(Partly from http://www.electronics-lab.com/blog/?p=18953 )
In deriving the formula for characteristic impedance resistance per unit length can be neglected. The impedance can be found from the values of capacitance per unit length (C/h) and the inductance per unit length (L/h):
where D is the diameter inside diameter of the shield, d is the outside diameter of inner conductor, h is the length of the cable, and e is the dielectric constant of the insulator.
The series inductance per unit length, in Henrys per meter, is given by: