Surface Transfer Impedance of Cable Shields Having a Longitudinal Seam

The magnitude of noise induced into communication cable from electromagnetic influences is reduced by the shielding properties of the metallic shield. One of these properties is the surface transfer impedance. The surface transfer impedance relates the current induced on one side of a shield to the longitudinal voltage appearing on the other side due to that current. At low frequencies the surface transfer impedance for nonpermeable materials is equal to the dc resistance of the shield. At high frequencies it decreases rapidly. The frequency at which the decrease begins is a function of the thickness and conductivity of the metal. For cylindrical shields having longitudinal seams, the transfer impedance increases at somewhat higher frequencies. The size of the seam opening determines the frequency at which the increase begins. Although the size of the seam opening is difficult to control, experimental results are in relatively good agreement with theoretical calculations.