One of the questions I normally get from students in the RTO Course once we move into HF is all about Baluns- what are they, what do they do, why are they needed and which one is the right one for my needs? The answer, like a lot of things in radio, can get pretty technical and go over people’s heads. The concept behind a balun is that its a type of transformer. BALanced to UNbalanced line. Dipole antennas are electrically balanced, and when they are fed with coax, they need a transformer because coax is electrically unbalanced. Without that transformer, residual current will cause your feedline to radiate as well- which causes a significant amount of loss and an unreliable radiation pattern.
The other reason you need a transformer is that the impedance (resistance) value, measured in Ohms, is usually never exactly 50. Your radio needs 50 ohms- too much resistance, and you’re not only inefficient but you’re generating heat. For that reason Baluns are measured in ratios- the most common ones are 1:1 (50 ohms:50 ohms, also known as a current choke). 4:1 (200:50), and 9:1 (450:50).
Anyway, that’s the quick and dirty from my end, here’s the rest: All About Baluns