Balun
From Wikipedia, the free encyclopedia
A balun (pronounced "bal-un") is a device designed to convert between balanced and unbalanced electrical signals, such as between coaxial cable and ladder line. Baluns can be considered as simple forms of transmission line transformers.
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[edit] Construction
Baluns can take so many forms that sometimes their presence is not obvious. They always involve some form of electromagnetic coupling, and therefore often include transformer structures. The most basic baluns use a transformer, with the unbalanced connection made to one winding, and the balanced to another. This first form spans a huge variety of sizes, types and materials, with many different winding schemes to achieve balance, impedance transformations, and signal couplings in circuits.
A large class of baluns use connected transmission lines of specific lengths, with no obvious "transformer" part. These are usually built for (narrow) frequency ranges where the lengths involved are some multiple of a quarter wavelength of the intended frequency in the transmission line medium. A common application is in making a coaxial connection to a balanced antenna, and designs include many types involving coaxial loops and variously connected "stubs".
A more complex (and subtle) type results when the transformer type (magnetic coupling) is combined with the transmission line type (electro-magnetic coupling). This is where whole transmission lines are used as windings, resulting in devices capable of very wideband operation. This whole class known generally as "Transmission Line Transformers" spawn their own huge variety. Very commonly, they use small ferrite cores in toroidal or "binocular" shapes. Something as simple as 10 turns of coaxial cable coiled up on a diameter about the size of a dinner plate makes an extremely effective choke balun to beyond 30 MHz. The magnetic material may be "air", but it is a transmission line transformer!
A balun's function is generally to achieve compatibility between systems, and as such, finds expansive application in modern communications, particularly in realising frequency conversion mixers to make cellular phone and data transmission networks possible. Baluns are everywhere: in radars, in transmitters, in satellites, in every telephone network, and probably in most wireless network modem/routers used in homes.
[edit] Radio equipment applications
In television, amateur radio, and other antenna installations and connections, baluns are used to convert between 300 ohm ribbon cable (balanced) and 75 Ω coaxial cable (unbalanced) or to directly connect a balanced antenna to (unbalanced) coax. To avoid EMC problems it is a good idea to connect a centre fed dipole antenna to coaxial cable via a balun.
The baluns used for home television antennas have a 4:1 impedance ratio, to match the standard 300 Ω twin-lead cable to 75 Ω coaxial cable.
[edit] Narrow-band designs
- One easy way to make a balun is a (λ/2) length of coaxial cable. The inner core of the cable is linked at each end to one of the balanced connections for a feeder or dipole. 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.
- Another narrow band design is to use a λ/4 length of metal pipe. The coaxial cable is placed inside the pipe; at one end the braid is wired to the pipe while at the other end no connection is made to the pipe. The balanced end of this balun is at the end where the pipe is wired to the braid. The λ/4 conductor acts as a transformer converting the infinite impedance at the unconnected end into a zero impedance at the end connected to the braid. Hence any current entering the balun through the connection which goes to the braid at the end with the connection to the pipe will flow into the pipe. This balun design is not good for low frequencies because of the long length of pipe which will be needed. An easy way to make such a balun is to paint the outside of the coax with conductive paint, then to connect this paint to the braid.
[edit] Wide-band designs
- Two coils on a ferrite rod can be used as a balun in the following way. The two windings need to be very tightly wound together, this can be done with enameled wire in two different ways. Either the two windings are wound with great care so that the two form a single layer where each turn is touching each of the adjacent turns of the other winding or the two wires are twisted together before winding the coil. The start of the first winding then needs to be connected to the end of the second winding. This connection then needs to be attached to the braid of the coaxial feeder, the core of the coaxial feeder is wired to one of the remaining connections. The two conductors for the balanced feeder are then wired to the coil connections which are not wired directly to the braid of the coax. This design can be thought of as an autotransformer. In addition to acting as a balun this design acts as a step up transformer which gives a 4:1 change in the impedance.
- An RF choke can be used in place of a balun. If a coil is made using coaxial cable near to the feed point of a balanced antenna then the RF current which flows on the outer surface of the coaxial cable can be attenuated. One way of doing this would be to wrap a lossy material, such as ferrite around the coaxial cable;
[edit] Audio equipment
In audio applications, baluns are used to convert between high impedance (see Nominal impedance) unbalanced and low impedance balanced lines.
Except for the connections, the three devices in the image are electrically identical, but only the leftmost two can be used as baluns. The device on the 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.
[edit] Video equipment
Baluns can be used to convert video coming from an S-video, RCA or VGA connector to run over Cat5 cables. Muxlab makes baluns that do this conversion.
[edit] Power line communications
In power line communications, baluns are used in coupling signals onto a power line.
In electronic communications, baluns are used to convert Twinax cables to Category 5 cables, and back.
[edit] References
- Building and Using Baluns and Ununs: Practical Designs for the Experimenter, Jerry Sevick (W2FMI), 1994.
- Radio communication handbook, Edition five, Radio Society of Great Britain (RSGB), 1976, pages 12.41 and 13.5
- Practical balun uses Basic uses of Baluns to transmit home A/V over Cat5
- SWDXER ¨The SWDXER¨ - with general SWL information and radio antenna tips.
