page_title: Space cloth
section_title: Calculating surface resistance from characteristic impedance
hierarchical_section_title: Space cloth / Examples / Calculating surface resistance from characteristic impedance
caption_attribution_description: English: Transmission line composed of two parallel wires terminated by space cloth.
context_page_description: 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 is similar to the cross section of a coaxial cable transmission line.
context_section_description: The characteristic impedance of a two parallel wire transmission line is given by
  where d is the diameter of the wire and D is the center to center separation between the wires.
  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:
caption: Transmission line composed of two parallel wires terminated by space cloth.
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