About Baluns - Balun Fundamentals and Terms

Balun is an acronym for BALanced to UNbalanced, which describes certain circuit behavior in a transmission line, source, or load. Most communications applications deal with two-terminal sources, transmission lines, and loads. This includes coaxial cables, open wire lines, and systems working against earth or a ground plane as the ”second conductor”.

Baluns are often selected based on the ratio between 50-ohm coaxial systems and the advertised impedance of another transmission line. As an example 9:1 baluns are often used to connect the normal 50-ohm impedance of our radios to a 450-ohm transmission line regardless of what the actual impedance is on the line or the impedance of the antenna. Sometimes, this is because designers and end-users fall into the habit of using the same balun others use. Sometimes, it is because of a mistaken idea of what a balun is supposed to do.

Balanced and Unbalanced

The difference between ideally operating unbalanced and balanced lines lies in system voltages, rather than currents. Balance is referenced to voltage, not current, in an ideal system.

An unbalanced (coaxial) line has significantly different voltage from each conductor to ground. In a perfectly working unbalanced (coaxial) line, the amount of voltage unbalance is infinite. One terminal (the shield) has zero voltage to the outside world, even while currents are equal and opposite. Current on the center conductor is balanced by an equal but opposite flowing current on the INSIDE of the innermost shield. No matter how we feed or connect a coaxial line, all current on the center conductor is always matched by an equal and opposite current on the shield. In figure 1, the 1-ampere shield current flows on the inside of the shield, nearest the center conductor.


Figure 1. Coaxial line currents

If the two terminals of the load or source do not carry equal currents, some current will flow in a loop through the ground or along the outside of the shield. The outside of the shield or shields is isolated by skin effect in the conductor wall. At radio frequencies the outside of the shield can be treated as an independent conductor connected to the “inside shield” at the ends of the coax. Most coaxial baluns make the outside shield connection a high impedance while not disturbing the inside operation.

Figure 2. Coaxial line current directions

Note in Figure 2 the direction of current outside the shield can be (and often is) opposite current on the inside. The shield’s outer surface current is independent of the balanced center conductor and inner shield current. The outside of the shield can easily pick up currents by passing too closely to an operating antenna. It will also radiate when excited with time-varying currents from an improper load or source.

A perfectly operating balanced line (Figure 3) has equal and opposite voltages, as well as equal and opposite currents, all along the length of the line. Any difference in opposing voltages along the line can cause the line to radiate, since that often means currents will become unbalanced. All operating balanced lines are surrounded by external magnetic and electric fields. This effect is caused by the necessary separation of conductors in the line. To minimize radiation, balanced lines should be twisted or transposed at fractional wavelength intervals. If you look at older open-wire telephone or signaling lines, they are periodically transposed.

Figure 3. Balanced line

(The portion of current not equal in amount and opposite in phase on any transmission line is called the common-mode current. The amount of opposing phase current is the differential-mode current. Differential mode operation, the normally desired method of operating a transmission line, has impedance. This is the impedance we talk about when we say a line is 50 ohms, or 450 ohms. This impedance is different than the common mode impedance.)

Most lines fall somewhere short of perfect examples, but the closer to perfect the less energy lost as unwanted radiation. Perfection also means the feedline does not pick up unwanted signals and noise, and RF will not appear on equipment near the transmitter unless it is from antenna or equipment radiation. In short, your antenna becomes the point of most signal reception and radiation. Most of us want the antenna to be an antenna, and the feedline (which often runs near computers, radios, TV sets, and noise sources) to NOT be an antenna!