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Magnetic Shielding Effectiveness and Simulation Analysis of Metalic
Enclosures with Apertures
Ibrahim Bahadir BASYIGIT
1
, Mehmet Fatih CAGLAR
2
and Selcuk HELHEL
3
1,2
Department of Electronics and Communication Engineering, Suleyman Demirel University, Isparta, Turkey
1
bahadirbasyigit@sdu.edu.tr ,
2
fatihcaglar@sdu.edu.tr
3
Department of Electrical and Electronics Engineering, Akdeniz University, Antalya, Turkey
3
selcukhelhel@akdeniz.edu.tr
Abstract
The effect of magnetic shielding effectiveness has been
investigated for apertures on metallic enclosure and has been
calculated as a function of enclosure dimensions, aperture
dimensions position within the enclosure. The calculation
magnetic shielding depends upon the frequency and polarization
of the applied field, the dimensions of the enclosure and the
apertures, the number of apertures, and the position within the
enclosure. Analytical formulation confirms that long thin
apertures are worse than round or square apertures of the same
area. For a typical sized enclosure, the theory predicts that
doubling the length of a slot reduces magnetic shielding
effectiveness and by about 12 dB, while doubling the width only
reduces magnetic shielding effectiveness and by about 2 dB.
Calculations using the new formulation show that doubling the
number of apertures reduces magnetic shielding effectiveness
and by about 6 dB. However, dividing a long slot into two
shorter ones increases magnetic shielding effectiveness and by
about 6 dB.
1. Introduction
Electromagnetic shielding is generally used to decrease the
emissions or to improve the immunity of the equipment. While
the integrity of shielding enclosure for a digital design is
compromised by slots and apertures for heat dissipation, airing
I/O cable penetration or other purpose. These apertures and slots
become the coupling route of electromagnetic interference
(EMI) from the interior to outside [1-4]. That’s why it effects
the shielding effectiveness.
Shielding effectiveness can be calculated by the numeric
methods are FDTD, FEM, MoM, TLM and etc., empirical /
semi-emprical methods or by some simulations which is one
that, CST Microwave Studio (CST-MWS) based on the Finite
Integration Technique (FIT) or by analytical metods are
comprised of theoretical formulas [5-9]. In this paper the
analytical formulation presented provides a fast means of
investigating the effect of design parameters on the shielding
effectiveness of an enclosure [1,2].
In section 2, an analytical formulation for electrical (ESE)
and magnetic shielding effectiveness (MSE) has been calculated
as function of frequency, aperture size, enclosure size. In section
3, according to the formula the effects of aperture size,
enclosure size, probe location, aperture shape, multiple
apertures, aperture configuration have been analyzed and the
results have been compared.
2. Analytical Formulation
The equivalent impedance of an aperture has been calculated
by the method of circuit modeling, and the voltage and current
have been calculated by transmission theory. Electrical shielding
effectiveness is belong to the voltage at point P and magnetic
shielding effectiveness is belong to the current at point P.
Figure1 shows metallic enclosure and its equivalent circuit. At
its equivalent circuit, the source impedance has been assumed as
free space impedance of Z
0
≈377Ω, and the source voltage is V
0
.
The first step is to define an equivalent impedance of slot
followed by transforming voltages and impedances to point P as
a second step. The enclosure has been represented as short
waveguide whose characteristic impedance Z
g
and propagation
constant k
g
are defined as below [1,2]:
/
1/2
(1)
1/2
(2)
Fig. 1. Rectangular enclosure and its equivalent circuit [1]
328
The aperture has been indicated as a length of coplanar strip
transmission line, and the total width is equal to the height of
enclosure b and aperture width is w. The transition between free
space and waveguide is identified that it is considered of
aperture as a length of coplanar strip transmission line. The
effective width is given as below:
1
(3)
where t is the enclosure wall thickness. If w
e
<b/√2, the
characteristic impedance of transmission line may
approximately be [1,2]:
120
ln 2
(4)
as discussed by Gupta at all [12]. The short circuits at the ends
of the aperture through a distance ℓ/2 are transformed to the
impedance at point A (on center of aperture) is Z
ap
. Also ℓ/a
factor must be included to account for the coupling between
aperture and enclosure. So short circuit impedance is:
tan
(5)
Z
0
, v
0
and Z
ap
are identified the source voltage v
1
and the
source impedance
Z
1
are shown as below:
/
(6)
/
(7)
Then an equivalent voltage v
2
, and source impedance Z
2
and
load impedance Z
3
are shown as below
(8)
(9)
tan
(10)
The voltage at P is:
/
(11)
The load impedance at P point is simply Z
0
without enclosure
means reference calculation. The MSE is given as below:
20log
20log
(12)
3. Result And Analysis
In this section; the affects of an enclosure size, aperture size,
probe location, aperture shape, multiple apertures and aperture
configuration have been analyzed by analytical formulation.
3.1. Affect of Enclosure Size
700 x 120 x 160 mm, 750 x 160 x 200 mm and 800 x 200 x
240 mm size enclosures having an aperture size of 150 x 37.5
mm have been investigated and Fig.3 indicates magnetic
shielding effectiveness. In the case of magnetic shielding
effectiveness, larger the enclosure size increases the shielding
effectiveness, and smaller the enclosure size increases the
resonance frequency as shown in Fig.3. After 900MHz, they
close each other about overlapping.
Fig. 3. Theoretical MSE for different enclosures having 150 x
37.5 mm size aperture.
3.2.
