Space cloth

Hypothetical plane with resistivity of 376.7 ohms per square.

Space cloth is a hypothetical infinite plane of conductive material having a resistance of η ohms per square, where η is the impedance of free space. η ≈ 376.7 ohms. If a transmission line composed of straight parallel perfect conductors in free space is terminated by space cloth that is normal to the transmission line then that transmission line is terminated by its characteristic impedance. 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. This equivalence can be used in reverse to calculate the resistance between two conductors on a resistive sheet if the arrangement of the conductors is the same as the cross section of a transmission line of known impedance. 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.

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