Electromagnetic Shielding
Understanding EM Shielding
Electromagnetic shielding is the primary method for achieving EMC compliance and protecting sensitive receivers from interference. The shield creates a Faraday cage that attenuates both electric and magnetic fields.
Shielding Mechanisms
- Reflection: Impedance mismatch at the shield surface reflects incident waves. Dominant for E-field and plane waves.
- Absorption: Energy absorbed as the wave passes through the conductive material. Increases with thickness and frequency.
- Multiple reflection: Internal reflections within the shield thickness. Important for thin shields at low frequency.
Practical Considerations
- Apertures: Any opening (ventilation holes, cable entry, seams) leaks radiation. The largest aperture determines the upper frequency limit of effective shielding.
- Seams: Metal-to-metal contact resistance at enclosure seams limits SE. EMI gaskets improve seam performance.
- Cable penetration: Cables entering the shield must be filtered or use shielded feedthrough connectors.
Frequently Asked Questions
What is electromagnetic shielding?
EM shielding uses conductive enclosures to block electromagnetic fields. SE = 20*log(E_unshielded/E_shielded). Continuous copper: > 100 dB SE. Practical enclosures: 40-80 dB. Limited by apertures, seams, and cable penetrations.
What limits shielding effectiveness?
Apertures are the primary limitation. A slot aperture resonates when its length = lambda/2, providing nearly zero SE at that frequency. Rule of thumb: maximum slot length must be < lambda/20 for 20 dB SE at that frequency.
What materials are used for shielding?
Copper, aluminum, steel, mu-metal (for magnetic fields), conductive coatings, and metallized fabric. Copper is most effective for E-fields. Mu-metal is needed for low-frequency magnetic fields. Aluminum is a good general-purpose shield material.