Autotransformer vs. Ruthroff Transformer in UNUNs and BALUNS

Both autotransformers and Ruthroff transformers are commonly used in UNUNs and BALUNS, but they function differently in terms of how they handle impedance transformation and common-mode currents.

1. Ruthroff Transformer (Tapped Winding, Isolated Secondary)

  • Type: Voltage Transformer
  • Common Use: Voltage Baluns (BALUN) and some UNUNs
  • Key Feature: Uses a separate winding (secondary side) for output, which provides isolation between input and output voltages.
  • Advantage: Helps transform impedance while keeping phase relationships intact.
  • Disadvantage: Does not suppress common-mode currents, meaning it can allow unwanted RF on the feedline.
  • Example:
    • 4:1 Ruthroff voltage balun for OCF dipoles.
    • 4:1 voltage UNUNs for end-fed antennas.
  • How It Works:
    • The transformer has a tapped winding where the input voltage is transformed based on the turns ratio.
    • The secondary side is not directly connected to the input but magnetically coupled.

2. Autotransformer (Tapped Coil, No Isolation)

  • Type: Voltage Transformer

    Common Use:     UNUNs (e.g., 9:1, 49:1, 64:1, etc.)

    Key Feature:     Uses a single winding (no secondary), meaning input and output share a common ground.
  • Advantage: Simpler and more efficient for impedance matching.
  • Disadvantage: Provides no isolation, meaning common-mode currents can be present unless an additional choke is used.
  • Example:
    • 49:1 or 64:1 UNUNs for EFHW antennas.
    • 9:1 UNUNs for random wire antennas.
  • How It Works:
    • A single tapped winding is used to create an impedance transformation.
    • The input (coax feedline) and output (antenna) are electrically connected, allowing for easy impedance conversion.

Key Differences

Feature

Ruthroff Transformer

Autotransformer

Type

Voltage Transformer

Voltage Transformer

Used in

Voltage Baluns & some UNUNs

Most UNUNs (49:1, 9:1, etc.)

Isolation?

Yes (separate secondary winding)

No (input & output share a common connection)

Common-Mode Suppression?

No, requires extra choke

No, requires extra choke

Efficiency

Slightly lower (due to secondary winding)

Higher (direct electrical connection)

Example Applications

4:1 Voltage Balun (BALUN)

49:1 UNUN for EFHW

 

Which One is Better?

  • For EFHW and random wire antennas → Autotransformer (49:1, 9:1, etc.) is simpler and works well.
  • For OCF dipoles and Windoms → A 4:1 Ruthroff voltage balun is common but not ideal for common-mode suppression.
  • For better performance, always consider adding a 1:1 current choke after an autotransformer or Ruthroff voltage balun. (Hybrid)

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.