of a radio frequency balun was found at the antenna terminals of a television receiver. Typically a 300-ohm balanced twin lead antenna input could only be connected to a coaxial cable from a cable TV system through a balun. Characteristic impedance The characteristic impedance of a transmission line is an important parameter at higher frequencies of operation. For a parallel 2-wire transmission line, where is half the distance between the wire centres, is the wire radius and , are respectively the permeability and permittivity of the surrounding medium. A commonly used approximation that is valid when the wire separation is much larger than the wire radius and in the absence of magnetic materials is where is the relative permittivity of the surrounding medium. Electric power lines In electric power transmission, the three conductors used for three-phase power transmission are referred to as a balanced line since the instantaneous sum of the three line voltages is nominally zero. However, balance in this field is referring to the symmetry of the source and load: it has nothing to do with the impedance balance of the line itself, the sense of the meaning in telecommunications. For the transmission of single-phase electric power as used for railway electrification, two conductors are used to carry in-phase and out-of-phase voltages such that the line is balanced. Bipolar HVDC lines at which each pole is operated with the same voltage toward ground are also balanced lines. See also Differential pair Twinaxial cabling Twisted-pair cable Balanced transmission standards Ethernet over twisted pair RS-422 RS-485 Low-voltage differential signalling (LVDS) References External links Balanced Lines, Phantom Powering, Grounding, and Other Arcane Mysteries – from Mackie; Communication circuits