Coaxial cable has the very useful property of being a
waveguide. Apply a voltage to one end and the signal flows down the cable via an
electromagnetic wave inside the cable, supported by
voltages between the
center conductor and the
shield (
braid) and currents existing on both.
When using coax to transmit a signal to an antenna, the transmitted signal flows along the inside of the cable. It must couple to the antenna at the feedpoint, where the unbalanced nature of coax appears. Consider the case of a coax connected to a dipole antenna. The user may connect the center conductor to one arm of the dipole and the shield to the other. However, the conducting shield has two sides. The inside of the shield carries the signal from the transmitter, however the outside of the shield exists exposed to the environment and can support radiating currents. Currents can form on the outside of the shield with the dipole arm it's in common with and begin to radiate. This is undesirable, and can produce an erratic, skewed radiation pattern.
A balun is often used to prevent this behavior. Typically the shield and center conductor are connected to leads that are then wound several times around a toroid before being connected to the antenna feedpoint. The toroid acts to channel the energy from the coax to the antenna and prevents currents from flowing on the outside of the shield.
These toroids may be created by winding wires around ferrite cores, or simpler ones created by winding many turns of the coaxial cable into a tightly bouncd air coil before connecting it to the leads of the antenna.
The windings in a balun may also be used to the operator's advantage by using them to transform the impedance from one side of the coil to the other. A 50-ohm transmitting system can be made to match a 200 ohm wire antenna by using a 4:1 transforming balun. One can buy commercial baluns with transforming ratios from 1:1 to 12:1 or greater.