EMC chamber types: SAC, FAR, OATS, GTEM, reverberation
Guide, EMC test environments
Choosing an EMC laboratory, or reading the test environment section of a report, requires understanding what each chamber type does and does not measure. CISPR 16-1-4 (2019) defines what a compliant emissions site is, IEC 61000-4-3 (2020) defines what a compliant radiated immunity setup is, and IEC 61000-4-21 (2011) opens a parallel route through reverberation. Around these reference texts orbit five physical environments, OATS (Open Area Test Site), SAC (Semi-Anechoic Chamber), FAR (Fully Anechoic Room), GTEM (Gigahertz TEM cell) and reverberation chamber, each with its validation rules, its frequency window, and its blind spots. This page maps them, explains how site validation works (NSA, sVSWR), how 3 m versus 10 m measurement distances are reconciled with the limits, what absorber types deliver across the band, and the recurring pitfalls that show up when a chamber is mis-selected for the EUT or the test type.
The OATS as the reference
Section titled “The OATS as the reference”The Open Area Test Site (OATS) is the historical reference site for radiated emissions. CISPR 16-1-4 specifies it as a flat ground plane (conducting, typically galvanised steel mesh or solid plate) of large area, with no overhead absorption and an open sky above the test volume. The receive antenna scans 1 m to 4 m in height, the EUT sits on a turntable on the ground plane, and the measurement integrates the direct ray and the ground-reflected ray. Site validation uses the Normalised Site Attenuation (NSA) method below 1 GHz, which compares the measured attenuation between two antennas against the theoretical OATS attenuation, with a tolerance of +/- 4 dB.
The OATS gives the closest representation of how the EUT will radiate in a real environment, free of indoor reflections and amplifier limitations. Two structural limits explain why it is rarely used today for routine emissions tests:
- Ambient noise. An outdoor site sits in the broadband and narrowband noise of the modern radio spectrum, with broadcast, cellular and Wi-Fi emissions that can mask EUT emissions by tens of dB. CISPR 16-1-4 requires the ambient noise floor to be at least 6 dB below the relevant limit at every measured frequency, a condition that no urban or peri-urban site meets reliably.
- Weather and availability. Outdoor measurement requires dry conditions, low wind, no precipitation, no thermal extremes. In Europe or North America the usable window is a fraction of the year and is unpredictable for industrial planning.
In contemporary practice, the OATS is used as the reference against which indoor chambers (SAC mainly) are validated, and for a small number of measurements that demand it (large EUT, military pre-compliance, calibration of reference antennas). The day-to-day emissions measurement happens indoors.
SAC, the workhorse for emissions
Section titled “SAC, the workhorse for emissions”The Semi-Anechoic Chamber (SAC) is the standard indoor emissions site for CISPR 32 (2015+A1:2019) and FCC Part 15 testing. It reproduces the OATS reflective geometry, conducting ground plane below the EUT, but absorbs reflections from walls and ceiling with RF absorber panels. The receive antenna still scans in height, the EUT still sits on a turntable, and the measurement still integrates direct plus ground-reflected rays. The chamber walls are a shielded enclosure (typically galvanised steel panels), which suppresses ambient noise by 80 dB to 120 dB.
SAC site validation
Section titled “SAC site validation”CISPR 16-1-4 recognises the SAC as a compliant alternative to the OATS provided two validations succeed:
- NSA (Normalised Site Attenuation) below 1 GHz. The measured site attenuation between two antennas at 1 m to 4 m height scan, at multiple positions across the turntable, must match the theoretical OATS attenuation within +/- 4 dB at every frequency. NSA is verified at five turntable positions (centre and four quadrants) for a fully validated test volume.
- sVSWR (site Voltage Standing Wave Ratio) above 1 GHz. A small probe moved along the test volume axis sees a residual standing wave that quantifies reflections from imperfect absorbers, antenna tower or turntable. CISPR 16-1-4 sets the acceptance criterion at 6 dB peak-to-peak ratio in the field, at 16 measurement positions.
Validation is performed at chamber commissioning, repeated typically every three years and re-performed after any modification (absorber replacement, antenna swap, turntable change). The validation certificate, kept by the lab, is what allows the chamber to be cited in a CISPR-compliant emissions report.
