The Guanella transformer (Current Balun/UNUN) explained

The Guanella transformer is a current-mode transmission line transformer (TLT) designed to force equal currents in both output conductors, which helps suppress common-mode currents. It is widely used in current baluns and current UNUNs, particularly for reducing unwanted RF on the feedline. However, while Guanella transformers excel at enforcing current balance, they are not always the most efficient for impedance transformation compared to voltage-mode transformers.

Invented by Guanella Gustav, Expired US Patent US2470307A

Key Characteristics of a Guanella Transformer

• Type: Current transformer
• Purpose: Provides impedance transformation while ensuring equal currents on both outputs.
• Core Concept: Uses two transmission lines wound on a ferrite core, allowing for an efficient impedance match while suppressing common-mode currents.
• Common Ratios: 1:1, 4:1, 9:1
• Best For: Dipoles, loops, OCF antennas, verticals, and UNUN impedance matching with minimal RF on coax.

How the Guanella Transformer Works

• The primary winding is directly connected to the input (e.g., coax feedline).
• The secondary consists of two independent transmission lines (coaxial or bifilar wire) that transfer power while maintaining equal currents at the output.
• Unlike a Ruthroff transformer (which works as a voltage transformer), the Guanella transformer forces current balance, preventing unwanted RF radiation on the coax.
• However, for certain impedance transformations, a Ruthroff transformer may be more efficient because it provides a more direct voltage transformation without the losses introduced by separate transmission lines.

Common Guanella Transformer Types

1:1 Guanella Current Balun (Choke Balun)

• Purpose: Blocks common-mode currents (RF on the feedline).
• Use Case: Used in dipoles, Yagis, and loops to ensure balanced feedline currents.
• Construction: A coaxial or bifilar wire winding on a ferrite toroid or core.
• Effect: Does not transform impedance, just removes common-mode currents.

4:1 Guanella Current Balun

• Purpose: Matches 200Ω antennas (e.g., folded dipoles, loops) to 50Ω coax.
• Use Case: Often used for center-fed and off-center-fed antennas.
• Construction: Two independent transmission lines wound on separate ferrite cores.
• Effect: Ensures equal currents while providing a 4:1 impedance transformation.
• Limitation: The efficiency of impedance transformation depends on the quality of the transmission lines and the ferrite material used. Ruthroff designs tend to have lower losses for the same impedance ratio.

9:1 Guanella Current UNUN

• Purpose: Matches 450Ω to 900Ω loads (random wire antennas) to 50Ω coax.
• Use Case: Random wires with a tuner.
• Construction: Three independent transmission lines, typically wound on ferrite toroids.
• Effect: Provides impedance transformation while reducing RF feedback.
• Limitation: The multiple transmission line design can introduce additional losses, making Ruthroff transformers sometimes more efficient for high-ratio impedance matching.

Guanella vs. Ruthroff Transformers

Feature	Guanella Transformer (Current Mode)	Ruthroff Transformer (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 Current Suppression	Yes	No
Construction	Uses independent transmission lines wound on ferrite cores	Uses a single tapped winding or autotransformer design
Efficiency in Impedance Transformation	Can introduce additional losses, especially at higher transformation ratios	More efficient for impedance transformation but allows common-mode currents
Best Used For	Dipoles, loops, verticals, folded dipoles, OCF dipoles	Voltage balancing, impedance transformation for Windom and EFHW antennas

When to Use a Guanella Transformer

• If common-mode current suppression is required (to prevent RF on the coax).
• If you need balanced current feed for a dipole, loop, or vertical antenna.
• If impedance transformation is needed (e.g., 4:1 for folded dipoles, 9:1 for long wires), but losses are acceptable.
• If a Ruthroff balun causes excessive RF feedback, switching to a Guanella balun is often a better solution.
• However, if maximum efficiency in impedance transformation is the primary goal, a Ruthroff transformer may be the better choice, provided common-mode current issues can be managed.

Final Takeaway

Guanella transformers are the preferred choice for RF applications where common-mode currents need to be eliminated. They enforce equal currents, making them more suitable for most practical antenna systems than Ruthroff voltage baluns. However, they are not always the most efficient for impedance transformation—Ruthroff transformers generally perform better in this regard but come with the trade-off of common-mode current issues.

Selecting the right transformer depends on whether common-mode suppression or impedance transformation efficiency is the priority.

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.