Ever heard the term “balun” and wondered what it meant?
If that’s the case, please read on.
Balanced + Unbalanced
The term balun is a portmanteau (a blending of two words) of balanced/unbalanced – BAL-UN.
A balun transformer is used in antennas for matching the wideband impedance of the antenna to that of the amplifier. It is often the balun that determines the maximum amount of power that can be delivered to an antenna used for EMC testing purposes. Balun equipped antennas can also be used for radiated emissions testing.
Bilog antennas (ironically also portmanteau for biconical/log-periodic – BI-LOG) or just a plain biconical or plain log-periodic antenna must be protected against accidental damage during EMC compliance testing. A balun transformer, located at the antenna’s feed-point, limits the power-handling ability of the antenna because some of the power delivered to the antenna ends up as heat in the core and windings.
The balun is usually comprised of a wideband ferrite core (1:1 transformer) that converts the balanced feed of the antenna to the unbalanced connection of the coax cable. As you may already be aware, coax is considered an unbalanced transmission line – the inner wire carries the intended signal, the shield carries it’s return.
The balun is typically supplied as part of the antenna – not added on later. Even though balun losses are minimal in respect to other losses in the system, the antenna calibration certificate includes a factor that accounts for losses of the balun. So, don’t fret about that, but do fret about the possibility of applying too much power to the balun that it gets fried when used for immunity testing. It may be a wise decision to use a separate antenna for emissions testing than immunity so that in the event that you damage the balun during immunity testing, you’ll still be able to perform radiated emissions testing.
Common-mode chokes are also used as baluns. Common mode chokes utilize ferrite material which absorbs common mode RF energy present and turns it into heat within the core.
How a Balun Works
A balun works by choking off the outer current sometimes found on the shields of coaxial cables. This function helps restore balanced antenna operation which is required for proper functioning. In respect to dipole antennas, the term balanced means currents flow equally on each side or arm of the antenna.
Now here’s where it gets a bit confusing…stay with me now.
When a coax is connected to a dipole antenna, the current on the center conductor has no place go, other than flow along the dipole arm that is connected to it. On the other hand, the current that travels along the inner side of the outer conductor (the shield) has two options: 1) flow down the dipole antenna arm like we want, or 2) flow down the reverse (outer) side of the outer shield conductor of the coaxial cable like we don’t want. Ideally, the current in item 2 should be zero, but often it’s not.
In the case of item 1, the current along the dipole arm connected to the outer conductor of the coax will be equal to the current on the other dipole arm. With currents in both arms of the dipole antenna being equal in magnitude, the dipole antenna operates correctly. It’s considered balanced. Life is good.
Now for the non-ideal situation…Sometimes coaxial cables do not allow balanced current flow. Some current will progress along the outside of the outer (shield) coax, leading to an unbalanced operation of the dipole (current not equal on each side or arm of the antenna). The solution to this problem is the balun. A balun forces an unbalanced transmission line to properly feed a balanced component, like the antenna. This is accomplished by forcing any current flowing down the reverse (outer) side of the outer shield conductor to zero. Hence the term “choke” meaning choking the current to zero and restoring balanced operation.
That’s about it – pretty simple huh?
Summary
Now that you have read this brief article you, too, should not only know what a balun is but be able to explain it in simple terms so someone else can understand it. If you’d like to learn more about this subject, please check out the references listed below.
