Electromagnetic shielding effectiveness of EMI materials is commonly measured based on MIL-DTL-83528 or ARP 1705.
Shielding effectiveness measurement procedure: (1) apply a 1” wide frame gasket of the material to the perimeter of a metallic cover plate; (2) bolt cover plate and cover the opening of a shielded enclosure that contains a receiver or spectrum analyzer; (3) a transmitter antenna is placed outside of the shielding enclosure and pointed in line with the cover plate and receiver/spectrum analyzer; (4) measure transmitted power from one antenna to the other through the opening (without gasket); (5) measure with the opening closed by means of a metal plate with the gasket to be tested mounted around and compressed
EM shielding effectiveness of an EMI material is calculated by: SE=20 log E1/E2 (H1/H2) or the difference between both measurements in dB from 20 MHz to 10 GHz, with possible extension to 18 GHz. The theoretic repeatability is +/-6-10dB but can be deteriorated to +/-20dB with position variation of antenna.
Reference: Leader tech; Schlegel Electronic Materials (SEM); evaluation engineering