Everything you always wanted to know
about baluns but were afraid to ask
The antenna terminal on a modern radio is coaxial (usually a PL259 connector) and therefore unbalanced. The radio chassis sits at zero potential (if all is well, that is) and the center pin of the coaxial connector carries the RF signal. A dipole antenna, on the other hand, which is often used for HF operation, is a balanced antenna. Simply connecting an unbalanced system to a balanced system is never a good idea! If coaxial cable is involved, its shield will carry strong RF currents which will be radiated where they will do the least good. This will cause RF in the shack, TVI, etc. The radio's chassis and everything connected to it will be "hot", resulting in blistered fingers due to RF burns.
Obviously some conversion from unbalanced to balanced is required in order to connect these two different systems. Enter the balun. As the name (bal-un) implies, a balun matches a balanced system to an unbalanced one. In addition to that, baluns can (but not necessarily do) perform impedance transformation as well. The 4:1 balun, which matches a 50 ohms unbalanced radio to a 200 ohms balanced dipole, is a popular example.
Voltage vs. current baluns
Baluns can employ either one of two principles. On the one hand there are voltage baluns, which are essentially auto-transformers. On the other hand there are current baluns which are based upon the principle of suppressing or neutralizing common-mode currents. And in-depth treatment of the theories behind the various balun systems and how they interact with feed lines is beyond the scope of this article. Instead, let's have a look at the four baluns that are most common in practice, and see how they compare.
The 4:1 voltage balun is the most commonly found balun. The voltage balun (also known as the "Ruthroff" balun) is actually a simple transformer. Current from the unbalanced terminal (the center pin of the coax) is fed directly to one of the balanced terminals, and runs through the top half of the transformer windings as well. This induces a similar current in the bottom half of the transformer windings, but because the top of the bottom transformer half is at zero potential, the lower balanced terminal develops a voltage equal but opposite to the upper one.
While this design is the most common one, it is by no means the best! The main problem is its flawed symmetry. The impedance of the top half of the transformer is switched in parallel with the (typically 50 ohms) impedance connected to the unbalanced terminal, while the lower half of the transformer is not. Especially at higher frequencies, where the inductance of the transformer windings increases, this means that the voltage developed across the upper transformer half is loaded by the unbalanced input impedance, and therefore drops. This is not true for the lower half of the transformer. As a result the signal across the balanced terminals is far less balanced for higher frequencies. This unbalance will cause a balanced feed line to radiate RF.
The 1:1 voltage balun is a variety on the previous one. Here the current drawn from the unbalanced terminal (the center pin of the coax) is fed not through one but through two transformer windings in series. The upper balanced terminal is connected to the connection point of the two windings. The third (bottom) transformer winding forms an arrangement similar to the one described above. In this configuration the amplitude of the unbalanced signal is equal to that of the balanced signal, since both of them are developed across the same number of transformer windings. In the 4:1 voltage balun the balanced signal was developed across twice the number of windings compared with the unbalanced signal, which means (this being a transformer) that the voltage is doubled while the current is halved, resulting in an impedance transformation factor of 4:1. The 1:1 balun performs neither voltage nor current transformation, hence the impedance transformation factor is 1:1. It does, however, experience the same problems with flawed symmetry and the resulting unbalance at higher frequencies!
The 1:1 current balun is also known as the Guanella balun or a choke. This is one of the simplest baluns. It is based on suppression of the common mode current in the feed line. The current through the upper winding induces an equal but opposite current in the bottom one, with a voltage developed across the bottom winding equal and opposite to the one across the top. This annulls the voltage on the bottom balanced terminal, thus keeping the chassis of the unbalanced side at zero potential. In practice this balun often takes the form of a coaxial cable wound into a coil. Its main disadvantage is that its efficiency depends entirely on the coil inductance, which should be infinite in order to achieve 100% suppression of feed line common mode currents. In the real world this inductance is finite and frequently low, which limits the efficacy of this balun.
