Shielding Effectiveness of a Box with a Slot
Let us calculate the shielding effectiveness of a PEC
box with a slot, excited with a plane wave incident in
the direction perpendicular to the slot.
A rectangular metal enclosure with an aperture on one
face can be modelled as a waveguide, shorted at the far
end, with the aperture at the entrance to thewaveguide. The model is quickly created with just afew plates in WIPL-D Pro (Fig. 1).

while it increases as we move away from the slot. The
subsidiary depthwise resonance shifts up in frequency
as the probe moves towards the back. The principal
resonance is due only to the box dimensions, so its
frequency is unaffected.

Fig 2. Explanation of geometry of the box with a slot (takenfrom [1])8060SE [dB]

40Fig 1. Rectangular box with an aperture200

The electric field shielding effectiveness is calculated
as the ratio of the impinging field to the field measured
at some point within the waveguide, distant from theslot.

The objective is to investigate the influence of changes
in box and slot geometry as well as in position inside
the box at which field is calculated on shieldingeffectiveness (Fig. 2).
For fixed dimensions of the box and of the aperture(a=30 cm, b=12 cm, d=30 cm, l=10 cm, w=0.5 cm),
shielding effectiveness at various positions within thebox (at the symmetry plane) is displayed in Figs 3 and4. The shielding effectiveness is worst close to the slot-40
-60
00.2

0.4
0.6
frequency [GHz]0.81Fig 3. Shielding effectiveness of electric field for p varying from5 cm to 20 cm.
806040SE [dB]

The theory assumes that a single TE10 waveguidemode propagates from the aperture and normal to it.
Higher order modes, and modes propagating in otherdirections may exist which will complicate the results,and introduce need for EM simulation in order topredict the shielding effectiveness. The box is taken tobe empty. The results of EM simulation are inexcellent agreement with results obtained by using
intermediate level simulation tools from University ofYork, and easily obtainable by using their online
calculator [1] .

p=5
p=10
p=15
p=20-20

200

p=5
p=10
p=15
p=20-20-40
00.20.4
0.6
frequency [GHz]0.81Fig 4. Shielding effectiveness of magnetic field for p varying
from 5 cm to 20 cm.

If we fix everything except the width of the box (a),then for b=12 cm, d=30 cm, l=10 cm, w=0.5 cm and
p=15 cm, and by varying a from 20 cm to 40 cm, wewww.wipl-d.com
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get Figs 5 and 6. The principal resonance frequency
shifts as we change the box size, as expected.

7060

By fixing all parameters except the length of the slot toa=30 cm, b=12 cm, d=30 cm, w=0.5 cm and p=15 cm
and varying l from 3 cm to 17 cm, we get Figs 7 and 8.
Shielding effectiveness is directly related to the size ofthe slot, and it decreases as the slot grows.

50SE [dB]

40

200

d=3
d=5
d=10
d=15
d=17-10
00.2

1080
a=20
a=25
a=30
a=35
a=406040
SE [dB]

30200.4
0.6
frequency [GHz]0.81Fig 8. Shielding effectiveness of magnetic field for l varying
from 3 cm to 17 cm

0-20-40-60
00.2

0.4
0.6
frequency [GHz]0.81Fig 5. Shielding effectiveness of electric field for box width avarying from 20 cm to 40 cm60a)b)c)d)e)f)50

SE [dB]

4030
20a=20
a=25
a=30
a=35
a=40

100
-1000.2

0.4
0.6
frequency [GHz]0.8

1Fig 6. Shielding effectiveness of magnetic field for box width avarying from 20 cm to 40 cm

A closer look at the fields inside the box at several
frequencies within the range from 700 MHz to 1.3
GHz was done in order to illustrate higher order modes
that form. Fig. 9 shows near fields at different
frequencies in case when the incident plane forms anangle of 75 degrees with the normal to the slot.
80

l=3l=5l=10l=15l=1760

SE [dB]

4020Fig 9. Electric field amplitudes inside the box at: a) 700 MHz,b) 800 MHz, c) 900 MHz, d) 1 GHz, e) 1.2 GHz, and f) 1.3 GHz.

All models used in this paper required around 750
unknowns (4.5 MB of RAM). In many examples, thiscan be further diminished using symmetry planes toabout 200 unknowns. Simulation in 100 frequency
points takes about 40 seconds on a Intel Core2 DuoCPU with 2.66 GHz clock.
References

0[1] http://www.emc.york.ac.uk/examples/screening/screening.html-20-40
00.2

0.4
0.6
frequency [GHz]0.8

1Figure 7. Shielding effectiveness of electric field for l varying
from 3 cm to 17 cm

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