Shielding Effectiveness – When to Stop Blocking and Start Absorbing

Definition and measurement of shielding effectiveness for electrically small enclosures presents some special challenges addressed in IEEE standard 299.1. Testing of small enclosures commonly gets to a point where blocking of penetration with gaskets and sealing tape might not improve shielding effectiveness. This paper uses a power balance model and simple enclosure Q measurements to explain this apparent limit. The same model is then used to design and quantitatively evaluate internal absorber solutions, which can overcome the apparent limit to achieve desired shielding effectiveness.