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 SE = 10 log [(E+1,.2-/Z+1,)/(E+2,.2-/Z+2,)] dB
EQUATIONS:
(3)
SE = 10 log [(H+1,.2-/Z+1,)/(H+2,.2-/Z+2,)] dB
(4)
where
P+1,
E+1,.2-/Z+1, = H+1,.2- Z+1,
=
P+2,
E+2,.2-/Z+2, = H+2,.2- Z+2,
=
E+1,
=
Incident Electric Field
E+2,
=
Transmitted Electric Field
H+1,
=
Incident Magnetic Field
H+2,
=
Transmitted Magnetic Field
When the wave impedance Z of the incident and transmitted electromagnetic
field is the same with and without the shielding in place the expression for
SE reduces to its familiar form:
SE = 20 log (E+1,/E+2,) dB
EQUATIONS:
(5)
SE = 20 log (H+1,/H+2,) dB
(6)
2.5
Shielding Material Characteristics. When an electromagnetic wave
encounters an enclosing conductive material shield, the portion of the wave
transmitted beyond the shielding barrier is reduced in magnitude by both
reflection and absorption by the shielding material. The reflection loss
occurs at the two interfaces between the transmitting medium (typically air)
and the shielding material (typically a conducting metal such as sheet steel,
copper, or aluminum). The absorption takes place as the wave passes through
the conductive material. The absorption loss in the wave energy results from
dissipated heat loss by currents induced in the conductive material by the
electric and magnetic fields of the wave passing through. The reflection loss
occurs because of the mismatch in wave impedance between the propagating
medium and the conductive material. The relationship for shielding
effectiveness of a conductive material is typically expressed as follows:
EQUATION:
SE = [R + A + C] dB
(7)
where
R = reflection loss
A = absorption loss
C = correction term for re-reflection within the metal surfaces
The correction term (C) is usually of small magnitude and ignored when the
absorption loss (A) is greater than about 10 dB.
The reflection loss (R+p,) for plane waves impinging on shielding material is:
8
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