Analytical and experimental study of the electric shielding effectiveness of a metallic enclosure for off-centred apertures
An electromagnetic shielding is frequently used to protect against external fields and leakage from electronic products to meet the electromagnetic compatibility (EMC) requirement. However, it is a great challenge to design a practical electromagnetic shield because its integrity is often comp...
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| Format: | Thesis |
| Language: | English English English |
| Published: |
2007
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| Subjects: | |
| Online Access: | http://eprints.uthm.edu.my/7644/1/24p%20FARHANA%20AHMAD%20PO%27AD.pdf http://eprints.uthm.edu.my/7644/2/FARHANA%20AHMAD%20PO%27AD%20COPYRIGHT%20DECLARATION.pdf http://eprints.uthm.edu.my/7644/3/FARHANA%20AHMAD%20PO%27AD%20WATERMARK.pdf |
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| Summary: | An electromagnetic shielding is frequently used to protect against external
fields and leakage from electronic products to meet the electromagnetic compatibility
(EMC) requirement. However, it is a great challenge to design a practical
electromagnetic shield because its integrity is often compromised by apertures and
slots used to accommodate ventilation or access to interior components. Such
openings allow exterior fields to be coupled onto printed circuit boards (PCBs), thus
inducing current and voltage on interior conductors. This phenomenon will degrade
the shielding effectiveness (SE) of the enclosure. This research is performed to
investigate the effects of apertures on the SE of a rectangular metallic enclosure for
off-centred apertures by using an analytical formulation and experimental study. The
theories developed by other researchers have been extended to take into account the
contribution of higher order modes and off-centred apertures in the enclosure. The
electric shielding effectiveness, Se were calculated as a function of frequency,
enclosure dimensions, aperture dimensions, aperture locations and various
observation points by employing a transmission line equivalent circuit approach. The
extended formulation applies only to rectangular enclosures with rectangular
apertures, but simple modifications were included to account for square, circular,
multiple apertures and the effect of the enclosure contents. Generally, the analytical
results of the Se are in good agreement with measurement results which were
conducted in a Gigahertz Transverse Electromagnetic (GTEM) cell in the range of
frequency from 10 MHz to 1 GHz. As an example, the Se at 600 MHz using
analytical formulation has 92% similarity compared to measurement results. It was
also found out that by taking into account five modes above the TEio will contribute
an additional 35 dB to the Se at 800 MHz. This indicates the significance of
multimode in determining Se for enclosure with off-centred aperture. In addition, the
results agree with present understanding on the dependence of SE on aperture sizes,
aperture shapes, aperture locations and effect of electromagnetic losses in the
enclosure. The reduction of Se by about 9.5 dB due to multiple apertures indicated by
20 log n where n is the number of apertures was also successfully endorsed in this
work. The findings generated from this research work can be used as design rules by
designers of practical shielded enclosures. Future works can be carried out to
incorporate enclosures of various shapes and sizes at frequencies greater than 1 GHz. |
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