[2][a] The calculation of the characteristic impedance of a transmission line composed of straight, parallel good conductors may be replaced by the calculation of the D.C. resistance between electrodes placed on a two-dimensional resistive surface.
For example, a pad surrounded by a guard ring on a printed circuit board (PCB) is similar to the cross section of a coaxial cable transmission line.
In the case of a closed structure like a coaxial cable, the space cloth may be trimmed to the boundary of the outer conductor.
This is exactly the equation for the characteristic impedance of a coaxial cable in free space.
If the second figure is taken to be two round pads on a printed circuit board that has surface contamination resulting in a surface resistivity of Rs (50 MΩ per square, for example) then the resistance between the two pads is given by: The figure shows the cross section of a three conductor transmission line.