Faraday cage shielding efficiency test: IEEE std 299 and electromagnetic attenuation

Shielding effectiveness

Shielding efficiency specifies the ability of a material or structure to reduce or attenuate electromagnetic wave propagation.

In environments like MRI or the anechoic chamber, where electromagnetic fields are generated and measured, it is necessary to have an environment free of ungovernable sources that would affect the measurement. To attenuate these signals as much as possible both environments are placed inside a Faraday cage that provides a shield.

The materials and type of construction of shielded chambers are different, but the purpose is to attenuate noise so that it is negligible for measurement. The better the attenuation capacity of the Faraday cage the better the shielding efficiency. This is defined as the ratio of the field strength at a given point to the field strength at the same point when a given electromagnetic shield is present. The measurement is expressed in decibels dB.

To verify the perfect operation of the Faraday cage, shielding efficiency is then tested according to the international IEEE std 299 standard.

This standard standardizes the procedure for measuring the shielding efficiency of a shielded environment from 9 kHz up to 18 GHz (one can extend the range down to 50 Hz and up to 100 GHz) for rooms greater than or equal to 2m.

The test involves generating a known signal from inside the shielded chamber and measuring it externally to evaluate its attenuation in decibels.

To test the shielded wall the measurement should be taken at several points (including floor and ceiling), paying special attention to critical points such as honeycombs, technical panels, joints between metal panels that make up the structure, the entrance door, etc.

Special attention should be paid to the frequency range and antennas to be used during the test:

Low frequency (9kHz - 16 MHz) loop antennas are used to measure the magnetic component
Resonant range (20 MHz - 300 MHz), biconical antennas or dipoles are used to measure the electrical component
High range (0.3 GHz - 18 GHz) dipole antennas are used up to 1 GHz and then horn antennas, for measurement of the electrical component

The IEEE standard, in addition to requiring various geometries (loops, dipoles, biconicals, horns), requires that both polarizations be realized: horizontal and vertical. This aligns the field lines at the various vertical and horizontal metal junctions of the shield and maximizes the penetration capacity of the electromagnetic wave by better revealing any holes in the shielding.

Each cage has its own attenuation specifications (multiple attenuation values on different frequency values depending on the application range).The final result of the shielding efficiency test certifies the degree of attenuation of the shielding environment during its implementation phase. The elements (if not properly installed) that can compromise the chamber shielding effectiveness are the EMI gaskets between panels, door and window fingers, engineering panels, and all those elements designed to ensure the RF sealing of the cage.