A balun is used to connect a balanced line to an unbalanced line. In a balanced line you have two signals working against each other where ground is irrelevant and in an unbalanced line you have just one signal working against ground. A balun will help reduce or eliminate RF current on the outside of a coaxial cable. A balun is also known as an air choke.
The word balun comes from the words "balanced" and "unbalanced." With RF a balun deals with impedance matching between antenna and line. Coaxial cable is a type of unbalanced feedline. Coaxial cable is the most common feedline in use today for RF. RF cable tends to travel both on the inside and the outside of a coax shield, which can give the coaxial cable characteristics of three independent wires.
Types of balun
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.
- Autotransformer balun: two coils on a ferrite rod
- Classical transformer: isolated transformers
- Transmission-line transformer: transformer type magnetic coupling combined with the transmission line type electro-magnetic coupling
- Delay line: connected transmission lines of specific lengths w/o transformer
- RF choke: coaxial cable near to the feed point of a balanced antenna
RF Balun Examples
To alleviate RF on the outside of a coaxial cable shield a simple three loop air choke can be introduced. You can make a simple current balun by coiling coaxial feedline in a particular way. The coil inductance and distributed capacitance resonate as a parallel trap whose high impedance inhibits unwanted shield current.
The coaxial cable can be wrapped three times around a piece of PVC that is 2.5" to 3". Basically, not so tight as to damage the cable, but no large a loop as to nullify the effective choking of the unwanted RF.
The air choke should be placed closely to the antenna rather than near the receiver. Place it at the feed-point of the antenna. Actually, all antenna matching is best done at the antenna, not the transmitter, despite the fact that HAM operators often place antenna matching units at the transceiver.
As an example, when a balun is used with a dipole antenna the balun effectively converts the dipole (balanced) for the unbalanced coaxial cable. Place the balun at the feedpoint of the dipole antenna by creating the coil or using a ferrite choke.
Rather than constructing an air coil you can place a ferrite choke on the coaxial cable. The ferrite will increase the cable common-mode impedance without affecting its differential impedance.
The ferrite choke has a ferrite bead or ferrite core which is a mix of material. The mix is referred to by a number, and each mix has different impedence properties which should be used at specific frequency ranges. An example is #43 ferrite mix, however, the mix number is not an industry standard. Palomar makes a type 43 ferrite and Fair-Rite makes a type 43 ferrite and they are not the same.
- Palomar mix #31 is for 1-10 MHz common mode suppression and acceptable for ham radio 1:1 feed line choke applications.
- Palomar mix #43 is for common mode chokes from 2-300 Mhz.
- Palomar mix #61 is for high power in multi ratio (2:1, 4:1, 9:1) impedance transformers.
To compare differences by manufacturer consider Palomar's type 43 material is good for attenuation from 2 MHz to 1000 MHz. Fair-Rite's type 43 is good for attenuation from 20 MHz to 300 MHz. As you can see this is significant on the HF band.
Fair-Rite 31 material - the best grade for general purpose HF and low-band chokes. Fair-Rite 31 clamp-on core, Part No 0431177081. An HF ferrite choke will typically need 6-8 turns passing through the center hole. Regardless of the type of core, one pass will probably not be very effective. The same Fair-Rite 31 material is also available in a "FT240" 2.4-inch toroid format Fair-Rite Part No 2631803802. 10-12 turns of coax on a FT240-31 makes a nice low-bands choke or balun.
Fighting RF Current Flow
Ferrite Chokes and Baluns may be employed to help stop RF currents from flowing where they aren’t wanted. Common-mode RF current on feedline increases the risk of TVI and other types of RF interference (RFI). RF current flows only on the surfaces of conductors. The outside surface of the coax shield is a completely separate conductor from the inside surface so you can think of the outside surface of the shield is part of the outside world. This is where the common-mode currents flow.
Common mode current means that the feedline is radiating RF. It is ideal to have only the antenna radiate. Excessive common mode current causes an excess of RF in the shack, which can get into all sorts of things including the building electrical system. To reduce this you can employee chokes and baluns.
- common mode choke
- feedline choke
- mains choke
They all do the same job – blocking unwanted common-mode RF current on the feedline cable. Chokes may be needed in several different places: at the antenna feedpoint which is considered a balun, at different points on the coax, on other wiring in the shack including computer cables, and on the mains electrical wiring.
Air-wound chokes are one of the least effective ways to stop common-mode current. They are also targeted to a narrow band because the way the winding is done dictates what band it will be effective on. They are cheap and easy to implement but have the aforementioned deficiencies.
Strings of ferrite beads may also have less effect at eliminating common-mode current. They are easy to employ and do not suffer from the narrow band shortfall. However, it takes a lot of ferrite beads to have much reduction impact and this can become expensive.
To have the most effective reduction of common-mode current you must employee ferrite cores of the specific mix for the application and use multiple turns. The best ferrite cores are made in the United States such as those made using Fair-Rite 31 and 43 grade materials. The best performance comes from large cores. The problem with this approach is cost. This is a very expensive way to deal with common-mode current.