AN AFFECT OF THE APERTURE LENGTH TO APERTURE WIDTH RATIO
ON BROADBAND SHIELDING EFFECTIVENESS
I. Bahadir BASYIGIT 1, P. Deniz TOSUN 1, Sukru OZEN 1, 2 and Selcuk HELHEL 1, 2
1
2
Akdeniz University, Engineering Faculty, Electrical–Electronics Engineering, 07058 Antalya, TURKEY
Akdeniz University, Industrial and Medical Application Based Microwave Research Centre (EMUMAM),
07058 Antalya, TURKEY
[email protected], [email protected], [email protected]
[email protected]
Abstract
The influence of an aperture area (between 2λ2 and 11λ2) on the Shielding Effectiveness of metallic
enclosures has been measured in an anechoic chamber, and wavelength dependent SE measurements are introduced.
Measurement setup was established operating between 6GHz to 13GHz. While the width/ length ratio varies from 1
to 16, shielding effectiveness gets better by about 6.5dB at low level frequencies, reaches about 10dB at higher
frequency band, and they get closer each other at around resonance frequencies.
1. Introduction
Shielding is one of the methods to protect electronic devices and systems to electromagnetic interference
[1-2]. Electromagnetic Shielding is generally used to reduce emissions. Shielding enclosures are important to protect
equipments from the effect of an exterior field, and filtering emission [2-6]. Aperture design on electronic
equipments’ shielding enclosures is a fundamental aspect of getting better electromagnetic emission and
susceptibility. Apertures, slots, cable penetrations, unused connector ports and others breaks in the shield can
influence significantly the effectiveness of shielding enclosures[7,8]. Thus, electromagnetic wave penetration into a
cavity that presents apertures, due to its relation with many practical problems involving EMC analysis, has been of
considerable interest.
The ability of an enclosure to reduce the emission or to improve the immunity of electronic equipment to
high frequency interference is characterized by its shielding effectiveness (SE) parameter. SE of an enclosure of
different dimension rectangular apertures is examined and their 6GHz to 13GHz broadband response have been
introduced. The affect of an aperture length to aperture width ratio increase has also been investigated in this study.
For these goals, dimensions of apertures were chosen such that aperture area kept constant (56.25cm^2), and
apertures are responding through broadband frequencies between 6GHz and 13GHz having different aperture area in
λ2 . This area varies between 2 λ2 and 10.5 λ2 .
2. Shielding Effectiveness Analyses
Shielding Effectiveness is a measure of the ratio of reference radio signal to test signal in decibel, and it is
expressed in Eq.1. Instead there are electrical and magnetic shielding effectiveness of an enclosure in the literature
as well as in an industry, an electrical shielding effectiveness has been examined in this study
SE = 20 log(
978-1-4244-6051-9/11/$26.00 ©2011 IEEE
Er
)
Ee
[dB]
(1)
where Er is the reference electrical field without an enclosure, and Ee is test electric field strength (V/m) with an
enclosure. Spectrum analyzer has been used, and recorded data is power in dBm. In order to obtain an electric field
equivalent, Equation 2 has been used where
power in dBm from mW, and
Pr,e is in dBm. Equation 3 is well known equation for calculating
ηo is the wave impedance which is assumed as 377 Ω .
Er ,e = Pr,e . ηo
P
Pr,e = 10 log(
r ,e
[mW]
103
[V/m]
(2)
[dBm]
(3)
3. Test Setup and Measurements
Measurements were carried out in an anechoic chamber in Akdeniz University, Industrial and Medical
Application Based Microwave Research Centre (EMUMAM). It is a full anechoic chamber has a dimension of
4mx4mx8m. Rohde-Schwarz (SMF-100A) signal generator operating between DC and 41GHz has been used a radio
source, and Agilent (E4405B-ESA-E Series) spectrum analyzer as a receiver. Transmitter unit was connected to
ultra wideband micro strip antenna operating between 5GHz-13GHz, and A-info (JXTXLB-90-20-C-SF) standard
gain horn antenna operating between 6GHz and 12 GHz was connected to spectrum analyzer as an ear. Both
transmitting and receiving antennas were attached to dielectric rope at 30cm above the wood made reference table.
