Shield Transfer Impedance Background
The module is used
to calculate shield transfer impedance over the frequency range selected in the
Frequency form. Several methods are available, depending on the
information available about the construction of the shield.
Cable shields may be either woven wire braid or solid.
Braided shields are constructed by weaving small gauge
wires into a tubular structure that surrounds the of an electrical cable. At low frequencies
braided shields have shield transfer impedance equal to their dc
resistance. Braid shields inherently have numerous small apertures as a
result of their construction. At high frequencies leakage through these
apertures results in an increase in shield transfer impedance with increasing
frequency.
Solid shields are generally used in permanent installations
where flexibility is not needed, or where high degree of high frequency
shielding effectiveness is required. Solid shields, as the name implies,
are constructed of solid, usually circular tubular conductors such as electrical
conduit or pipe, but may be any solid conductive material. At low
frequencies solid shields have shield transfer impedance equal to their dc
resistance. Shield transfer impedance for solid shields decreases with
increasing frequency due to skin effect.
Tech Note - Shield Transfer Impedance
Shield
transfer impedance is the quotient of the voltage induced on the wires within
the shield and the current flowing on the shield. The lower the shield
transfer impedance, the better the shield's performance. Unlike shielding
effectiveness, shield transfer impedance is independent of the conditions under
which it is used. Shielding effectiveness is affected by numerous factors,
including the circuit impedances at each end of the line and the physical layout
of the cable. Transfer impedance is an intrinsic property of the shield,
so unlike shielding effectiveness, its value is not a function of the
application. The voltage induced on the wires, due to current induced on
the shield by an incident field, appears as a voltage in series with the wires. At low frequencies
this voltage divides between the terminations at the ends of each wire. As
frequency increases, and the wires are no longer short compared to the
wavelength of the incident field, the induced voltage is not in phase across the
length of the wire, so the voltage appearing at each end may be lower.
Where the cable length corresponds to even multiples of one-quarter wavelength,
the voltage at the terminations is a minimum. At odd multiples of
one-quarter wavelength, the voltage at the terminations is a
maximum.
Select the tab on the Shield Transfer Impedance form that allows
entry of the information that best fits available parameters. More:
 Braided Shields
Solid Shields
Shield Parameters
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