Copyright (c) Ralph Holland 1996
A Cost Effective Current-mode 1:1 Balun
describes a 1:1 balun.
A cost effective current-mode 1:1 balun can be constructed from a length of coax and a rod typically used for a broadcast antenna loop-stick, some electrical tape, cable ties, a length of PVC water-pipe and some connectors. The balun is formed by winding several turns of coax on the ferrite rod.
The operating principle is that the inner conductor and the inside of the braid act as two opposing bifilar windings with substantial inductance inserted in the outside of the braid. Differential current passes through such a transformer with little insertion loss as the opposing windings of the transformer mode effectively eliminate the winding inductance. If you want to run an unbalanced differential current through the transformer then substantial inductance will be present. Thus the current balun suppresses common-mode current. Since current flowing on the outside of the braid, is referenced to ground it must flow through the impedance resulting from the winding inductance formed by the outside of the braid and the core. This inductance will reduce the current if the impedance is high enough.
The same principle applies in the common-mode choke where two or more wires pass through a ferrite core. A typical example is seen in the ferrite chokes clamped on the monitor cable of computers.
High permeability cores can be used for current-mode baluns or common-mode chokes as there is not net magnetic field around the bifilar winding even though substantial currents are flowing.
Figure 1 - Schematic
A ferrite rod is easier to wind and cheaper than a toroid. At 160m I found that I needed 30 turns of RG58 U/C to ensure that I obtained equal, but opposite current, in each leg of an asymmetrically mounted dipole. To place 30 turns you will need to wind more than one layer across the core. The turns can be held by insulation tape and by applying two cable ties on the ends of the last layer. One end of the coax is terminated in a connector while the braid and centre conductor are split out and used as the balanced feed at the other end. You should use coax with adequate breakdown voltage to avoid damage when operating into mismatched loads.
Figure 2- Construction details for 1:1 balun
The balun can be housed in PVC water pipe. Cut a section large enough to make two end pieces which can be flattened with the aid of the hot air from a hair-drier or heat-gun. The circular end-sections can be cut with tin-snips. I drilled a hole for a panel-mount connector in one end and used banana connectors for the balanced feed on the other end. The end sections should be inserted inside each end of the pipe and held in place with the PVC glue. I have found hot-melt glue adequate and easily removable. Extra protection is obtained for the ends if you leave an overhang by inserting the ends further into the pipe. My balun has survived several four-wheel-driving desert trips and is still intact and operating after five years.