December 3, 2008 – Educational Radio Net
Jim Hadlock K7WA
What does a balun do?
What happens if you don't use one?
Bal-Un is a term formed from the words balanced and unbalanced. It refers to a device used to couple an Unbalanced transmission line to a Balanced load. In the real world, we use a balun to couple a coaxial transmission to a balanced antenna, such as a dipole.
Coaxial transmission lines are commonly used to connect our transceivers to antennas. Coax comes in several sizes and types for different applications. It consists of an inner conductor with an insulated covering (dielectric), which is then covered with a braided wire sheathing (shield). The sheathing is covered with a flexible outer jacket. Coax is weatherproof and may be buried underground, run inside a metal mast or taped to a tower without harmful effects. At the transceiver, the center conductor is connected to the transmitter output (or receiver input), and the shield is connected to the chassis. This arrangements works well with an unbalanced load, such as a vertical monopole antenna fed against a ground plane or radials. However, when coax is used to feed a balanced load, such as a dipole antenna, some provision should be made for converting from the unbalanced transmission line to the balanced load. Otherwise, RF currents will flow on the outer conductor of the coax, compromising the effectiveness of the antenna.
To understand this problem, think of a coaxial transmission line as a wire centered inside a metal pipe. When we connect the coaxial transmission line to our transmitter, the RF current flows on the center wire and on the inside surface of the pipe. This is due to what's called the "skin effect". The "skin effect" describes how RF currents flow in a thin layer on the surface of a conductor, proportional in depth to the wavelength of the signal. If we connect the other end of the coaxial transmission to a balanced antenna, such as a dipole, RF current from the center wire flows to one side of the antenna. The current from the inside surface of the pipe however, is connected to two conductors: the other side of the antenna and the outside surface of the pipe. Current flowing on the outside of the pipe is subtracted from the current that should be flowing on the antenna creating voltage and current nodes on the outside surface of the pipe back down to the transmitter where it is grounded. To go back to our coax fed dipole example, RF current on the outside surface of the coaxial transmission line shield will distort the radiation pattern of the antenna and detract from its effectiveness. It may also contribute to television interference.
A properly connected balun will reduce or eliminate the RF current flow on the outside surface of the coaxial transmission line shield. While the most common use of a balun is at the feedpoint of a balanced antenna, they are also used at the output of an antenna tuner to feed a balanced transmission line (Twin Lead) and even part way down a feedline to convert from balanced transmission line to coaxial transmission line (as in the G5RV antenna).
There are several types of baluns available to radio amateurs and described in the literature. Let's begin with the Current Balun (also called the Choke Balun). Current Baluns have become popular for application in the high frequency range (1.8 mHz to 30 mHz) because they are simple, cheap, and effective. In its simplest form, a Current Balun consists of a number of turns of coaxial cable wound into a close coil at the feedpoint of the antenna. The size of the coil is determined by the operating frequency. For example, the installation directions for the Cushcraft A3S tri-band yagi specify eight turns of RG8/U coaxial cable with a six inch diameter. This coil is a high impedance RF choke at the operating frequency of the antenna and prevents RF current from flowing on the outside of the coaxial transmission line shield. Another approach to the Current Balun was introduced by Walter Maxwell, W2DU. This involves slipping a stack of high-permeability ferrite beads over the coaxial transmission line at the feedpoint of the antenna. The stack of ferrite beads creates a high impedance effectively suppressing any RF current from flowing down the outside surface of the transmission line. Current Baluns and ferrite bead kits are available from many sources.
Another approach is the Voltage Balun as described by Jerry Sevick, W2FMI, and others. This design uses inductors to produce equal, opposite phase voltages into the two resistances, or halves of the antenna. An additional feature of the Voltage Balun is that, by using a combination of inductors as a broad-band RF transformer, it can accommodate impedance conversion in addition to balancing the RF voltages. Typical impedance conversion is 4:1, although Sevick describes transmission line transformers with many other ratios in his classic book: Understanding, Building, and Using Baluns and Ununs.
A third balun technique, most often used at VHF and UHF, is the Coaxial Balun made from a half wavelength loop of coaxial transmission line and presenting a high impedance to any RF current that might otherwise flow on the outer shield of the coaxial transmission line. The half wavelength Coaxial Balun gives a 4:1 impedance step-up.
While I have described how a balun improves the effectiveness of a coax fed balanced antenna, it also has other uses. Consider a vertical antenna with elevated radials. The outer surface of the coaxial transmission line shield will "look" to the antenna like another radial. A Current Balun at the feedpoint of the vertical will prevent RF current from flowing on the feedline. According to author John Devoldere, ON4UN, in Low- Band DXing: "Is it harmful to put a current balun on all the coaxial antenna feed lines for all your antennas? Not at all. If the feed point is symmetric, there will be no current flowing and the beads will do no harm. As a matter of fact they may help reduce unwanted coupling from antennas into feed lines of other nearby antennas."
Baluns are an effective means of preventing unwanted RF current on the outer shield of coaxial feedlines from distorting antenna patterns, as well as reducing TVI (radiation coupling into nearby television sets, house wiring, etc.) and RF in the shack.
ARRL Technical Information Service: An Analysis of the Balun, by Bruce A. Eggers
Some Aspects of the Balun Problem, by Walter Maxwell W2DU:
Baluns: What They Do and How They Do It, by Roy W. Lewallen W7EL:
Understanding, Building, and Using Baluns and Ununs, by Jerry Sevick W2FMI, CQ
Low-Band DXing (4th Edition), by John Devoldere ON4UN, The ARRL, Inc.
The ARRL Antenna Book (21st Edition), The ARRL, Inc.
The ARRL Handbook, The ARRL, Inc.
Palomer Engineers (1:1 Current Balun Kit): www.palomer-engineers.com
The Radio Works (Baluns, Coax, Antenna Parts, etc.): www.radioworks.com
Wednesday, December 3, 2008
Subscribe to: Post Comments (Atom)
Post a Comment