The Autotransformer (Voltage Unun transformer) explained
The Autotransformer (Voltage UNUN Transformer) is a voltage-mode impedance matching transformer designed to match unbalanced loads to unbalanced sources efficiently. Unlike current-mode transformers such as Guanella baluns, autotransformers operate in voltage mode, ensuring effective impedance transformation while lacking common-mode current suppression in standard configurations. However, an autotransformer can exhibit common-mode resonance (CMR) suppression capabilities if the primary is wound using bifilar or, even better, trifilar techniques, which can improve common-mode rejection.
Key Characteristics of an Autotransformer (Voltage UNUN)
- Type: Voltage transformer (UNUN - Unbalanced to Unbalanced)
- Purpose: Provides impedance transformation in unbalanced systems.
- Core Concept: Uses a single tapped winding on a ferrite core, making it simple and efficient for voltage step-up or step-down.
- Common Ratios: 4:1, 9:1, 12:1
- Best For: End-fed antennas, random wire antennas, and impedance transformation in RF systems.
How the Autotransformer (UNUN) Works
- The primary and secondary windings share a common winding with a tapped point that determines the impedance transformation ratio.
- Voltage transformation occurs directly through inductive coupling, adjusting the impedance seen by the coaxial feedline.
- Unlike Guanella transformers, which enforce current balance, the autotransformer focuses purely on voltage transformation.
- Standard configurations do not suppress common-mode currents, meaning additional ferrite chokes may be required to mitigate unwanted RF.
- Bifilar or trifilar winding on the primary can enhance common-mode rejection, reducing unwanted noise and improving CMR performance.
- Depending on the winding and grounding configuration, the autotransformer may exhibit common-mode resonance (CMR) effects, leading to unintended common-mode choking on the primary.
Enhancing Common-Mode Rejection with Bifilar and Trifilar Windings
When using a single-wire winding, common-mode currents can easily form along the length of the conductor due to the lack of a dedicated return path, making the transformer susceptible to RF interference. However, by implementing a bifilar (two-wire) or trifilar (three-wire) winding, the transformer’s ability to reject common-mode signals is significantly enhanced. In a bifilar winding, the two conductors are twisted together and wound around the core, ensuring tight magnetic coupling between the wires, which helps cancel common-mode currents by providing a more controlled impedance path for differential-mode signals. A trifilar winding further improves this effect by introducing an additional conductor, creating a more balanced environment that further reduces unwanted noise. The increased coupling between the windings also lowers leakage inductance, improving overall efficiency while providing some level of common-mode suppression. This technique is particularly useful in high-power RF applications where both efficient impedance transformation and common-mode noise reduction are critical.
Common Autotransformer Types
4:1 Autotransformer UNUN
- Purpose: Matches a 200Ω antenna (e.g., off-center-fed antennas, some verticals) to 50Ω coax.
- Use Case: Ideal for impedance matching in multi-band antenna setups.
- Construction: A single tapped winding around a toroidal ferrite core.
- Effect: Provides an efficient voltage transformation but may require an additional common-mode choke.
9:1 Autotransformer UNUN
- Purpose: Matches high-impedance antennas (450Ω–900Ω) to 50Ω coax.
- Use Case: Random wire antennas, end-fed half-wave (EFHW) antennas, and long-wire setups.
- Construction: A single-turn winding with a tapped secondary on a toroidal core.
- Effect: Efficiently converts impedance but requires counterpoises or grounding to minimize RF feedback. May exhibit common-mode resonance, potentially requiring additional chokes.
12:1 Autotransformer UNUN
- Purpose: Matches very high-impedance loads (e.g., 600Ω–1200Ω) to 50Ω coax.
- Use Case: Used in long-wire antennas or high-impedance antenna feeds.
- Construction: A single tapped winding with a larger transformation ratio.
- Effect: High impedance transformation efficiency but sensitive to placement and grounding. The risk of common-mode current issues increases with higher transformation ratios.
Autotransformer vs. Guanella vs. Ruthroff Transformers
| Feature | Autotransformer (Voltage UNUN) | Guanella Transformer (Current Mode) | Ruthroff Transformer (Voltage Mode) |
|---|---|---|---|
| Type | Voltage Transformer | Current Transformer | Voltage Transformer |
| Impedance Matching | Yes (4:1, 9:1, 12:1) | Yes (1:1, 4:1, 9:1) | Yes (1:1, 4:1, 9:1) |
| Common-Mode Current Suppression | No (unless bifilar/trifilar winding is used) | Yes | No |
| Common-Mode Resonance Issues | Possible (depending on grounding and winding configuration) | No | No |
| Construction | Uses a single tapped winding on a ferrite core | Uses independent transmission lines wound on ferrite cores | Uses a single tapped winding or autotransformer design |
| Efficiency in Impedance Transformation | High | Can introduce losses at higher transformation ratios | More efficient than Guanella but does not suppress common-mode currents |
| Best Used For | End-fed antennas, random wires, impedance matching | Dipoles, loops, verticals, folded dipoles, OCF dipoles | Windoms, EFHW antennas, impedance transformation applications |
When to Use an Autotransformer (Voltage UNUN)
- If impedance transformation is needed for end-fed antennas or random wire antennas.
- If voltage transformation is the primary goal rather than current balance.
- If a simple, efficient solution is preferred for impedance matching.
- If common-mode currents are not a concern or can be handled separately.
- If a bifilar or trifilar winding is used on the primary, allowing for some level of common-mode rejection.
- If common-mode resonance (CMR) is understood and mitigated with proper grounding and ferrite chokes.
- If a Guanella or Ruthroff transformer introduces unnecessary complexity for the specific application.
Final Takeaway
Autotransformer UNUNs are an efficient choice for unbalanced-to-unbalanced impedance transformation, particularly for end-fed and random wire antennas. They provide superior voltage transformation efficiency compared to Guanella transformers, but they do not suppress common-mode currents in standard configurations, which may require additional chokes. However, if the primary winding is bifilar or trifilar, an autotransformer can exhibit some level of common-mode rejection, improving performance in certain scenarios. Moreover, depending on the construction, common-mode resonance effects can lead to unintended choking on the primary, making careful design and grounding essential. When designing an RF system where impedance transformation is key, an autotransformer UNUN is often the simplest and most effective solution.
Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.