The 4:1 current balun (Guanella balun) may look a bit confusing, but in fact isn't all that hard to understand. It consists essentially of two 1:1 Guanella (current) baluns, the unbalanced sides of which have been switched in parallel, while the balanced sides are in series. This means that both 1:1 baluns will each develop the same voltage on the right hand side as on the left hand side (they have a transformation factor of 1:1 after all) but because on the balanced side these two voltages are switched in series, the voltage on that side doubles. With twice the voltage (and therefore half the current) at the balanced side, the result is a 4:1 impedance transformation. Both 1:1 baluns which make up the 4:1 variety may be wound on different cores, or on the same core. The main disadvantage of this particular balun is that the characteristic impedance of the windings themselves should be twice that of the unbalanced impedance and half that of the balanced impedance. In other words, in order to get a 1:1 SWR at the 50 ohms side, the characteristic impedance of the windings ideally should be exactly 100 ohms, which is extremely hard to achieve in practice. On the other hand, while the SWR meter may not show a 1:1 match with a dummy load, the symmetry of the signals at the balanced end of this balun will always be 100%, which cannot be said for the other three balun designs shown above!
Balun cores: air, ferrite or powdered iron?
Many balun designs can be found on the Internet. A number of these are "air wound" baluns, consisting of wire or coax wound across a former (e.g. a plastic tube) filled with air. This makes sense in countries where four figure power levels are legal (or at least common) since an air core is, for all practical purposes, impossible to magnetically saturate. In other countries such as South Africa, however, where 100W is the most common maximum power level, and with a happy few owning a "foot warmer" that pumps out 200 or even 400W, it is very well possible to use properly selected powdered iron or ferrite cores without causing these cores to saturate. Since the proper functioning of all baluns discussed here depends on self-induction and magnetic coupling, the higher magnetic permeability of a core is preferable to the low permeability of air. Baluns based on air cores often leave a lot to be desired, with their efficacy being limited by the low induction and loose coupling of the windings, especially on lower frequencies.
Various options are available for balun cores. The top of the range (and of course the most expensive option) is the ferrite toroid core. These can be obtained in South Africa from suppliers such as Mantech or RS components. They can be hard to find and somewhat pricey, but they are available. Ordering ferrite cores in larger batches may be easier than obtaining single ones, so consider making it a club project! Various ferrite mixes are available. Without going into the details of ferrites (about which many volumes have been written), an excellent all-round ferrite mix for balun cores is 4C65. The amount of power the balun can handle is in no small measure determined by the size of the toroid - the bigger, the better. In practice a 36mm toroid is more than sufficient to handle 100W, provided that the impedance mismatch isn't too great.
(Updated, 1 Jan. 2011) The biggest problem with ferrites is that there is no generally accepted standard for type marking. An unknown ferrite is just that, and if no information about its composition is available (colour is not a reliable indication!) then measuring the ferrite's properties at various frequencies is the only way to find out. This can be done by putting a primary and secondary winding onto the ferrite core, and testing the properties of the transformator thus obtained at various frequencies. Experiment with different windings and different power levels to test for permeability and saturation. This procedure is a whole different subject, though, and beyond the scope of this article. (End of update.)
As an alternative to ferrite, powdered iron cores are available. These tend to be a bit cheaper than ferrites. A well-known type is the T-200. These cores have a much lower permability than the 4C65 ferrite, which means that they won't saturate as easily at higher power levels. On the other hand, the price we pay for that lower permeability is a lower induction and less tightly coupled windings.
(Updated 1 Jan. 2011) The anonymous toroids found at ham radio flea markets and in most hams' junk boxes are generally salvaged from switched mode power supplies and the like. They are intended for RFI suppression. These toroids (in fact, all toroids found in power regulation circuits) are almost invariably powdered iron cores and not ferrite. Their composition has been optimized for RFI suppression, and is generally not well suited for use in baluns. (End of update.)
