Long wire antennas remain one of the simplest and most effective solutions for HF reception and transmission. However, directly feeding a long wire into modern 50-ohm coaxial systems results in severe impedance mismatch, high SWR, and unwanted RF currents on the feedline. The 9:1 UN-UN long wire transformer, shown in the attached image, is a widely used solution that bridges this mismatch efficiently and economically.
This article explains the construction, operation, and real-world usage of a 9:1 impedance transformer (UN-UN) for long wire and random wire antennas, making it ideal for SWL listeners and amateur radio operators.
What Is a 9:1 UN-UN Transformer?
A UN-UN (Unbalanced to Unbalanced) transformer is used when both the antenna and the feedline are unbalanced systems. In this design, a toroidal ferrite core is wound with multiple turns of insulated wire to form a broadband impedance transformer.
The 9:1 ratio converts a high-impedance long wire (typically 300–600 ohms or higher) down to a value closer to 50 ohms, which is suitable for coaxial cable and modern HF receivers or transceivers.
9:1 UN-UN – Understanding the Connections
The image clearly illustrates the three essential terminals of a 9:1 UN-UN:
- Antenna (Ant.) – Connected directly to the long wire element
- Center (Cent.) – Connected to the center conductor of the coaxial feedline
- Ground (GND) – Connected to an RF ground, counterpoise, or earth system
The transformer windings on the ferrite toroid provide broadband matching across a wide HF range, typically from 1.8 MHz to 30 MHz, depending on core material and winding technique.
Long wire antennas are inherently high-impedance and frequency-dependent. Without matching, most of the RF energy is reflected back to the source. A 9:1 UN-UN offers a practical compromise that significantly reduces SWR across multiple bands without the complexity of a full antenna tuner.
This makes it especially popular for:
- HF shortwave listening (SWL)
- Multiband random wire antennas
- SDR receivers and QRP rigs
- Portable and field radio setups
Ferrite Core and Winding Considerations
The performance of a 9:1 UN-UN depends heavily on the ferrite core material and winding layout. Commonly used cores include FT-140-43, FT-240-43, or similar ferrites optimized for HF frequencies.
Uniform winding, tight coupling, and good insulation help minimize losses and extend low-frequency performance. The red and black wires in the image indicate multiple turns forming the transformer’s primary and secondary windings.
Grounding and Counterpoise Importance
Although often overlooked, the ground or counterpoise connection plays a critical role. A poor ground can result in:
- Increased noise pickup
- RF on the coax shield
- Reduced antenna efficiency
A short counterpoise wire, ground rod, or radial system significantly improves performance, especially on lower HF bands.
A counterpoise is an electrical network of wires or metal acting as a substitute for a ground connection in antenna systems
Performance Considerations
While the 9:1 Unun is incredibly versatile, remember that it is not a “magic bullet.” Because it is an unbalanced system, the outside of your coax shield may carry RF current. To keep your signal clean:
- Always use a 1:1 Choke (Common Mode Filter) on the coax line before it enters your radio room.
- Ensure your ground/counterpoise is as robust as possible to improve radiation efficiency.
While a 9:1 UN-UN does not provide perfect matching on all bands, it delivers excellent real-world results with minimal complexity. For transmit applications, power handling depends on core size and wire gauge, but many designs comfortably handle QRP to 100 W when properly built.
For best results, an external antenna tuner can be added after the transformer to fine-tune SWR.
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