Winding Rod and Toroidal TransformersThe rod transformer, although not as popular as its toroidal counterpart, is the easier transformer to wind. This is because it can be wound one wire at a time and hence, work-hardening becomes less of a problem. Further, the turns can be wound much more tightly (i.e., less space between adjacent turns). This allows one to easily obtain a characteristic impedance of 25 ohms which is necessary with transformers matching 50 ohms to 12.5 ohms. With toroidal cores, one has to resort to low- impedance coaxial cable or stropline or a floating-third-wire.
The major trick in ending up with a tight bifilar winding on a rod is to put on the first winding as tightly as possible. The second wire is then fastened firmly at its beginning by lightly soldering it to the first wire. It is then wound around the rod by actually stuffing it between the turns of the first winding. ne then finds it becomes fast to the rod and results in the best electrical and mechanical condition. For a trifilar winding, the first winding should have about 1-wire-diameter spacing between turns. For a quadrifilar winding, the spacing should be 2-wire-diameters, etc.
The best procedure for toroidal transformers is to make a ribbon out of the wires and them wind them all at the same time. This assures their spacing and hence performances. The ribbon can be easily constructed by using thin sections of Scotch #27 glass gape as clamps every 5/8- to 3/4- inches. The tapes should be about 1/4 inch wide and long enough to wrap around the wires twice. These dimension were achieved by first cutting the 1/2-inch wide tape lengthwise for the proper width.
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