Three enclosures having a dimension of 150mmx150mmx800mm were used for measurements. Each enclosure has
different apertures on it as given in Table 1.
(a) Reference Measurement without enclosure
(c) Signal Generator and Spectrum Analyzer
(b) Measurement with enclosure (7.5x7.5cm)
(d) Measurement with enclosure (30x1.875cm)
Figure1. Test Equipments
Table 1. Enclosure Details
Enclosure Dimension
Aperture Dimension
Width(mm)
Length(mm)
150mmx150mmx800mm
300
18.75
150mmx150mmx800mm
150
37.5
150mmx150mmx800mm
75
75
4. Results and Conclusion
There, shielding effectiveness at 71 local frequencies were recorded as shown in Fig.2. While the width/
length ratio varies from 1 to 16, shielding effectiveness gets better by about 6.5dB at low level frequencies, reaches
about 10dB at higher frequency band, and they get closer each other at around resonance frequencies. Performance
collapses at around 3.5GHz.
Shielding Effectiveness [dB]
5
0
-5
-10
7.5cm x 7.5cm
15cm x 3.75cm
30cm x 1.875cm
-15
2
3
4
5
6
7
8
9
10
11
Aperture Area
Figure2. Broadband shielding effectiveness variation depending on aperture area ( λ2 ).
For selected frequencies (6GHz, 8GHz and 12GHz), results indicate that the increase of the ratio of an
aperture length to aperture width results in SE improvement, and SE decreases as the frequency increases as shown
in Fig.3.
6
Shielding Effectiveness [dB]
4
2
0
6 GHz
8 GHz
12 GHz
-2
-4
-6
-8
-10
0
2
4
6
8
10
12
14
16
Aperture Length to Aperture Width Ratio
Figure 3. Shielding Effectiveness variation with respect to the ratio of aperture length to aperture width
Increase of the ratio of an aperture length to aperture width results in better shielding effectiveness. One of
the main reason of keeping apertures on an enclosure is cooling electronics by air circulation. While cooling
electronics require bigger aperture area, SE measurements indicates an the ratio of an aperture length to aperture
width. Optimum solution for determining the aperture of an enclosures can be obtained by calculating both SE and
heat transfer analyses of an enclosure.
5. Acknowledgements
This study is supported by Akdeniz University, Scientific Research Projects Supporting Unit (BAPYB),
Industrial and Medical Application Based Microwave Research Center (EMUMAM) and and State Planning
Organization (Project number: DPT-2007K120530).
6. References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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Shielding Effectiveness of Metalic Enclosures", APMC 2005 Proceedings, 2005.
G. Chunhong and L. Shufang, "Shielding effectiveness of an enclosure with apertures", 2005 IEEE
Internetional Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless
Communications Proceedings, 2005, pp. 614-618.
M. Bahadorzadeh and M. N. Moghaddasi, ''Improving the Shielding Effectivenessof a Rectangular Metallic
Enclosure with Aperture by Using Extra Shielding Wall", Progress In Electromagnetics Research Letters,
Vol.1, 2008, pp. 45-50.
I. Belokour, J. LoVetri, "Shielding Effectiveness Estimation of Enclosures with Apertures", IEEE 2000.
A.Ciccomancini Scogna, G.Antonini, A.Orlandi, "Analysis of Radiated Emissions and Shielding
Effectiveness for a Metallic Enclosure with Shielding Springs", IEEE 2007.
S. Ozen, N. Arı, Elektromanyetik Uyumluluk, ISBN 978-9944-341-72-1, Palme Yayıncılık, Ankara, 2008.
S. HELHEL, Elektromanyetik Uyumluluk El Kitabı, TÜBİTAK-UEKAE, 1997