The Balun Transformer

4:1 Balun Transformer

Many modern HF transceivers come fully equipped with built in tuners. While these tuners are great for changing bands, the manufacturers left out a very important accessory; the 4 to 1 balun. With out a balun the transceiver can only feed an antenna which uses coaxial cable. While this may be satisfactory for some operators, this is a real problem for those of us who prefer the super low loss ladder line or wish to use coaxial cable and feed a folded dipole element which presents a feed impedance in the region or 200 Ohms. Ideal for transforming to 50 Ohms with a 4:1 balun. Fortunately, a 4 to 1 balun can be easily home brewed as illustrated in Figures 1 and 2.

Figure 1 shows a bifilar winding on a toroid. The toroid should be of a ferrite type material and can be any of the following sizes but the number of turns should be adjusted accordingly:

The exact number of turns is not critical but the numbers listed in the preceding table should yield optimum results. It is possible to exceed the power ratings listed above but the performance of the balun may be degraded during high SWR causing heating of the core. Figure 1 shows the construction details of the balun and I've colour coded the windings for ease of display. Ensure you mark the end of the wires to avoid confusion when assembled on the toroid.

As mensioned before, The Exact number of turns is NOT critical. However, as with everything you tend to build, you may find it neccesary to add or remove a turn or two if the desired effect is not reached. One factor which effects the performance of the balun is wire size. Most small constructions use #16, #14, #12 or #10 guage enameled wire.

1:1 Balun Transformer

As with the 4:1 balun, the number of turns can be adjusted accordingly shown in the previous table above. Figure 3 shows a trifilar transformer that produces a 1:1 impedance ratio, but will transform an unbalanced transmission line (e.g. Coaxial cable) to a balanced antenna (e.g. Dipole).

Although it provides no impedance transformation, it does tend to balance the feed current to the antenna. Many operators (me Included) make it standard practice to use a balun at the feed point to an antenna.

The balun should be housed in a suitable metal enclosure such as those available at Maplin Electronics. Use a SO239 or BNC connector for the unbalanced input. M4 - M6 bolts, insulated from the enclosure work fine for the balanced output terminals. Figure 5 shows the general arrangment.