A Common-mode currents on antenna feed lines are often the culprits when directional patterns are distorted or SWR readings become unpredictable. The solution is a balun—a contraction of the words balanced to unbalanced. Baluns eliminate these common- mode currents while making the transition between an unbalanced feed line (such as coaxial cable) and a balanced load (such as an antenna).
Figure 2—The W2DU balun consists of ferrite cores over a length of coax.
There are many types of balun designs, but Walt Maxwell, W2DU, developed the one you’re asking about. It is a choke-type balun that consists of a number of small ferrite cores placed directly over a section of coaxial cable where it is connected to the antenna. The W2DU balun in Figure 2 is a low-power version using 50 Amidon FB-73-2401 cores slipped over a 1-foot length of RG-58 coax. Twelve Amidon FB-77-1024 cores on RG-8 or RG-213 will do the same job. The 70-series cores are best for HF work. Type 43 or 61 is best for VHF. The W2DU balun is very effective and, best of all, very easy to make.
From QST March 1999
