Page 115 - Power Electronics Handbook

108 Electromagnetic compatibility
material. As an example into the use of equations (4.4) and (4.5), a 1 mm
aluminium sheet at lwIz would have a skin depth of 2.7mm and an
absorption loss of 3.2dB, whereas at 1oOMHz the skin depth would be
8.3 p, and the absorption loss 1047dB. Reflection loss, which together
with absorption loss determines the shielding effectiveness, occurs
whenever there is a discontinuity in the characteristic impedance between
the shield and its surrounding. This is illustrated in Figure 4.5(b). When
the field reaches the point of discontinuity, at surface A, some of the
incident field is reflected and the rest penetrates, to be attenuated by the
absorption loss in the material of impedance Z2. When the field reaches
surface B it will exit, again with reflection loss at this interface.
The characteristic impedance of an electromagnetic wave is the ratio of
the electrical to magnetic fields. This impedance depends on the properties
of the shield material and the distance from the source of interference to
the measurement point. If D is the distance from the source and h the
wavelength, then, defining K by the value shown in equation (4.7), the
impedance of the electromagnetic wave is given by equations (4.8) and
(4.9), for a magnetic and an electric source, respectively.

2rrD
K=- (4.7)
h

(magnetic source) (4.8)

2, = 120n [l + 4 (electric source) (4.9)
M

These equations are shown graphically in Figure 4.5(c), which illustrates
that close to the source of EM1 the impedance is high if the source is
electrical, whilst it is low if the source is magnetic. As the distance from the
source increases, the impedance reaches asymptotically towards the free
space wave impedance of a plane wave, of 120n, or 377 P.
For a conductor the charactertistic impedance is given by equation
(4.10).

(4.10)

This impedance is very low, compared to the free space wave impedance of
377!2, therefore boundary reflections can result in high losses, which is
desirable.
Returning to the model given in Figure 4.5(b), and considering an
electric field for the present, even if absorption loss is ignored the fields El,
E2 and E3 will be different, the difference being due to reflection losses at
the two interfaces A and B. The relationships between these fields are