The role of the drain wire in modern foil cable shields

Theoretical and experimental analysis of a wide range of shields that combine an electrically thin cylindrical shield with a drain wire/current diverter shows that the surface transfer impedance can be characterized as a solid foil shield in parallel with the impedance of the drain wire. The drain wire/current diverter should be modeled as an inductor and a frequency-dependent resistor in series. The resistance is frequency-dependent because of skin depth effects. Neglecting high-frequency aperture coupling, the surface transfer impedance of a foil shield with drain wire can be characterized in three frequency regimes. At low frequencies, the transfer impedance is determined by the resistance of the drain wire. At intermediate frequencies, the resistance of the drain wire increases due to skin depth and inductive effects. For most cable shields, the skin depth effect will dominate. At higher frequencies, for cable samples that use good (circumferential) shield termination techniques, the surface transfer impedance is determined by the transfer impedance of the foil shield