Classical, impedance-quadrupling BalUn transformer.

Some readers might ask themselves, ‘What the heck is a Balun transformer?’ And the answer is that, in certain high-frequency applications, this term gets used for a Balanced-to-Unbalanced (impedance-matching) transformer, often implemented as a transmission-line transformer. One common place they did get used in years gone by was, to allow people to connect 300Ω twin-lead TV antenna cable, to the 75Ω coax inputs of more-recent TVs. Actually, what was inside those little adapters was, a toroidal ferrite core, with a piece of sheet-metal (probably aluminum) stamped around it in a clever way, so that this stamped sheet of metal also acted as the ~windings~ of the transformer.

Really, this type of transformer does the same thing that an ‘autotransformer’ does, only, at much higher frequencies. If the reader is picturing a (center-tapped) autotransformer with many windings, then he or she should also picture how many implicit, internal capacitors those have (between the windings), and how capacitors become increasingly conductive, at higher frequencies… Traditional, wound transformers start to become useless well before 100MHz has been reached.

If people look this subject up elsewhere on the Web, They might find diagrams of various types of transmission-line transformers. But, it’s easy to get confused about the way those need to be connected, so that one possible result could be, a transformer that does not work correctly. For that reason, I have just reconstructed how I remember them to have been configured in the past:




I suppose that another piece of possibly related trivia could be, that an impedance of, say, 150Ω, connected to a voltage of zero, is equivalent to 300Ω, connected to a relative voltage of (-1). Another related assumption is, that such transmission lines are indeed wound on effective ferrite cores, capable of choking their net current to zero.


Now, there’s another, related application of transmission-line transformers, which could be, that a number of transistorized output drivers might only be able to handle some higher (load-) impedance (each), but that the goal is to combine their amperage, so that a divided output-impedance also results, at minimal waste of energy. Additionally, some small mismatch in the outputs could be expected, which should be absorbed, and not result in reflected waves…

(Updated 6/02/2021, 9h15… )

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