The Ruthroff transformer (Voltage Balun/UNUN) explained

The Ruthroff transformer is a voltage-mode transmission line transformer (TLT) that works by balancing voltage between input and output terminals. Unlike the Guanella transformer, which focuses on current balance and common-mode suppression, the Ruthroff transformer is primarily used for efficient impedance transformation. However, it does not inherently suppress common-mode currents, which can lead to unwanted RF on the coax if not properly managed.

Invented by Clyde L Ruthroff, Expired US Patent US3037175A

Key Characteristics of a Ruthroff Transformer

• Type: Voltage transformer
• Purpose: Provides impedance transformation using a tapped winding or autotransformer design.
• Core Concept: Uses a single winding or tapped coil, making it more efficient in voltage transformation but prone to common-mode currents.
• Common Ratios: 1:1, 4:1, 9:1
• Best For: End-fed half-wave (EFHW) antennas, Windom antennas, and applications where impedance transformation is the priority.

How the Ruthroff Transformer Works

• The primary winding receives input power (e.g., from a coax feedline).
• The secondary winding is typically an autotransformer or a tapped winding, stepping up or down the voltage as needed.
• Unlike a Guanella transformer, the Ruthroff transformer does not enforce equal current distribution, making it less ideal for applications where current balance is necessary.
• This design makes impedance transformation more efficient but can lead to increased common-mode current problems, requiring additional mitigation like ferrite chokes.

Common Ruthroff Transformer Types

1:1 Ruthroff Voltage Balun

• Purpose: Matches balanced and unbalanced systems.
• Use Case: Often used in voltage baluns for antennas like Windoms and EFHWs.
• Construction: Uses a single winding with a center tap to create a balanced output.
• Effect: Converts an unbalanced feed to a balanced load, but does not eliminate common-mode currents.

4:1 Ruthroff Voltage Balun

• Purpose: Matches 200Ω antennas (e.g., Windoms, center-fed antennas) to 50Ω coax.
• Use Case: Commonly used for multi-band antennas where impedance transformation is required.
• Construction: A single autotransformer winding with a tap for the secondary.
• Effect: Provides efficient voltage transformation but requires additional chokes to prevent common-mode currents.

9:1 Ruthroff UNUN

• Purpose: Matches high-impedance antennas (450Ω–900Ω) to 50Ω coax.
• Use Case: Used with random wire antennas and end-fed antennas where high voltage transformation is needed.
• Construction: Uses a tapped or autotransformer winding.
• Effect: Efficiently converts impedance but may require additional common-mode current suppression.

Ruthroff vs. Guanella Transformers

Feature	Ruthroff Transformer (Voltage Mode)	Guanella Transformer (Current Mode)
Type	Voltage Transformer	Current Transformer
Impedance Matching	Yes (1:1, 4:1, 9:1)	Yes (1:1, 4:1, 9:1)
Common-Mode Current Suppression	No	Yes
Construction	Uses a single tapped winding or autotransformer design	Uses independent transmission lines wound on ferrite cores
Efficiency in Impedance Transformation	More efficient for impedance transformation	Can introduce losses at higher transformation ratios
Best Used For	Windoms, EFHW antennas, impedance transformation applications	Dipoles, loops, verticals, folded dipoles, OCF dipoles

When to Use a Ruthroff Transformer

• If efficient impedance transformation is required (e.g., 4:1 for Windoms, 9:1 for long wires).
• If voltage balancing is more critical than current balancing.
• If Guanella transformers introduce too much loss for the application.
• If additional common-mode current suppression measures (e.g., ferrite chokes) are available.
• If a balanced-to-unbalanced connection is needed without concern for common-mode current suppression.

Final Takeaway

Ruthroff transformers are ideal for applications where efficient impedance transformation is the primary goal. They provide better voltage transformation efficiency than Guanella transformers, but they do not suppress common-mode currents, which can lead to RF issues on the coax. When connecting an unbalanced antenna to a coaxial feedline, a Ruthroff voltage transformer is often the best choice, provided additional measures are taken to manage common-mode currents like the hybrid approach.

Article written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru. With extensive experience in active and passive antenna systems, high-power RF transformers, and custom RF solutions, Joeri shares insights into cutting-edge radio communication technologies.