Page 116 - Power Electronics Handbook

EM shielding techniques 109
equations (4.11) and (4.12), when multiple reflections, as described later,
are ignored.

(4.11)

(4.12)

If Z2 is a shield placed in air, then 23 equals Z1, and equation (4.12)
reduces to that given by equation (4.13). This is furtber simplified if the
shield is a conductor and Z2 is small, as shown in equation (4.14).

(4.13)

(4.14)

Using equation (4.14) the reflective component of the shielding
effectiveness, as in equation (4.1). is given by equation (4.15).
RE = 201og (&) (4.15)

This indicates that the smaller the conductor's characteristic impedance,
the greater the effectiveness of the sbielding.
The characteristic impedance of a conductor increases with frequency,
bing given by proportionality (4.16), so that the shielding effectiveness is
as in propOrtiOn&ty (4.17). However, abQorption loss decreases with
frequency since the skin depth decreases, so that the total shielding
effectiveness, as indicated by equation (4.3), varies as in F!i 4.5(d).
Generally, a conductive sheet gives very good electric field shielding,
except for very thin coatings.
22 (Y wl'z (4.16)

Re (Y (4.17)
For an electric field most of the reflections occur at boundary A, in
Figure 4.5(b), giving high reflection loss and low penetration. For a
magnetic field most of the refkcticmi occur at boundary B, and there is
therefore high penetration. Low-frequency magnetic fields have low
reflection loss and depend on absorption loss for their shielding
effcctivcntss. For a -tic field equations (4.11) to (4.14) are equally
applicable, where the eiectric fields El, 4 and E3 are replaced by magnetic
fields Ht, Hz and H3. The refldve component of the shielding loss is
given by equation (4.18) ignoring multiple reflections, which is similar to
that of the electric field of equation (4.15).
RH = 20 log (5- (4.18)