A brief overview of UNUN types and their applications

What is an UNUN?

An UNUN (UNbalanced to UNbalanced) transformer is used to match impedance between an unbalanced antenna and an unbalanced feedline (coax), improving efficiency and reducing losses.

Are UNUNs Voltage or Current Transformers?

Most UNUNs function as voltage transformers (Ruthroff or Autotransformer), similar to voltage baluns, because they rely on impedance transformation without enforcing equal currents.

• Voltage UNUNs (most common): Transform impedance but do not suppress common-mode currents unless designed as an autotransformer unun with a bi- or trifilar-wound or litz wire primary on a high-impedance ferrite core.
• Current UNUNs (less common): Some 4:1 UNUNs are built using a current-balun design to help suppress common-mode currents, though this is rare.

Autotransfer vs traditional Ruthroff

An autotransformer balun (autotransfer) and a traditional Ruthroff balun serve similar impedance transformation purposes but operate differently. The Ruthroff balun is a transmission-line transformer that relies on bifilar or trifilar windings on a ferrite core, utilizing separate primary and secondary windings to achieve impedance transformation with galvanic isolation. This makes it useful in applications where isolation is required to prevent ground loops or unwanted common-mode currents.

In contrast, an autotransformer balun (sometimes called an autotransfer or “auto-unun” in unbalanced systems) has a single winding with a tap, where part of the winding serves as both primary and secondary. This results in a more efficient design with lower losses and better high-frequency response due to reduced leakage inductance, but it does not provide galvanic isolation between input and output.

A crucial factor in common-mode rejection (CMR) is how the primary is wound. If a single wire is used for the primary, there is minimal capacitive and magnetic coupling between adjacent turns, which results in better CMR performance by reducing the ability of common-mode currents to propagate. However, when bifilar, trifilar, or litz wire is used, the increased magnetic coupling enhances common-mode impedance, making it easier for common-mode signals to propagate through the transformer, thus reducing CMR performance.

One effective solution to handle CMR issues while maintaining the efficiency of an autotransformer is to use a hybrid approach, where a current balun (choke) is placed in front of the unun. This current balun acts as a common-mode choke, suppressing unwanted common-mode currents before they reach the autotransformer, ensuring that the system benefits from both efficient impedance transformation and strong CMR performance. This hybrid approach is commonly used in high-power end-fed half-wave (EFHW) transformers and loop antenna feed systems, where CMR suppression is crucial to avoid unwanted noise pickup and RF feedback.

The choice between these designs depends on the application: Ruthroff baluns are preferred when isolation is needed, whereas autotransformer baluns are often used for efficient impedance matching where isolation is not a concern. However, in cases where both impedance transformation and common-mode rejection are needed, a hybrid design with a current balun ahead of the unun provides the best of both worlds.

Common UNUN Types & When to Use Them:

1. 9:1 UNUN

• Purpose: Matches high impedance (400–900Ω) random wire antennas to 50Ω coax.
• Use case: Random wire antennas with a tuner.
• Best practice: Requires a good counterpoise for optimal performance.

2. 49:1, 56:1, 64:1, 68:1, 70:1, 75:1 UNUN

• Purpose: Matches End-Fed Half-Wave (EFHW) antennas (~2.5–3.2kΩ) to 50Ω coax.
• Use case: EFHW antennas for multi-band operation.
• Best practice: Works best with minimal counterpoise or ground connection.

3. 4:1 UNUN

• Purpose: Matches End-Fed Off-Center, verticals, or monopoles with ~200Ω impedance to 50Ω coax.
• Use case: Ground-plane verticals or impedance matching in unbalanced systems.
• Best practice: Often used in conjunction with radials or a ground plane.

When to Use an UNUN Instead of a BALUN

• BALUN (BALanced to UNbalanced): Used for dipoles, loops, and other balanced antennas.
• UNUN (UNbalanced to UNbalanced): Used for end-fed wires, verticals, and other unbalanced antennas.

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