Rich Seifert wrote:
(snip)

You can use a transformer, but wired as a *balun* (coils in series with
the lines instead of across them); this provides the required impedance
transformation while still passing DC.

So that is how they do it.

TV baluns, from 300 ohm balanced to 75 ohm coax, don't do that.

http://en.wikipedia.org/wiki/Balun

Does that restrict which impedance transformation you can make?

-- glen