Making a 9:1 Unun for Amateur Radio

A 9:1 unun (unbalanced to unbalanced transformer) is a useful device for matching high impedance antennas, such as end-fed wires, to a standard 50-ohm coaxial feed line. This article will guide you through the process of making a 9:1 unun, including the materials needed and step-by-step instructions.

Materials Needed

1. Ferrite Toroid Core: A common choice is the FT-240-43 toroid.
2. Enamel-Coated Wire: AWG 18 or similar.
3. Heat Shrink Tubing: For insulation.
4. Plastic Enclosure: To house the unun.
5. Connectors: SO-239 or similar for coaxial connections.
6. Soldering Iron and Solder: For connections.

Step-by-Step Instructions

1. Prepare the Toroid Core:
   • Take the ferrite toroid core and wind three lengths of enamel-coated wire around it. This is known as a trifilar winding. For a 9:1 unun, you typically need 9 turns. Ensure the turns are evenly spaced.
2. Wind the Wires:
   • Cut three equal lengths of enamel-coated wire (about 3 feet each). Twist these wires together to form a trifilar winding. Wind this trifilar bundle around the toroid core 9 times.
3. Connect the Wires:
   • Identify the ends of the wires. Label them as A1, A2, B1, B2, C1, and C2. Connect A1 to B2, B1 to C2, and C1 to A2. This configuration creates the 9:1 impedance transformation.
4. Assemble the Unun:
   • Place the toroid core with the wound wires inside the plastic enclosure. Secure it with cable ties if necessary. Attach the connectors (SO-239) to the enclosure and solder the wire ends to these connectors. Typically, the junction of A1 and B2 is connected to the antenna terminal, and the junction of B1 and C2 is connected to the ground terminal. The remaining wire ends (A2 and C1) are connected to the coaxial connector.
5. Insulate the Connections:
   • Use heat shrink tubing to insulate the soldered connections and prevent short circuits.
6. Test the Unun:
   • Before using the unun, test it with an antenna analyzer to ensure it is working correctly. The impedance should be close to 50 ohms across the HF bands.

This diagram shows the toroid core with the trifilar winding. The connections are made to the coaxial cable’s inner conductor and shield.

Conclusion

Building a 9:1 unun is a rewarding project that can enhance your amateur radio setup by ensuring efficient transmission and reception. By following these steps and using the right materials, you can create a reliable unun that will serve you well in your radio communications.

If you have any questions or need further assistance, feel free to ask! Happy building!