Leitungstheorie
A more subtle type results when the transformer type (magnetic coupling) is combined with the transmission line type (electro-magnetic coupling). Most typically the same kind of transmission line wires are used for the windings as carry the signal from the radio to the antenna, although these baluns can be made using any type of wire. The resulting devices have very wideband operation. "Transmission line transformers" commonly use small ferrite cores in toroidal rings or two-hole, binocular, shapes. Something as simple as 10 turns of coaxial cable coiled up on a diameter about the size of a dinner plate makes an effective choke balun for frequencies from about 10 MHz to beyond 30 MHz. The magnetic material may be air, but it is a transmission line transformer.