Before revision: When the separation is many times greater than the wire diameter then the function can be approximately replaced by a natural logarithm (with its argument doubled): The exact and approximate formulas for the separation needed to achieve some given characteristic impedance through a pair of wires are therefore The dielectric material between the two conductors with either twin-lead or ladder line is not all air .
After revision: When the separation is many times greater than the wire diameter then the function can be approximately replaced by a natural logarithm (with its argument doubled): The exact and approximate formulas for the separation needed to achieve some given characteristic impedance through a pair of wires are therefore The dielectric material between the two conductors with either twin-lead or ladder line is not all air