What is the proper procedure for measuring the shielding effectiveness of an enclosure?
Shielding Effectiveness Testing
Shielding effectiveness testing is essential for ensuring that electronic enclosures meet EMC requirements and protect sensitive electronics from external interference or prevent internal emissions from radiating.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
Calibration Procedure
(1) Transmit side: a signal generator drives a transmit antenna positioned outside the enclosure at a specified distance (typically 30 cm to 2 m from the enclosure wall, depending on the test standard and frequency). (2) Receive side: a receive antenna inside the enclosure is connected to a spectrum analyzer or EMI receiver outside via a feedthrough connector. The feedthrough must have SE greater than the enclosure (> 100 dB) to avoid leakage through the cable entry. Use a fiber-optic link or a shielded bulkhead connector with proper bonding. (3) Reference measurement: remove a panel of the enclosure (or use a reference measurement without the enclosure) to establish the unshielded received level. SE = P_reference - P_shielded. (4) Frequency sweep: measure at discrete frequencies or sweep across the range. Key frequencies: below 100 kHz: magnetic field shielding (most challenging for thin shields). 100 kHz - 30 MHz: transition region (both E and H fields important). 30 MHz - 1 GHz: plane wave shielding (most enclosures perform well). Above 1 GHz: aperture leakage dominates (seams, ventilation, cable entries). MIL-STD-461G test method RE101/RE102: measures radiated emissions from equipment, which indirectly tests shielding effectiveness.
Error Sources
SE is limited by the weakest path through the enclosure: (1) Apertures: any opening in the shield (ventilation hole, slot, seam gap) becomes the dominant leakage path at frequencies where the opening is comparable to lambda/2. A 1 cm slot: SE ≈ 0 dB at 15 GHz (lambda/2 = 1 cm). At 1 GHz (lambda/2 = 15 cm): SE ≈ 20×log10(15/1) = 23 dB from the slot alone. Rule of thumb: SE from an aperture ≈ 20×log10(lambda/(2 × aperture_dimension)). (2) Seams: the joints between enclosure panels create long, narrow apertures. A 30 cm seam with 0.1 mm gap: behaves as a slot antenna at frequencies where 30 cm > lambda/2 (> 500 MHz). Mitigation: use conductive gaskets (beryllium copper fingerstock, conductive elastomer, or knitted wire mesh) at all seams to ensure metal-to-metal contact. (3) Cable penetrations: every cable entering the enclosure is a potential leakage path. The cable shield must be bonded to the enclosure wall at the entry point (360° bonding using shield clamps or feedthrough filters). A pigtail ground (connecting only the cable shield wire to the enclosure) has 10-30 dB less SE than a proper 360° bond. (4) Ventilation: air openings can be shielded with honeycomb waveguide panels (waveguide-below-cutoff attenuators). A honeycomb with 3 mm cell size and 10 mm depth: SE > 50 dB above 1 GHz.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Fixture Considerations
Shield material affects SE at different frequencies: (1) Electric field (high frequency, > 10 MHz): any conductive material provides excellent SE (copper, aluminum, steel). SE ∝ thickness × conductivity. 0.5 mm copper: SE > 100 dB from 1 MHz to 10 GHz. (2) Magnetic field (low frequency, < 100 kHz): high-permeability materials are needed. Mumetal (mu_r = 20,000-80,000): SE = 30-60 dB at 60 Hz for 1 mm thickness. Steel (mu_r = 200-500): SE = 10-30 dB at 60 Hz. Aluminum and copper (mu_r ≈ 1): SE < 5 dB at 60 Hz (poor magnetic shielding). For applications requiring both high-frequency and low-frequency SE: use a composite shield (inner layer of Mumetal for magnetic shielding + outer layer of copper or aluminum for electric field shielding).
Frequently Asked Questions
What SE do I need for my application?
Application-specific guidelines: commercial EMC compliance (FCC Part 15, CISPR 32): SE > 20-40 dB across 30 MHz - 6 GHz (sufficient to meet emission limits with 10 dB margin). Military (MIL-STD-461G): SE > 60-80 dB across 10 kHz - 18 GHz. Medical devices (IEC 60601-1-2): SE > 20-40 dB (depends on the device immunity level). TEMPEST (eavesdropping protection): SE > 80-100 dB (classified requirement). HEMP (high-altitude EMP protection): SE > 80 dB at 100 kHz, > 60 dB up to 1 GHz. Sensitive RF receivers: SE > 80-100 dB (to prevent desensitization from external signals).
How do I test shielding of small components like PCB shields?
For PCB-level shields (stamped metal cans soldered to the PCB): use a TEM cell or GTEM cell instead of IEEE 299. The shield can is placed inside the cell, and the SE is measured by comparing the coupled power with and without the shield. Alternatively: use a reverberation chamber (mode-stirred chamber) which provides a statistically uniform field environment. The SE is determined from the average received power inside the shield relative to outside. For quick screening: a near-field probe scan over the PCB with and without the shield provides a qualitative SE map, showing where leakage occurs.
Why does SE decrease at higher frequencies?
SE can decrease at higher frequencies due to: (1) Aperture resonance: slots and seams become resonant when their length ≈ lambda/2, allowing maximum coupling. A 10 cm slot resonates at 1.5 GHz (lambda/2 = 10 cm). (2) Reduced skin depth: higher frequency means smaller skin depth, which increases absorption loss (good) but also means that thin shields become transparent (bad if thickness < skin depth). For copper: skin depth at 1 GHz = 2.1 um (any thickness > 10 um provides excellent absorption). Rarely a problem for metal shields. (3) Cavity resonances: the enclosure acts as a resonant cavity. At cavity resonance frequencies: the internal fields are amplified, and the SE appears to decrease (the field inside is enhanced by the quality factor of the cavity). Mitigation: add absorber inside the enclosure to damp cavity resonances. (4) Cable and connector coupling: at higher frequencies, cables act as more efficient antennas, coupling more energy through cable penetrations.