When specifying a semi-anechoic chamber (SAC), one of the first decisions is measurement distance: 3m, 5m, or 10m. This determines chamber size, cost, standards supported, and the range of products testable — and is a defined parameter in your test standard affecting result validity, not just a physical dimension.
What the Measurement Distance Actually Means
In CISPR 16-1-4, measurement distance (r) is the distance between the antenna reference point and the EUT boundary. A larger distance means lower received signal, though CISPR 16-1-1 provides correction factors normalising limits between distances. The distance also determines minimum quiet zone size, absorber requirements, and chamber dimensions.
3m Semi-Anechoic Chamber
The entry-level, most widely deployed configuration, suitable for consumer electronics (CISPR 32), household appliances (CISPR 14), lighting (CISPR 15), and ISM equipment (CISPR 11). Quiet zone diameters range 1.5–8.0m; a 1.5m quiet zone chamber typically needs approximately 6m × 7m internal floor dimensions. Worth specifying a quiet zone 30–50% larger than your current largest EUT for future growth.
Standards supported: CISPR 16-1-4, IEC 61000-4-3, MIL-STD-461, GJB151C, CISPR 25 component-level, DO-160 Section 20.
Limitations: Can't support vehicle-level automotive testing, which requires 10m.
5m Semi-Anechoic Chamber
Bridges the gap between 3m and 10m configurations, suited to labs offering both distances, OEMs with larger products, and organisations planning future 10m capability. Quiet zones run 2.0–4.0m, with internal floor dimensions typically 10–13m × 8–10m.
Achieving tight NSA (±3.0 dB rather than the ±4.0 dB CISPR minimum) requires careful absorber specification, since the end and side walls must perform over a wider incident angle range at 5m distance. Always ask for NSA data at the actual chamber size being specified.
10m Semi-Anechoic Chamber
The reference standard for automotive EMC testing, required for CISPR 25 vehicle-level radiated emission and immunity testing, full vehicle testing, and large industrial equipment. Quiet zones run 3.0–8.0m; a 3.0m quiet zone chamber needs approximately 20m × 12m floor dimensions and 8–10m height.
Typical accessories: vehicle-rated turntable (3,000–5,000 kg), motorised antenna mast, EUT ramp, and vehicle exhaust extraction.
Standards supported: all 3m/5m standards plus CISPR 25 vehicle-level, ISO 11452-2, and UN ECE Regulation 10.
Normalised Site Attenuation and Measurement Distance
NSA figures always apply at the measurement distance the chamber is designed for. A 10m chamber can't be characterised at 3m and claimed equivalent to a 3m chamber measurement — antenna positions and ground-plane geometry differ. Testing at both distances requires a chamber supporting the larger geometry, or two facilities.
Comparing the Options
| Specification | 3m SAC | 5m SAC | 10m SAC |
|---|---|---|---|
| Typical quiet zone | 1.5–4.0 m | 2.0–4.0 m | 3.0–8.0 m |
| Approx. internal floor area | 40–70 m² | 80–130 m² | 240–400 m² |
| Vehicle-level automotive | No | No | Yes (CISPR 25) |
| Relative capital cost | Reference | 1.5–2× | 4–8× |
Making the Right Decision
The measurement distance choice cascades through building space, structural requirements, HVAC, and total project cost. Finch works with our chamber development partner to provide site survey, design, and performance modelling before you commit to a specification.
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