Balun

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.

The baluns used for home television antennas have a 4:1 turns ratio, to match the standard 300-ohm ribbon cable to 75-ohm coaxial cable.

Three audio transformers. Except for the connections, all three are electrically identical, but only the leftmost two can be used as baluns. The one at left would normally be used to connect a high impedance source, such as a guitar, into a balanced microphone input. The one in the centre is for connecting a low impedance balanced source, such as a microphone, into a guitar amplifier. The one at the right is not technically a balun, as it provides only impedance matching.

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