In the world of amateur radio, the terms balun, unun, common mode choke, line isolator and impedance transformer are often used. But also often mixed up and sometimes lead to heated discussions on forums and social media groups. For your convenience (you don’t have to ask on a forum or in a social media group) I have listed all four.
What is a balun?
Balun is an amalgamation of the words ‘balanced’ and ‘unbalanced’. The balun is used to decouple, as it were, the balanced (or symmetrical) antenna from the unbalanced (asymmetrical) feed line. For example, a dipole fed with coaxial cable. If you don’t do this, the coax will form an active part of the antenna. This can cause the antenna to produce an unfavorable radiation pattern and cause high frequency currents to flow over the coax screen. The latter leads to radiation on electrical appliances and thus interference in your own shack, home or even with your neighbors.
What is a unun?
The unun basically does the same thing as a balun, decoupling the antenna from the coax, so that the coax is no longer an active part of the antenna. But the unun is there for an asymmetrical antenna on an asymmetrical feedline. For example, a groundplane or an EndFed antenna fed with coax cable.
What is a common mode choke?
Actually, you can also think of the balun and unun as a common mode choke, provided the impedance of the antenna is not transformed, 1:1. The common mode choke is mainly used to suppress common mode currents. These can be produced by the antenna itself. In that case, the common mode choke is placed close to the antenna’s feed point. But it can also suppress common mode currents induced by sources nearby like electrical equipment. That will cause ‘common mode noise’ on reception, usually a a raised noise floor. In that case, the common mode choke is placed close to the receiver.
What is a line isolator?
This is often sold as some special device for EndFed antennas, but is actually exactly the same as a common mode choke, serving the same purpose.
What is an impedance transformer?
The impedance transformer is often confused with the balun and unun, while the impedance transformer does only one thing. Transforming the impedance of the antenna to that of the feed line and thus the receiver and transmitter. Unfortunately, impedance transformers are still often used as 1:1 baluns or common mode chokes, while they are often unsuitable for this purpose. A balun is not always capable of suppressing common mode currents and decoupling. A ‘4:1 balun’ to connect a dipole to 50 Ohm coax will adjust the impedance, but cannot adjust the symmetrical dipole to an asymmetrical feedline. Often, an impedance transformer is then combined with a 1:1 balun to form a “hybrid balun”.
1:4 or 4:1 balun?
It won’t be the first time that you think you are buying a 1:4 balun, to adapt a 12.5 Ohm antenna to 50 Ohm coaxial cable, but get your hands on a 4:1 balun to adapt a 200 Ohm antenna to 50 Ohm coaxial cable. The only correct designation for impedance transformation is Impedance antenna : Impedance feed line or Z ant : Z feed line.
Different versions of baluns, ununs and common mode chokes with different results
The balun, unun or common mode choke comes in different versions. With and without toroid, coax wound or bifilar with wire. Read more about the different types here.