A balun is a device designed to convert between balanced and unbalanced electrical signals, such as between coaxial cable and twin-lead (pronounced lēd like reed, not lĕd like red).

This is almost always done through the use of a small isolation transformer, with the earth ground or chassis ground left floating or unconnected on the balanced side. This transformer can then also perform impedance matching at the same time. A balun generally has no other functional purpose except for compatibility between systems.

The most common uses of baluns are:

• In television, amateur radio, and other antenna installations and connections, to convert between ribbon cable (balanced) and coaxial cable (unbalanced) or to directly feed a balanced antenna with (unbalanced) coax.
• In audio applications, to convert between high impedance unbalanced and low impedance balanced lines.
• In power line communications, baluns are used in coupling signals onto a power line.

A 300-to-75-ohm balun

Audio transformers

Simple homemade balun

Simple homemade 1:1 balun using a toroidal core and coaxial cable.

References

• Building and Using Baluns and Ununs: Practical Designs for the Experimenter, Jerry Sevick, 1996.

• Balun design for radio-communications and scientific purposes