Balun is short for Balance To Unbalance Transformer. Balun can use to transmission between balanced and unbalanced, impedance transformation, and so on.There are current and voltage balun to select for us.
Current balun suppresses the common mode current generated by the balanced-unbalanced conversion by increasing the outer surface impedance of the coaxial cable shielding layer, so it is also called Choke. Common current baluns is the magnetic rings baluns, magnetic beads baluns and hollow baluns.
Many hams basically use current-type baluns when they use Yagi antennas, which are simple in structure and very effective. Take a hollow balun as an example. Find a cylinder with a suitable diameter as the skeleton, wind the coaxial cable neatly on it and fix it to make a balun. In this way, it can be made using the feeder we usually use, and the loss is relatively small. But this kind of balun covers a relatively narrow frequency range, and it works best for single-band Yagi. If it is used in multiple bands, it is best not to exceed the use range of one octave, that is, 3.5-7Mhz.
Voltage Balun is made of insulated wires wound on magnetic materials in different ways, and the realization principle is more similar to that of a transformer. Common voltage baluns is the auto-coupling baluns and the classic transformer baluns. Among them, the most typical transformer balun achieves the purpose of balanced-unbalanced conversion through two sets of mutually isolated coils.
As a lot of ham said, the voltage baluns only works when the antenna is very symmetrical, in fact it is often impossible. However, voltage balun is very useful when the antenna is used in a wide band and the impedance changes greatly. Currently, most of the baluns sold on the market are voltage baluns.