Affect of Aperture Size
160 x 160 x 800 mm size enclosures having square type
aperture size of 55 x 55 mm, 75 x 75 mm and 95 x 95 mm have
been investigated Fig.4 indicates a magnetic shielding
effectiveness. For the magnetic shielding effectiveness
performance, smaller the aperture size increases shielding
effectiveness, and they track each other with 10dB difference for
whole band. Resonance frequencies are so close to each other
around 960MHz
.
Fig. 4. Theoretical MSE for 160 x 160 x 800 mm size enclosure
having different apertures.
3.3. Affect of Probe Location
A 160 x 160 x 800 mm size enclosure having an aperture size
of 75 x 75 mm has been investigated for three different probe
locations. D=160 mm is the depth of an enclosure, and p
indicates the probe location chosen as 40, 80 and 120 mm,
respectively (see Fig.1). Calculated magnetic shielding
effectiveness is shown in Fig.5 for the probe distance of 80mm
that it is at the center of an enclosure. 40dB of dramatic
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-20
-10
0
10
20
30
40
50
60
70
Frequency [GHz]
Magnetic Shielding Effectiveness [dB]
700 x 120 x 160 mm
750 x 160 x 200 mm
800 x 200 x 240 mm
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
10
15
20
25
30
35
40
45
50
55
Frequency [GHz]
Magnetic Shielding Effectiveness [dB]
55 x 55 mm
75 x 75 mm
95 x 75 mm
329
deviation in magnetic shielding effectiveness (MSE) has been
observed with short probe location between 300 and 770 MHz
band. At 770MHz, MSE for 80mm location deviates by about
40dB then others.
Fig. 5. Theoretical MSE for varying probe location in
160x160x800 mm size enclosure having 75 x 75 mm aperture.
3.4.
Affect of Aperture Shape
Same sizes of three different enclosures have been chosen
with constant probe location. While varying aperture dimensions
were chosen, aperture area was fixed as 100mm
2
. MSE has been
investigated, and results are shown in Fig.6, MSE for whole
three has a resonance frequency of 960MHz. Square type
aperture has 14dB better MSE then 25 x 4 mm size rectangular
aperture, and 13dB worse MSE then 4x25mm size rectangular
aperture. MSE responses are more smooth and about flat for
whole band.
Fig. 6. Theoretical MSE for 160x160x800 mm size enclosure
having different aperture shape
3.5. Affect of Multiple Apertures
In this section, 160 x 160 x 800 mm size enclosures having
different number of apertures on them were chosen. Aperture
size was chosen as 25 x 4 mm Fig.7 for indicates MSE for n={1,
3, 5} apertures positioned on an enclosure. MSE for all options
become smooth and deviate by about 10dB from each other.
3.6. Affect of Aperture Configuration
While the total aperture area was kept constant as 400
2
m
m
,
MSE has been investigated for varying aperture dimensions.
Those are one 100x 4 mm size aperture, 4 times 25x4 mm
aperture and 8 times 10x5 mm aperture, respectively. Results
show that increased number of holes on an enclosure decreases
MSE for a constant total aperture size. As shown in Fig.8, it is
observed that there is a dramatic decrease in MSE, while the
numbers of holes increase.
Fig. 7. Theoretical MSE for 160x160x800 mm size enclosure
having 1,3 and 5 times 25x4 mm apertures on it
Fig. 8. Theoretical MSE for 160x160x800 mm size enclosure
having different number of aperture (total area equals 400mm
2
).
4. Conclusions
For magnetic shielding effectiveness (MSE); larger the
enclosure size increases the shielding effectiveness, and smaller
the enclosure size increases the resonance frequency. Smaller
the aperture size increases shielding effectiveness, and they
track each other with 10dB difference for whole band. When the
probe distance is in center of enclosure depth (it is 80mm here),
shielding effectiveness deviates by about 40dB then others.
Square type aperture has 14dB better MSE then 25 x 4 mm size
rectangular aperture, and 13dB worse MSE then 4x25mm size
rectangular aperture. MSE decreases with the number of
apertures. that increased number of holes on an enclosure
decreases MSE for a constant total aperture size and it is
observed that there is a dramatic decrease in MSE, while the
numbers of holes increase.
Acknowledgement
We would like to thank to The Directorate of Industrial and
Medical Applications Microwave Research Center
(UMUMAM), Akdeniz University for allowing us to use EMC
Pre-Compliance Test Laboratory facilities granted by State
Planning Organization (Project Number: 2007K120530-DPT).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-20
0
20
40
60
80
Frequency [GHz]
Magnetic Shielding Effectiveness [dB]
p = 40 mm
p = 80 mm
p = 120 mm
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
70
75
80
85
90
95
Frequency [GHz]
Magnetic Shielding Effectiveness [dB]
n = 1
n = 3
n = 5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
70
75
80
85
90
Frequency [GHz]
Magnetic Shielding Effectiveness [dB]
n = 1 ( 1 aperture with 100 x 4 mm )
n = 4 ( 4 apertures with 25 x 4 mm )
n = 8 ( 8 apertures with 10 x 5 mm )
330
Funding
This work was supported by The Departmant of Scientific
Research Projects in Suleyman Semirel University named as
‘Determination the effect on total electromagnetic emission
distribution of internal computer components and location of
internal computer components’ and in Turkish ‘Bilgisayar içi
Donanım Bileşenleri ile Bileşen Yerleşiminin, Toplam
Elektromanyetik Emisyon Dağılımına Etkisinin Belirlenmesi’
[Project Number: 4384-D2-15].
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331
