|A Balun is an electrical device that converts between a balanced signal (two signals working against each other where ground is irrelevant) and an unbalanced signal (a single signal working against ground or pseudo-ground). A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. The origin of the word balun is “balanced to unbalanced”. Baluns can take many forms and their presence is not always obvious. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals. A variation of this device is the UNUN, which transfers signal from one unbalanced line to another.
|Types of balun
Classical transformer type
This type is sometimes called a “voltage balun”. The ‘primary’ winding receives the input signal, and the ‘secondary’ winding puts out the converted signal. The core that they are wound on may either be empty (an “air core”) or equivalently a magnetically neutral material like a porcelain support, or it may be a material which is good magnetic conductor like ferrite in modern high-frequency baluns, or of ‘soft iron’ as in the early days of telegraphy.In classical transformers, there are two electrically separate windings of wire coils around the transformer’s core. The advantage of transformer-type over other types is that the electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or otherwise electrically incompatible; for that reason they are often calledisolation transformers.
The electrical signal in the primary coil is converted into a magnetic field in the transformer’s core. When the electrical current through the primary reverses it causes the established magnetic field to collapse, and the collapsing magnetic field induces an electric field in the secondary winding.
The ratio of loops in each winding and the efficiency of the coils’ magnetic coupling determines the ratio of electrical potential (Voltage) to electrical current (Amperage) and the total power of the output. For idealized transformers, although the ratio of Voltage to Amperage will change in exact proportion to the square of the winding ratio, the power (measured in Watts) remains identical. In real transformers, some energy is lost inside due to heating of the metallic core of the transformer, and lost outside to the surrounding environment imperfect magnetic coupling between the two coils.
Generally a balun consists of two wires (primary and secondary) and a toroid core: it converts the electrical energy of the primary wire into a magnetic field. Depending on how the secondary wire is done, the magnetic field is converted back to an electric field. Anautotransformer balun has only one coil, or is made of two or more coils that have an electrical connection. The coil is typically wound on aferrite rod or doughnut-shaped toroid. One can also make an autotransformer from an ordinary transformer by cross-wiring the primary and secondary windings. Baluns made with autotransformer windings are also called “Voltage baluns”, since they produce balanced output voltage, but not necessarily balanced current.
As with a two-winding transformer balun, the ratio of voltage to current changes in proportion to the square of number of windings between the two input wires divided by the number of windings between the two output wires.In all autotransformers, the single winding must have at least one extra electrical connection – called a tap or tap point – between the two ends of the winding. The current sent into the balun through one pair of connections acts as if it were a primary coil, and magnetizes the entire core. When the electric current in the input segment of the coil changes, the induced magnetic field collapses and the collapse of the magnetic field in the core induces an electric current in the entire coil. Electrical connections to parts of the coil different from the input connections have higher or lower voltages depending on the length of the coil that the output is tapped from.
Unlike transformer-type baluns, autotransformer baluns connect all the wires connected to them to a single ground voltage level. Since outdoor antennas are prone build-up of static electric charge, the path for the static to drain to ground through an autotransformer balun can be a distinct advantage.
Transmission-line transformer type
Transmission line or “choke” baluns can be considered as simple forms of transmission line transformers. This type is sometimes called a “current balun”, since it ensures equal current on both sides of its output, but not necessarily equal voltage.
The Guanella transmission line transformer (Guanella 1944) is often combined with a balun to act as an impedance matching transformer. Putting balancing aside a 1:4 transformer of this type consists of a 75 Ω transmission line divided in parallel into two 150 Ω cables, which are then combined in series for 300 Ω. It is implemented as a specific wiring around the ferrite core of the balun.A more subtle type results when the transformer type (magnetic coupling) is combined with the transmission line type (electro-magnetic coupling). Most typically the same kind of transmission line wires are used for the windings as carry the signal from the radio to the antenna, although these baluns can be made using any type of wire. The resulting in devices have very wideband operation."Transmission line transformers" commonly use small ferrite cores in toroidal "rings" or two-hole "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 effective choke balun for frequencies from about 10 MHz to beyond 30 MHz. The magnetic material may be "air", but it is a transmission line transformer.
Delay line type
A large class of baluns uses 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".
One easy way to make a balun is a one-half wavelength (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. 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 that 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.