The Autotransformer (Voltage Unun transformer) explained

The Autotransformer (Voltage UNUN Transformer) is a voltage-mode impedance matching transformer designed to efficiently match unbalanced loads to unbalanced sources. Unlike current-mode devices such as Guanella baluns, autotransformers operate in voltage mode, providing highly effective impedance transformation but lacking common-mode suppression in standard single-wire winding configurations. However, when wound bifilar or trifilar, they can exhibit some level of common-mode rejection, improving RF performance in practical installations.

Key Characteristics of an Autotransformer (Voltage UNUN)

• Type: Voltage transformer (UNUN – Unbalanced to Unbalanced)
• Purpose: Impedance transformation in unbalanced RF systems
• Core Concept: A single tapped winding on ferrite core
• Common Ratios: 4:1, 9:1, 12:1
• Best For: End-fed antennas, random wires, EFHW matching

How the Autotransformer (UNUN) Works

• Primary and secondary share a winding, with taps setting the ratio
• Voltage transformation occurs via inductive coupling
• Efficient for step-up/step-down matching, but does not enforce current balance
• Additional ferrite chokes often required to control common-mode currents
• Bifilar or trifilar winding improves CMR and reduces leakage inductance

Enhancing Common-Mode Rejection

Single-wire wound autotransformers are prone to common-mode currents, but by twisting bifilar or trifilar conductors on the core, tight magnetic coupling is achieved. This improves noise rejection, lowers leakage inductance, and provides some suppression of common-mode RF without sacrificing efficiency. This makes such designs valuable in high-power HF systems where both impedance matching and noise control are required.

Common Autotransformer Types

4:1 Autotransformer UNUN

• Purpose: Match 200 Ω antennas (e.g., OCF dipoles, verticals) to 50 Ω
• Construction: Single tapped winding on ferrite core
• Note: May require an external choke for common-mode control

9:1 Autotransformer UNUN

• Purpose: Match 450–900 Ω random wires or EFHW to 50 Ω
• Effect: Efficient but needs counterpoises or grounding to avoid RF feedback
• Limitation: Can exhibit common-mode resonance unless choked

12:1 Autotransformer UNUN

• Purpose: Match very high impedance wires (600–1200 Ω) to 50 Ω
• Limitation: Higher ratios increase risk of RF feedback and grounding sensitivity

Autotransformer vs Guanella vs Ruthroff

When to Use an Autotransformer

• For end-fed or random wire antennas needing impedance matching
• When voltage transformation is the main requirement
• Where a simple, efficient solution is preferred
• With bifilar/trifilar winding for improved CMR
• When common-mode issues can be controlled separately with a choke

Final Takeaway

Autotransformer UNUNs provide efficient impedance matching for end-fed and random wire antennas. They are simpler and more efficient than Guanella transformers, but do not inherently suppress common-mode currents. Adding bifilar or trifilar windings improves their performance, but external chokes are often still required. Careful grounding and layout help avoid common-mode resonance effects. For many HF setups, they remain the most effective and compact solution.

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