biferi,
Really want to understand it all? Oh boy, get ready to do a lot of learning! Not here, and not from me though.
Very simply, a balun changes between a 'balanced' and 'un-balanced' state. Between an antenna that may or may not be 'balanced', and a feed line which also may or may not be 'balanced'. There are balanced and un-balanced antennas AND feed lines. Putting one of those baluns in the appropriate place is certainly what they are designed to be used for. The 'trick' is in knowing which thingy is, and is not, balanced. That's where that learning comes in, knowing just what 'balanced' means, and which thingy is and isn't, and why.
Coaxial feed line is not a balanced feed line. If you feed an unbalanced antenna with it, there's no reason to use a balun. If you feed a balanced antenna with coax, there is a reason to use a balun. Depending on a whole lot of things, sometimes using a balun is more trouble than it's really worth, too. Lots of opinions about that particular aspect, so draw your own conclusions on what is 'necessary'.
The 'other' aspect of a balun is that it's a transformer, and can change impedances from one side to the other side of it. Exactly like a voltage transformer can change voltages (240Vac to 120Vac). That give you an option when using 50 ohm coax and having an antenna that isn't close to 50 ohms, but to maybe 100, or 300, or whatever. Handy at times, and also misunderstood at times. Knowing which way that transformation needs to go determines which way around that balun has to connect. Otherwise, it's like getting that 240/120Vac transformer turned in the wrong direction... not very good, right? And if that ain't enough, baluns are frequency determined. May work well in one range of frequencies, crappy in a different range of frequencies. Great, just great, more crap to learn! I warned ya'...
- 'Doc
I just love this s4#^! Don't you?