Baluns, Ununs and Isolators What You Need To Know Hap Griffin WZ4O

The Standard Half-Wave Dipole Presents a balanced load to the transmission line

Balanced vs Unbalanced Transmission Lines In a balanced line, the balanced and 180 degree out of phase currents exactly cancel at any point in space outside of the line (no radiation). In a coaxial line, the fields are contained inside the shield and do not radiate to any point in space outside of the line (no radiation).

How a Dipole is Designed to be Fed

Dipole Fed Directly by Coaxial Cable

What is a Balun? A Balun is a special type of RF transformer (consisting of transmission line elements) that perform two functions: Primarily – Balanced to Un-balanced transition (or Vice-Versa) to reduce or eliminate Common Mode current on the shield of unbalanced transmission line (coax) Secondarily – Can be used to transform impedance The word BALUN is a contraction of “BALanced to UN-balanced transformer)

Why Might Impedance Transformation Be Required?

A Voltage Balun attempts to provide equal and opposite VOLTAGES at the two output terminals. A Current Balun attempts to drive equal and opposite CURRENTS into the two output terminals.

Typical Voltage Balun – W2AU Note the Bifilar or Trifilar windings – these are actually short transmission line elements The transmission line elements are physically short compared to the wavelength of operation, so are broadbanded and operate over a wide frequency range.

Voltage Balun with Impedance Transformation 4:1 “Ruthroff” Voltage Balun

1:1 Ruthroff Voltage Balun

1:1 Ruthroff Voltage Balun

4:1 Ruthroff Voltage Balun

4:1 Ruthroff Voltage Balun

Voltage Baluns Using ¼ Wavelength Sections of Transmission Line - Narrowband

Current Balun, or Guanilla Balun

1:1 Current Balun

1:1 Current Balun

4:1 Current Balun

4:1 Current Balun

4:1 Current Balun

Choke Balun, or “Ugly Balun”

Choke Balun Made With Ferrite Cores

W2DU Choke Isolator “Balun”

Conclusions: If you are NOT worried about pattern accuracy, and DO NOT have RF in the shack problems, you can get away without using a Balun. Conversely, if you DO care about pattern accuracy, or have issues with RF in the shack, DO use a Balun. For typical wire antennas, where the legs are likely to have unequal environments, a CURRENT Balun is the best choice. For yagi’s where the environment is more likely to be symmetrical around the elements, a VOLTAGE Balun is OK.