Space cloth

[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.

Coaxial cable terminated with space cloth (green).
Resistance of an annular ring of material having a surface resistance of η per square. The radius of the outer surface of the inner electrode (yellow) is r 1 . The radius of the inner surface of the outer electrode (magenta) is r 2 .
Transmission line composed of two parallel wires terminated by space cloth.
Transmission line composed of two parallel wires cross section
Cross section of a three conductor transmission line composed of two parallel plates and a rectangular shield.