First, a gentle reminder - this thread is "Anecdotes & Gripes" - you
might do better posting this on the "Circuits" or "Antennas" threads...
a balun converts a signal from unbalanced (2 wires, one "ground", the other "signal")
to balanced (AGAIN, 2 wires - not 2 signals, as a general rule), usually not referenced
to "ground". (Ground is often no more than a convenient fiction at UHF and microwave
frequencies, anyway (Grin!)
Baluns are implemented in a variety of technologies,
some more frequency-sensitive than others. So you can find both narrow-band and wide-band
balun designs. (For the latter, see Jerry Sevick's book "Transmission Line Transformers).