What NSA, FU, and Shielding Effectiveness Mean — and What Figures to Ask For

When specifying or evaluating an EMC anechoic chamber, three performance parameters appear on every datasheet: NSA, FU, and shielding effectiveness (SE). Understanding what these figures actually measure puts you in a much better position to evaluate proposals and avoid being misled by impressive-looking numbers that mean little in practice.

Shielding Effectiveness (SE)

SE measures how well chamber walls, doors, and penetrations attenuate electromagnetic signals passing through the shield — the ratio (in dB) of signal received outside versus inside when transmitting from outside.

How SE is measured

SE is measured per EN 50147-1 (Europe/Australia) and GB/T 12190-2021 (China), comparing received signal with the door open versus closed across a broad frequency range using different antenna types — loop antennas below 16MHz, biconical/dipole from 20MHz–1GHz, and horn antennas from 1–18GHz. The test is conducted before absorbers are installed, since SE is a property of the shielded enclosure itself.

What SE figures should you ask for?

For a well-constructed EMC shielded room, minimum SE requirements typically run ≥70dB at 14kHz (magnetic), ≥100dB from 150kHz through 18GHz (magnetic/plane wave/microwave). Defence and high-security applications call for tighter figures across the board, often ≥100dB uniformly from 1MHz to 18GHz.

Key point for procurement: Always ask for a qualification report showing SE at the specific frequencies relevant to your test standards. A chamber meeting 100dB at 1GHz but dropping to 60dB at 14kHz isn't suitable for low-frequency magnetic immunity testing, regardless of the headline specification.

Normalised Site Attenuation (NSA)

NSA is the primary parameter for semi-anechoic chambers used for CISPR 16-1-4 emission testing and IEC 61000-4-3 immunity site validation. It measures whether the site performs like a theoretically ideal open-area test site by comparing measured attenuation between transmit/receive antennas against calculated ideal values.

NSA isn't a shielding parameter — it indicates how effectively wall and ceiling absorbers suppress reflections, leaving only the direct signal and intended ground-plane reflection.

What NSA figures should you ask for?

The CISPR 16-1-4 acceptance criterion is ±4.0 dB from calculated theoretical values. Better chambers achieve ±3.0 dB, worth specifying for accredited labs pursuing ILAC or NATA accreditation where measurement uncertainty is critical, and increasingly important for 5m and 10m chambers where deviations at the quiet zone edge have proportionally larger effects. Ask for NSA plots across 30MHz–1GHz minimum, ideally to 18GHz, in both polarisations.

Field Uniformity (FU)

FU characterises how consistent field strength is across the EUT face during immunity testing. IEC 61000-4-3 requires a 16-point grid (4×4) on a 1.5m × 1.5m plane, passing if at least 75% of points fall within a 6dB window of the target field strength.

Why FU matters

Poor field uniformity creates hot and cold spots, meaning results are less reproducible and less representative of real exposure. FU depends on chamber geometry, absorber performance, and antenna-to-EUT distance.

Interpreting FU figures

A chamber quoting the minimum 75%/6dB threshold only tells you it meets the standard, not by how much. A chamber achieving 90% compliance at 3dB is significantly better than one scraping 75% at 6dB — ask suppliers for representative plots across the frequency range, not just pass/fail statements.

Putting It Together: What to Include in Your Chamber Specification

Shielding effectiveness: frequency range 9kHz–18GHz, minimum SE at each test point per EN 50147-1.

NSA: 30MHz–1GHz minimum, horizontal and vertical polarisation, ±4.0 dB (or ±3.0 dB) acceptance criterion.

Field uniformity: IEC 61000-4-3, minimum 75% of 16 points within 6dB, 80MHz–6GHz (or 18GHz for full-band systems).

All three parameters should be documented in a chamber qualification report with measurement plots and calibration records delivered as part of commissioning.

Summary

All Finch EMC chambers are qualified to these standards before delivery, with full measurement documentation provided.