Ken Hough's Website
The Need For Baluns:
Many kinds of antenna in general use are BALANCED antennas. That is they are essentially symmetrical and are not fed against an RF ground connection. Dipoles, doublets, yagis arrays, and most vertical antennas that do not include ground plane elements are examples of balanced antennas which should ideally be connected via appropriate balanced twin feeder lines. This kind of arrangement ensures that RF energy is not radiated from the feeder lines.
Balanced feeders are intended to be used in free space and should not pass close to other conducting materials, which can be a problems in confined spaces. Mainly for this reason, many folks prefer to use coaxial feeder cables.
Coaxial cables restrict electrical fields associated with the passage of RF signals to the space between central conductor and the inner surface of the braid. RF fields should not appear on the outside of the coax. Coaxial cables are not affected by proximity to other conducting materials. Very convenient, but there is a problem when connecting coax feeders to balanced antennas.
Imagine the core conductor of a coax feeder connected to one side of a dipole and the braid of the coax connected to the other side of the dipole. On the core side, currents that are generated in the dipole will flow into the core of the coax and will be screened from outside influences by the braid of the coax. However, currents generated on the braid side can flow into the braid which on the outside is NOT screened, nor is this current balanced by an adjacent anti-phase current as is the case with a balanced feeder. Current flow on the outside of the coax (sometimes referred to as common mode current) will cause significant radiation of RF to occur from the outer surface of the coax feeder, and is likely to be noticeable as high and varying SWR readings when the feeder is moved about, or even as very high levels of RF in the shack!
Note: Common mode currents can also be caused by antennas are which not properly balanced. This could happen as a result of poor design or a fault.
Common mode currents can be practically eliminated by including a BALUN (BALanced to UNbalanced) device between feeder and antenna.
Several kinds of balun are in use, but only "choke baluns" and transformer baluns are discussed here. Choke baluns are used specifically to block the flow of RF current on the outsides of coax feeders. Transformer baluns can also include impedance matching.
Choke Baluns work by effectively increasing the inductance of the outer surface of the braid to the extent that current flow is blocked. This has no effect on the normal flow of RF current via the core and inside surface of the braid.
A choke balun can be realised by winding several turns of coax feeder nearest to an antenna into a coil. However, because of inter-turn capacitance, there will be an optimum number of turns and diameter for a given frequency. Best results are obtained if the turns are arranged side by side rather than being bunched up. Actually, useful performance can be expected over a moderately wide frequency range. Both the ARRL Handbook (2009 ed.) and the RSGB Communications Handbook (11th ed.) give dimensions for coax baluns from which the following table was derived:
Frequency (MHz): Number of turns: Diameter of turns: Number of turns: Diameter of turns:
3.5 8 837 6-8 870
7 10 670 6 767
14 8 437 8 300
21 6-8 343 8 225
28 6-8 257 6-8 174
G4FGQ gives details for a 2m choke balun as follows:
6 turns of RG-58
Length = 30 mm = 1.2"
Diameter = 37.5 mm = 1.5"
Operation over extended frequency ranges can be achieved by incorporating ferrite material into a choke balun. Coax feeder can be wound onto a short length of ferrite rod, or onto a toroidal ferrite core. Similar effects can be achieved by stacking small toroids side by side onto the outside of coax feeder. The latter solution does work well, but can be expensive to implement.
Some examples of baluns are shown below. Note that these images have been taken from the RSGB Communications Handbook -- 11th edition, which includes an excellent review of the subject. Recommended reading!
How effective is that choke balun?
A clip-on RF current measuring device can be used to determine the magnitude of common mode currents on coax feeders. Measuring this current before and after fitting a choke balun will show how effective the balum is.