Shielding Effectiveness Measurements: traditional system vs SEMS and calibration criteria

The SEMS (Shielding Effectiveness Measurement System) has been designed by MPB to perform attenuation measurements of Faraday cages in the frequency range from 10 kHz to 300 MHz.

The system consists of two units:

• TX Unit (transmitter);
• RX Unit (receiver).

The fully automatic system allows shielding effectiveness measurements to be performed with no possibility for the operator to modify the output data. This ensures high reliability and excellent repeatability of the results.

The procedure is divided into two phases.

Phase 1 - System zeroing

In this phase, initial calibration is carried out by characterizing the entire transmission/reception path without any shielding element between the transmitting and receiving antennas.

Phase 2 - Measurement with shielding in place

The shielding material (Faraday cage or shielded enclosure) is then placed between the antennas. In fully automatic mode, the receiver measures the transmitted signal attenuated by the shielding. The measured value is compared with Phase 1 and processed to determine the shielding attenuation.

Due to its operating principle, SEMS is classified as a relative measurement system, not an absolute one.

The final result is obtained from the difference between two measurements. It represents a relative value (attenuation introduced by the shielding), not an absolute measurement referenced to a metrological standard. Therefore, ISO/IEC 17025 accredited calibration is not applicable to the SEMS system as a whole.

Similarly, traditional systems (RF generator, amplifier, spectrum analyzer) used according to the IEEE 299 standard also rely on relative measurements between reference and shielded conditions.

Metrological traceability is ensured through calibration of the individual instruments in the measurement chain and the equipment used to verify system performance. SEMS undergoes functional calibration using reference instruments with ISO/IEC 17025 accredited certificates issued by accredited laboratories.