Understanding Shielding Effectiveness

Shielding effectiveness (SE) quantifies how well a shield attenuates electromagnetic fields. It's the ratio of field strength without the shield to field strength with the shield, expressed in decibels (dB).

SE = 20 × log₁₀(E₁/E₂) for electric fields SE = 20 × log₁₀(H₁/H₂) for magnetic fields

Higher dB values indicate better shielding. A shield with 40 dB SE reduces field strength by a factor of 100.

The decibel scale is logarithmic, which means each 20 dB increase represents a 10× improvement:

For most commercial electronics, 40-60 dB provides adequate shielding. Medical and military applications often require 80+ dB.

• *MIL-STD-285
• Reference standard for shielded enclosure testing
• Measures SE at discrete frequencies
• Requires specialized test chamber

• *IEEE 299
• Similar to MIL-STD-285 with updated procedures
• Applicable to room-size enclosures

• *Coaxial Transmission Line Method
• For material-level testing
• ASTM D4935 standard
• Fast, repeatable results

• *Board-Level Testing
• Near-field probing before/after shield installation
• Application-specific, provides real-world data
• POCONS can provide SE data for specific configurations

• *Material Properties
• Conductivity: Higher conductivity improves SE
• Permeability: Important for low-frequency magnetic shielding
• Thickness: Thicker materials provide better SE, especially at low frequencies

• *Shield Construction
• Seam quality: Overlapping seams outperform butt joints
• Apertures: Openings degrade SE—keep smaller than λ/20
• Ground contact: Poor grounding dramatically reduces SE

• *Frequency
• SE typically varies with frequency
• Different mechanisms dominate at different frequencies
• Specify SE at your frequency of concern

• *Assembly
• Installation matters as much as the shield itself
• Consistent ground contact around perimeter
• Proper clip selection for two-piece shields

• *Step 1: Define Your Environment
• What frequencies are you concerned with?
• Is this containment (emissions) or immunity (susceptibility)?
• What are the regulatory limits?

• *Step 2: Calculate Required SE
• Measure or simulate unshielded emissions/susceptibility
• Calculate margin needed to meet requirements
• Add safety margin (typically 6-10 dB)

• *Step 3: Specify Clearly
• State SE requirement at specific frequency/frequencies
• Reference test method
• Include acceptance criteria for incoming inspection

Example Specification: "Shield assembly shall provide minimum 50 dB shielding effectiveness at 2.4 GHz per IEEE 299 test methodology."

These are general guidelines. Contact us for application-specific recommendations based on your regulatory and performance requirements.