Baluns (balanced to unbalanced transformers) and ununs (unbalanced to unbalanced transformers) belong to a class of matching devices known as transmission line transformers. They transmit the energy from input to output by a transmission line mode instead of by flux linkages as in the case of conventional transformers. When properly designed, they can have extremely high efficiencies and very broad bandwidths. The theory of operation of these devices rests chiefly on that of chokes and transmission lines. A balun or unun is simply a choke that isolates the input from the output (thus only allowing transmission currents to flow) and a configuration of transmission lines.
A major difficulty (even today) is that this very important and popular class of matching devices is not included in the curriculums at universities. In fact, there are not standards for the specification and testing of these devices by any professional group! As a result, practically everyone still perceives these devices as conventional transformers and not transmission line transformers. To remedy this, it has been suggested that this class of transformers be called broadband transmission line matching networks.
For more detailed information, please read the following written by Dr Jerry Sevick, and available from ByteMark.
1) Transmission Line Transformers, 2nd Edition, published by ARRL. Contains analysis, characterization and designs of baluns and ununs. Good for students, engineers and designers.
2) Transmission Line Transformers Design Handbook. Contains 48 improved designs of baluns and ununs covering all impedance rations. Good for "do-it-yourself" specialists, hobbyists and amateurs.
3) Building and Using Baluns and Ununs: Practical Designs for the Experimenter, published by CQ Communications. The all new definitive source for his latest practical information and designs. Unique opportunity to learn about the application of baluns and ununs for dipoles, yagis, log periodics, beverages, antenna tuners and countless other examples.
4) Series on baluns and ununs in Communications Quarterly and CQ Magazine. Describes various types of balun and unun designs. Good for experimenters and "do-it-yourself" specialists.
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