🛠️ DIY Project: Building a 9:1 UnUn Using a Toroid or a Ferrite Rod from an Old Radio Receiver
Hello, fellow hams!
Following our previous discussion on optimal lengths for a "Random Wire" antenna, today we will dive deep into its heart—the 9:1 UnUn (Unbalanced-to-Unbalanced) transformer.
Its job is to step down the high impedance of the wire (typically between 300 and 900 ohms across the HF bands) close to the standard 50 ohms that our transceiver expects to see.
Here is a step-by-step guide to building one yourself using parts you probably already have lying around in your junk box.
🧲 Core Selection (Two Options)
Depending on your operating power and available junk box materials, you have two great choices:
Option 1: Ferrite Toroid (Classic and Most Efficient)
A toroid keeps the magnetic field tightly contained within itself, ensuring minimal loss and zero external leakage.
- For QRP (up to 5-10W): Use a small Amidon T80-6 (yellow) or T106-6 iron powder core. Ferrite rings harvested from old PC power supplies (yellow/white or solid black/grey) will also work, though Amidon cores are preferred for predictable HF behavior.
- For 100W (e.g., FT-450D): You must use a larger, robust ferrite toroid with Mix 43 material. The FT140-43 or FT240-43 (solid black) are the industry standards.
Option 2: Ferrite Rod from an Old Radio (Excellent Alternative for QRP/RX)
If you have an old dismantled broadcast radio receiver (like old European/Soviet Selena, VEF, Sokol, or any Medium Wave/Long Wave radio), you will find a ferrite rod inside that was used for the MW/LW internal loop antenna.
- Characteristics: This rod works perfectly across the HF bands (especially up to 14-21 MHz). Because the magnetic circuit is open, it has slightly lower efficiency than a toroid, but it is ideal for SWL (listening) and QRP transmitters up to 5-10W (like the FT-817).
- Preparation: Cut a piece of the rod to a length of about 6 to 10 cm (approx. 2.5 to 4 inches). You can snap it cleanly after scoring it lightly with a file.
🧵 Required Materials for Winding:
- Wire: Solid enameled copper wire (magnet wire) with a diameter of 0.6 mm to 1.0 mm (approx. 22 to 18 AWG). If you don't have any, you can strip individual strands from a standard LAN internet cable (UTP)—their insulation can handle 10-15W of power with ease.
- Enclosure: A small plastic project box (like an outdoor electrical wall box) to keep it weatherproof if the UnUn is staying outside.
- Connectors: One SO-239 (female UHF chassis mount for the coax) and two binding posts/screws (one for the long wire, one for the counterpoise).
🛠️ Step-by-Step Winding (Trifilar Method)
Whether you use a toroid or a ferrite rod, the winding method is identical: trifilar (using three wires simultaneously).
- Prep the wires: Cut 3 identical pieces of wire, each about 40-50 cm (15-20 inches) long.
- Twist them (optional): You can lightly twist the three wires together (like a loose braid) to keep them neat, but don't make it too tight. To make things easy, mentally number them or use three different colored wires (e.g., Blue, Green, Orange from a LAN cable).
- Number of Turns:
- For a Toroid: Wind 9 to 11 turns of the twisted bundle around the core. (Every time the bundle passes through the center hole of the toroid counts as 1 turn). Space the turns evenly around the ring.
- For a Ferrite Rod: Wind 10 to 12 turns tight and close to each other in the middle section of the rod. Secure the ends with electrical tape or masking tape so they don't unwind.
🔌 Connecting the Leads (The Critical Part!)
You now have a total of 3 wires at the start (A, B, C) and 3 wires at the end (A', B', C'). We need to connect them in series to create an autotransformer.
Follow this configuration:
- Connection 1: Connect the end of the first wire (A') to the start of the second wire (B). Solder them together.
- Connection 2: Connect the end of the second wire (B') to the start of the third wire (C). Solder them together.
You are now left with two free single ends: the start of the first wire (A) and the end of the third wire (C').
Connecting to the Enclosure Hardware:
- Chassis Ground (GND): Connect the start of the first wire (A) to the ground/shield flange of the SO-239 connector. The Counterpoise binding post also connects directly to this exact same point.
- Coax Center Conductor: Connect Connection 1 (A'+B) to the center pin of the SO-239 connector (this is your 50 Ohm input).
- Long Wire (Antenna): Connect the final free end (C') to the antenna binding post (this is your high-impedance 9:1 transformed output).
- Note: Connection 2 (B'+C) remains isolated. Cover it with heat-shrink tubing and leave it floating inside the box. It simply serves as the internal middle tap of the transformer.
📊 How the Impedance Transformation Works
Because we connected the three windings in series, the turn ratio between the input (coax) and the output (antenna) is 1 to 3. Since impedance changes with the square of the turns ratio (3² = 9), we achieve the exact 1:9 transformation ratio we need.
If your random wire antenna presents an impedance of roughly 450 ohms on a certain frequency, the UnUn will step it down precisely to 50 ohms, making your radio happy and keeping your SWR low.
Good luck with your builds, everyone! Making your own UnUn is a fantastic, cheap way to learn how magnetic circuits function in radio equipment. If anyone gets stuck tracing the wires, just drop a comment below—we'll help you out!
This article was co-authored and formatted with the help of AI (Artificial Intelligence).
73! de LZ4NS
