I hear all the time, "you need a balun between your balanced dipole and unbalanced coax to stop the common-mode current" when it seems the proper advice is "you need something to stop common-mode current (full stop)." Sure, attaching a balanced system (e.g. dipole) to an unbalanced system (e.g. coax) is a frequent source of common-mode current. But, this answer points out that even with a so called "balanced" antenna connected to a balanced feedline, the antenna often isn't really balanced, so some sort of common mode choke is a good idea no matter what.
So, if the same advice applies either way, why do we make such a big distinction between balanced and unbalanced? My assumption is that it's one of a couple things:
- Misconception: A lot of people may not realize a common-mode choke is recommended anyway.
- Old knowledge: This is sort of a special case of misconception. Maybe in the past, we didn't think of common-mode suppression as a generally recommended thing, and only later did it become common knowledge that this is a good thing even in balanced systems. Meanwhile, outdated information persists.
- Other reasons to differentiate: Even though common-mode suppression is a good idea regardless of balanced vs. unbalanced, there may be other ways we do treat them differently, or other reasons to distinguish between them.
My suspicion is the first one is true to some degree, but that the third case is the main reason.
To further muddy the waters, I'm seeing the word "balanced" used in two seemingly different ways. One is in the sense above where, for example, we have a "balanced" dipole that's actually unbalanced because one side is close to a hunk of metal, causing it to present an unbalanced impedance at the feed point. The other way I've seen it used is to represent a system where the two conductors have equal and opposite voltages relative to ground, versus unbalanced where one of the conductors is grounded. Are these two uses of the word actually the same thing? If so, how?