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Revision as of 22:47, 10 December 2005
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 300 ohm ribbon cable (balanced) and 75 ohm coaxial cable (unbalanced) or to directly connect a balanced antenna to (unbalanced) coax.
One easy to make balun is a (λ/2) length of coaxial cable, the inner core of the cable is linked at each end to the one of the balenced connections for a feeder or diople, one of these terminals should be connected to the inner core of the coaxial feeder. All three braids should be connected together. This then forms a 4:1 balun which works at only one frequency.
Other designs are wide band, and are transformers wound with either ferrite or air cores.
- 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.
- In electronic communications, baluns are used to convert TWIN-X cables to Category 5 cables, and back.
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, serving as a passive DI unit. 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.
This is a simple RF choke which works as a balun by preventing signals passing along the outside of the braid. Such a device can be used to cure TVI by acting as a braid-breaker.
References
- Building and Using Baluns and Ununs: Practical Designs for the Experimenter, Jerry Sevick, 1996.
See also: Transmission line transformers
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