4:1 UN-UN Transformer for End-Fed Antennas

A 4:1 UN-UN (Unbalanced-to-Unbalanced) transformer is a versatile impedance-matching device used in many different antenna systems, not just end-fed wires. The diagram above shows a classic broadband design using a T200-2 toroidal core with bifilar winding, suitable for a wide range of HF antenna applications.

What Does a 4:1 UN-UN Do?

A 4:1 UN-UN transforms impedance by a factor of four, typically converting:

• 200 Ω ↔ 50 Ω
• 300 Ω ↔ 75 Ω (in some receiving systems)

Because both sides are unbalanced, it interfaces naturally with coaxial cable and antennas that do not require balanced feed.

4:1 UN-UN Construction Overview

• Core: T200-2 powdered iron toroid
• Winding: 19 turns of bifilar (red/black) insulated wire
• Feed Connector: SO-239 for 50-ohm coax
• Ground / Counterpoise: Required for current return

This design offers broadband HF performance, low loss, and good power handling.

4:1 UN-UN Connections and Wiring

• Radiator Antenna Connection:
  The red wire output is connected directly to the radiating element (long wire or end-fed antenna).
• Coaxial Input:
  An SO-239 connector is used for the 50-ohm coaxial cable from the transceiver (RTX).
• Ground / Counterpoise:
  The coax shield is tied to a ground or counterpoise wire, which is essential for proper current return and stable SWR.

Step-By-Step Assembly

1. The Winding: Take your bifilar (two-conductor) cable and wrap it 19 times around the T200-2 toroid. Ensure the windings are tight and evenly spaced. 19 turns on a T200-2 core provides excellent inductance for the HF bands.
2. The Tap: Look closely at the diagram. You aren’t just connecting two ends. One end of the “Black” wire connects to the Ground/Shield of the SO-239, while the other end of that same black wire joins the “Red” wire to create a jumper to the Center Pin.
3. The Output: The remaining Red wire lead goes directly to your Radiator Antenna (the long wire).
4. Counterpoise: For the best results, connect a ground wire or counterpoise to the ground lug of the SO-239. This provides the “other half” of the antenna system and reduces RF in the shack.

Important Modification for FT240-43

While the original diagram specifies a T200-2 (Iron Powder), many modern antenna designers prefer the FT240-43 (Ferrite) for 4:1 Un-Uns and Baluns used on the HF bands (1.8–30 MHz).

If you swap the T200-2 for an FT240-43, do not use 19 turns. Because the ferrite has much higher permeability, 19 turns would create excessive capacitance, which will ruin your SWR on the higher bands (10m/12m).

• Recommended Turns for FT240-43: Use 9 to 12 turns of bifilar wire instead of 19.
• Winding Style: You can still use the same bifilar (side-by-side) method shown in your image.
• Use the T200-2 if you already have it and primarily want to operate on the mid-to-high bands (20m–10m) where its lower inductance is less of an issue.
• Upgrade to the FT240-43 if you want a true “all-band” transformer with better performance on 40m/80m and higher overall efficiency.

Why the FT240-43 is often the better choice

1. Magnetizing Inductance: To work effectively as a transformer, the windings need enough “choking” impedance (usually 500 ohms) to prevent the core from becoming part of the antenna. The low permeability of the T200-2 makes this difficult to achieve on lower bands like 80m or 160m without an impractical number of turns. The FT240-43 hits this target easily with fewer turns.
2. Lower Loss: Ferrite Mix 43 is designed for broadband transformer use. In a 4:1 configuration, it typically offers lower insertion loss than iron powder across the standard HF spectrum.
3. Physical Size: The “240” in FT240 means it is 2.4 inches in diameter (slightly larger than the 2.0-inch T200). This extra mass helps with heat dissipation if you are running high power (100W+).

Related Posts

• 9:1 UN-UN Long Wire Transformer for HF Antennas
• 1:49 UNUN using two stacked FT240-43 cores for endfed halfwave antenna (February 18, 2022)
• Aperiodic Multiband Wire Antenna