The term balun is an acronym for BALanced-to-UNbalanced. This usually means connecting a balanced load such as a dipole antenna to an unbalanced input such as 50 ohm coaxial cable. The shield side of the cable is usually grounded.
The balun belongs to a class of devices known as “transmission line transformers”. Transmission line transformers are formed by winding bifilar turns, multifilar turns, coaxial cable, or strip-line cable (two strips of flat conductor with a dielectric material between the strips) on a core having high permeability. Because of the high impedance of the winding, the balanced output terminals are isolated from the unbalanced input terminals.
There are two general classes of baluns: the voltage balun and the current balun. The voltage balun forces equal voltage across the two sides of a balanced load and the current balun forces equal current through the two sides of a balanced load. The 1:1 current balun is usually nothing more than a simple transmission line (e.g. bifilar winding or coaxial cable) wound on the core. Voltage baluns are a little more complicated.
When to use Baluns?
Baluns are usually used to feed dipole antennas with 50 ohm coax and to transpose from ladder line feeding a multi-band antenna to the unbalanced input of an antenna tuner.
The primary reason for using a balun at the feedpoint of a dipole antenna is to prevent RF currents from flowing on the outside of the shield of the coax feeding the antenna. RF current on the outside of the coax shield causes the shield to radiate. This may cause a distortion in the radiation pattern of the antenna and may cause TVI and telephone interference if the coax is passed close to a TV cable or telephone line. If the “hot” shield is brought into the shack, one might encounter the “hot mike” problem where the mike bites one’s lips when touched. Also interference to PC’s and other devices attached to the transmitter are common.
Let’s look at how these things occur and decide whether a balun is needed.
Figure above is the classic diagram describing what happens to RF currents at the feed point of a coax fed dipole. This diagram can be found in most books on antennas. The RF current from the transceiver is contained within the coax flowing on the center conductor- I1 and the inside of the coax shield-I2.The current on the inside of the shield does not reach the outside because RF currents only flow on the surface of conductors. This is the so called “skin effect”. When the RF currents reach the antenna, the current on the center conductor flows into one side of the dipole. The current on the inside of the shield flows into the other half of the dipole and down the outside of the shield of the coax-I3. The amount of current taking each route will depend upon the impedance of each route. For example, if the impedance of the outside of the shield is very high, most of the current will flow into the antenna. However, if the impedance of the shield is low, the current will be divided between the antenna and the outside of the shield. If currents exist on the outside of the coax shield, a 1:1 balun at the feed point will likely correct the problem. A current balun is best for this use.
Usually the shield of the coax feed line is grounded somewhere near the input. Preferably, it will be grounded before the coax enters the shack such as at an antenna switch. If the length of the coax is ½ wavelength from the ground point to the antenna feed point, the outside shield will reflect the low ground impedance at the feedpoint and a large amount of current will flow on the shield. Conversely, if the length of the coax is ¼ WL from the ground point to the antenna feedpoint, the shield will present a very high impedance at the feedpoint and most of the RF current will flow into the dipole antenna. Coax lengths that are not a multiple of ¼ or ½ WL will have a current level on the shield somewhere between the two extremes. The velocity factor of the coax is not involved in determining the length in wavelengths. One half wavelength will be approximately 468/frequency in MHz.
If the feed line is not grounded outside the shack but is brought to the transceiver and the transceiver is grounded, the length of the ground wire must be added to the length of the coax plus some allowance for the equivalent length of the transceiver case. In this case one is bringing the radiating coax into the shack which is not good. Also, if the length of the ground wire is ¼ WL, the case of the transceiver and the mike will be very hot. Some Hams have also experienced computer problems because of a high amount of RF.
To cure this one need to install additional current balun at the transceiver end of coaxial cable which presents high impedance to currents flowing on the outside of shield. This Balun is NOT actually Balun in this case but Line Isolator or most known as Common Mode Choke or shortly CMC. Read more about CMC’s at here.
- ARRL Antenna Handbook