. The tail of the wire antenna is more negotiable.

2 thoughts on “Project: 9:1 UnUn Transformer

    • Author gravatar

      So, did your 107′ long wire come from this:


      How did you know that the R = 450 ohms?


      • Author gravatar

        Hey Karl… No I didn’t use that page for my reference on the random wire. The guide I followed is already linked in the 2nd paragraph. I believe the guide I linked above goes into the science of it, but the length of a random wire is very intentional to result in the approximate same mis-match on all bands. This happens to be 9:1 SWR or 450ohms. The 9:1 transformer matches the impedance so the antenna becomes a much better match on all bands. From there, your radio’s antenna tuner can do the rest.

        Like most compromise antennas, you give up some things, in this case, radiated power… to gain other things, like multi-band ease of use.

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