EMI Shielding Effectiveness

SE = 20 × log10(Ei / Et)

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Result
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Formula

SE = 20 × log10(Ei / Et)

Description

Shielding effectiveness quantifies how well an enclosure or shield attenuates electromagnetic fields, expressed as the ratio of incident to transmitted field strength in decibels. A higher SE means better shielding. Shielding works through three mechanisms: reflection (impedance mismatch at the shield surface), absorption (energy converted to heat as the wave passes through the conductor), and multiple reflections (re-reflection within the shield material). At low frequencies, magnetic shielding is more difficult than electric field shielding.

Variables

  • SE — Shielding effectiveness (dB)
  • Ei — Incident electric field strength (V/m)
  • Et — Transmitted electric field strength (V/m)

Practical Notes

Typical SE values: unshielded enclosure 0 dB, painted enclosure 0-10 dB, conductive gasket sealed metal enclosure 40-80 dB, welded enclosure 80-120+ dB. Shield effectiveness degrades at seams, joints, and apertures. Any opening larger than λ/20 significantly compromises shielding. Use conductive gaskets, finger stock, or EMI tape at enclosure seams.

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