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Electronics & Antennas for Ham Radio

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The Guanella transformer (Current Balun/UNUN) explained

The Guanella transformer is a current-mode transmission line transformer (TLT) designed to enforce equal currents in both output conductors. This property makes it highly effective at suppressing common-mode currents, which are the primary cause of unwanted RF on coaxial feedlines. While Guanella transformers excel at balancing currents, they are not always the most efficient choice for impedance transformation compared to voltage-mode designs like the Ruthroff transformer.

Related reading:
The Ruthroff Transformer (Voltage Balun/UNUN) Explained
The Autotransformer Voltage UNUN Explained
The Hybrid Transformer Explained
Autotransformer vs Ruthroff Transformer

Invented by Gustav Guanella

Originally patented in US Patent 2470307A, the Guanella transformer introduced a practical way to achieve current balance in HF matching networks. Unlike traditional autotransformers, it relies on transmission line sections wound on ferrite cores.

Key Characteristics of a Guanella Transformer

  • Type: Current-mode transformer
  • Purpose: Provides impedance transformation while ensuring equal currents
  • Core Concept: Uses parallel transmission lines wound on ferrite cores
  • Common Ratios: 1:1, 4:1, 9:1
  • Best For: Dipoles, loops, OCF antennas, verticals, and UNUNs

How the Guanella Transformer Works

  • The primary connects directly to the feedline.
  • The secondary uses transmission line sections (bifilar wire or coaxial lines).
  • Enforces current balance — reducing RF on the coax shield.
  • Compared to a Ruthroff transformer, it is less efficient for large ratio impedance transformations but better at CMC suppression.

Common Guanella Transformer Types

1:1 Guanella Current Balun (Choke)

  • Purpose: Blocks common-mode currents without impedance transformation
  • Use Case: Dipoles, Yagis, loops
  • Construction: Bifilar wire or coaxial line wound on ferrite

4:1 Guanella Current Balun

  • Purpose: Matches 200 Ω antennas (loops, folded dipoles) to 50 Ω
  • Construction: Two independent transmission lines on ferrite
  • Effect: Provides a 4:1 transformation with current balance
  • Limitation: Higher loss than Ruthroff at the same ratio

9:1 Guanella Current UNUN

  • Purpose: Matches high-impedance long wires (450–900 Ω) to 50 Ω
  • Construction: Three transmission lines wound on ferrite
  • Limitation: Losses accumulate with more lines — sometimes better to use a Ruthroff-type for high ratios

Guanella vs. Ruthroff Transformers

Feature Guanella (Current Mode) Ruthroff (Voltage Mode)
Type Current transformer Voltage transformer
Impedance Matching Yes (1:1, 4:1, 9:1) Yes (1:1, 4:1, 9:1)
Common-Mode Suppression Yes No
Construction Multiple parallel transmission lines on ferrite Autotransformer winding or tapped coil
Efficiency Lower at high ratios Higher efficiency, but allows CMC
Best Use Dipoles, loops, OCFs, verticals EFHW, Windoms, high-ratio matching

When to Use a Guanella Transformer

  • If common-mode current suppression is required
  • If feeding a balanced load (dipole, loop)
  • If moderate impedance transformation is needed (4:1, 9:1)
  • If a Ruthroff balun causes excessive RF feedback

Final Takeaway

Guanella transformers are the preferred choice when common-mode suppression is the priority. They provide better current balance for antennas like dipoles, loops, and verticals. However, when maximum efficiency in impedance transformation is required (such as in end-fed half-wave designs), a Ruthroff transformer may perform better — provided that common-mode issues are controlled separately with a choke.

Mini-FAQ

  • Does a Guanella transformer always reduce RF on the coax? — Yes, it enforces current balance and suppresses common-mode currents.
  • Is a 4:1 Guanella better than a 4:1 Ruthroff? — It depends: Guanella reduces RF feedback, Ruthroff is more efficient but noisier.
  • When should I use a 9:1 Guanella UNUN? — For long wires with tuners, where balance and stability are more important than minimal loss.

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Questions or experiences to share? Feel free to contact RF.Guru.

Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru, specializing in high-performance HF/VHF antennas and RF components.

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