TABLE OF CONTENTS The balun ( balanced-to- unbalanced) transformer is a subset of the general class known as transmission line transformers. This class differs greatly from the conventional class of transformers where energy is transmitted from input to output by flux linkages. Here, energy is transmitted by a transmission line mode, and the conventional current flow is prevented by the choking action of the coiled transmission lines. This conventional current flow can lead to high flux densities and saturation when the balun is improperly designed or the impedance, as seen at the output, is much greater than expected. The balun, as well as other forms of the transmission line transformer, does not have a primary winding or a secondary winding as such. Its operation in the passband, where only transmission line currents flow, is analyzed purely by transmission line theory. The objectives then are: (A) to have sufficient reactance in the coiled transmission lines to prevent the unwanted conventional current. (B) to select the proper characteristic impedance in order to optimize the high-frequency response and (C) to minimize the parasitics which eventually reduce the inductive reactance of the coiled transmission lines at high frequencies to unacceptable values.
To date, practically all of the 1:1 and 1:4 balun designs, excluding baluns using ferrite beads or 1/2- ? and 1/4- ? sections of transmission line, have used the schematics of Ruthroff which were presented in his classic paper in 1959 (ref 9). [1] But the earlier paper...
