balun?

The most common ones have 75 ohm coax on one end and 300 ohm flat lead on the other. . .

US TV Channel 13 == 216 MHz Channel 83 == 890 MHz

Balun = Balanced to Unbalanced. It is used to convert a balanced line to single ended, or vice versa.

The answer in this case is in the name: a "balun" is a device for converting between an unbalanced line and a balanced load, or vice-versa. Hence, BALanced-to-UNbalanced (transformer, although not all "baluns" are transformer-like devices - you can make a balun for a given frequency from the right length of transmission line, f'rinstance). Pretty common in antenna/transmission line work.

For the transformer type, it's not, really, in the basic theory of the thing - it's more in how it's connected, and in practice that generally it is intended for high-frequency, fairly broadband operation (very high, compared to power transformers and the like).

Bob M.

Indeed. A typical balun would appear as a dead-short to a mains frequency signal.

You've never seen a line isolation transformer?

So the primary has one side grounded, while the secondary goes to the inputs of a diff. amp, with center tap to ground?

Mark

Why assume a grounded center tap? Better to say that the primary - or input side - has one end connected to the "ground" or reference used for the unbalanced output of whatever you're connecting to, and the output side is connected to a balanced input - for instance, perhaps it feeds a dipole antenna. No need for a "ground" at all on the output side!

Bob M.