Before revision: The characteristic impedance of any transmission line is given by where for twin-lead line the primary line constants are where is the wire diameter and is the separation of the wires measured between their centre-lines , is the absolute permittivity between the wires , is the length of the wire, and where the surface resistance of the wires is given by Neglecting the wire resistance and the leakage conductance , this gives where is the impedance of free space (approximately 376.74 Ω), is the relative permittivity (which for air is ).
After revision: The characteristic impedance of any transmission line is given by where for twin-lead line the primary line constants are where is the wire diameter and is the separation of the wires measured between their centre-lines ; is the absolute permittivity between the wires ; is the absolute permeability; is the surface conductivity between the wires;operating at frequency or radian frequency and where the surface resistance of the wires is given by Neglecting the wire resistance and the leakage conductance , this gives where is the impedance of free space (approximately 376.74 Ω), is the relative permittivity (which for air is ).