Fortunately there is a cheap alternative to toroid cores, which can be found in just about every radio ham's junk box. These are the ferrite rods salvaged from the old ferrite "loopstick" antenna's in AM radio's! While the ferrite mix of these rods is generally unknown, the fact that they have been designed for use as an antenna in the broadcast bands makes them generally well suited for use on the HF bands at moderate power levels, and their permeability can be expected to be more or less in the desired range as well. A ferrite rod with a length of 10 or 12 cm or so and a diameter of 10-15mm (not critical) will do very nicely for power levels up to 100W. Multiple ferrite rods can be taped together to get a chunkier core that will handle even higher power levels. If laquered copper wire is used for the windings, put some plastic tape or heat shrink tube over the ferrite for extra insulation.
Baluns can be wound on a ferrite core using insulated wire, which can be held in place with cable ties. Laquered copper wire can also be used, but this is more difficult to apply since it is less flexible. When winding laquered copper wire onto a ferrite rod, be careful not to apply too much force in shaping the wire, because ferrite rods are a brittle ceramic and break easily!
(Updated 1 Jan. 2011) The balun's properties at various frequencies (i.e. its useable bandwidth) are determined by two factors: the inductance of the windings, and the capacitance between the windings. The latter can be especially detrimental at the higher frequencies. The capacitance can be reduced by increasing the spacing between the windings. Some experimentation with varying numbers of windings at different spacings may be necessary to find the optimum configuration in a particular situation.
Often a "choke" to suppress coaxial shield currents (i.e. a 1:1 Guanella balun) is made by winding the coaxial cable into 10 or 12 loops, either at the antenna or at the foot of the tower. Generally this is not very effective. On low frequencies the impedance of the coil (which is essentially wound on an air core with very low permeability) is insufficient. On higher frequencies the capacitance between the windings (made worse by the fact that both ends of the coil are usually close together) becomes a problem.
A better 1:1 Guanella (a.k.a. choke) balun can be made by winding an odd number of layers of coax onto a ferrite rod. The ferrite's permeability is much higher than that of air, which improves its performance on the higher bands. The opposite ends of the coil are on opposite ends of the rod, which reduces capacitance and improves performance on the higher bands. A balun with low capacitance, intended for use on the high bands (e.g. 10m and 6m) may be constructed from coax wound onto a toroid core as shown in the photo. Note the spacing and the direction of the windings. The size and permeability of the toroid (which determines when magnetic saturation will occur) and the breakdown voltage of the coax determine the maximum power level this balun can handle. When teflon-based coax and a large powdered iron core are used, baluns constructed in this manner can easily handle several kW. (Note: a balun faced with a serious impedance mismatch will saturate and overheat at lower power levels than one that "sees" the correct impedance at both sides!) (End of update.)
Baluns are generally easy to build. If they are intended for outdoor use, they need to be weatherproofed. A good way to do this is to use plastic drain pipe, which can be cheaply obtained from a local plumber, along with matching end caps. Baluns wound on ferrite rods can be housed in pieces of pipe with a length of 10-20cm (depending on the size of the rod) while toroids may be housed in short sections of pipe which essentially only serve to hold two end caps tightly together. A small amount of silicone sealant can provide further waterproofing. Use as little sealant as possible, so that you can remove the end caps later if repairs or changes should be necessary.
Baluns for indoor use can be housed in just about any enclosure that is on hand. Plastic is preferable, though. If metal housings are used, ensure that all wires and cores are kept well clear of any metal parts.
Below are the generic winding diagrams for typical balun types. The number of windings is intended as a starting point only. The ideal number depends on the desired frequencies and bandwidth, as well as on the core material used, i.e. the ferrite or powdered iron mix. The bandwidth can be changed by varying the spacing between the windings. Some experimentation may be necessary.
(This article was compiled over time from many different sources in literature and on the Internet. Special thanks to PA0FRI.)