Baluns
The word BALUN is a shortened form of the words BALanced to UNbalanced. It describes a passive device intended to connected a "balanced above ground" differential pair of wires, i.e. zip-cord, to an unbalanced wire in his case coaxial cable. It can also be used to connect a balanced pair to another balanced pair having a different characteristic impedance. Most balun use ferrite or powdered-iron cores and cores shaped as toroids or binocular cores to keep their physical dimensions manageable. Air core balun a rather large but have the advantage of less losses.
NOTE: A balun is a passive device, like any transformer, and hence couples in both directions. So a 4:1 balun can became a 1:4 balun in the blink of an eye.
Amateurs use the term Voltage Balun and Current Balun to describe two forms a Balun. Here is a very general definition of Voltage vs Current Balun. A Voltage Balun at the balanced port has equal voltage magnitude but opposing phase, and a Current Balun has the current at the balanced port with equal current magnitude but opposing phase.
Some balun have no transformer connection, just a length of coaxial cable wound around a former or strung with large ferrite beads to break up any RF currents on the outer conductor of the coaxial cable. These are called Choke Balun and are generally categorized as a current balun.
Below you find a few pictures of some of my experiments with Transmission Line Transformers, be it a balun, a balbal or an unun.