Test distance and limit correction
Section titled “Test distance and limit correction”CISPR 32 and FCC Part 15 limits are stated at a reference distance. CISPR Class B limits are stated at 10 m, FCC Part 15 Class B limits at 3 m for most bands. When the lab measures at a different distance, the limit is corrected assuming an inverse-distance scaling in the far field, which gives -10.5 dB for a 3 m to 10 m transition (20 log10(10/3)). The corrected limit is what appears in the report.
| Test distance | Typical chamber size | Limit correction vs CISPR 10 m | Notes |
|---|---|---|---|
| 3 m | 7 m x 6 m x 6 m typical | +10.5 dB (or limit measured at 3 m) | Default for small consumer EUT, FCC Part 15 default |
| 5 m | 9 m x 7 m x 7 m typical | +6 dB approximately | Intermediate, less common |
| 10 m | 19 m x 12 m x 9 m typical | Reference, no correction | Gold standard for CISPR Class B, large EUT |
A 3 m chamber that does not fit the EUT in its validated test volume is not usable for compliance, even if the EUT physically fits between turntable and antenna. The validation defines the volume in which NSA and sVSWR were measured, and beyond that volume the chamber is not a CISPR site.
Absorber types in a SAC
Section titled “Absorber types in a SAC”The absorber stack determines the chamber's frequency window. Three families combine.
| Absorber type | Frequency range | Reflection loss | Notes |
|---|---|---|---|
| Ferrite tiles | 30 MHz to 200 MHz | 18 to 25 dB | Heavy, expensive per square metre, indispensable at low frequency |
| Hybrid (ferrite + pyramidal foam) | 30 MHz to 18 GHz | 20 dB typical | Standard for CISPR full-band chambers |
| Pyramidal foam alone | 200 MHz to 40 GHz | Greater than 30 dB above 1 GHz for tall cones | Used in FAR and high-frequency chambers, cone height drives the lower cutoff |
A 30 MHz to 1 GHz emissions chamber needs ferrite at minimum on the side walls and ceiling. A full CISPR 30 MHz to 18 GHz chamber needs hybrid panels (ferrite below, foam cone above) on five surfaces, with the floor left as a conducting ground plane.
FAR, the option above 1 GHz
Section titled “FAR, the option above 1 GHz”The Fully Anechoic Room (FAR) extends the SAC by absorbing the floor as well. There is no ground reflection, the field at the EUT is closer to a free-space plane wave, and the antenna no longer scans in height. The FAR is required or strongly recommended for several test types:
- Radiated emissions above 1 GHz under certain RED test schedules and certain CISPR 32 annexes, where the ground reflection adds uncertainty that exceeds the budget.
- Radio type approval tests that demand free-space conditions, in particular RSE (Radiated Spurious Emissions) for cellular bands per EN 301 908 family.
- Antenna pattern measurements when used as an antenna characterisation environment.
The FAR is validated by sVSWR per CISPR 16-1-4, as the NSA model (which assumes a ground reflection) does not apply. The acceptance criterion remains 6 dB sVSWR at 16 positions in the test volume.
A side benefit of the FAR is its suitability for radiated immunity. IEC 61000-4-3 immunity setups are typically performed in a FAR or in a SAC with the floor temporarily absorber-lined (carpets or panels), because field uniformity calibration assumes a single plane wave at the EUT and a ground reflection corrupts that assumption.
GTEM cell, the compact bench-top alternative
Section titled “GTEM cell, the compact bench-top alternative”The GTEM (Gigahertz Transverse Electromagnetic) cell is not a chamber in the room sense. It is a tapered transmission line, asymmetric, where a central septum and an outer enclosure form a coaxial waveguide that supports a TEM (Transverse Electromagnetic) mode from DC to about 18 GHz. The EUT is placed inside the cell, in the volume between septum and outer conductor, and the radiated emission couples into the line.
How an EUT is characterised in a GTEM
Section titled “How an EUT is characterised in a GTEM”The TEM field in the cell is linearly polarised, in one direction at the EUT position. To obtain emissions equivalent to an OATS measurement (where the antenna scans both polarisations and the EUT rotates), the EUT is measured in three orthogonal orientations. An algorithm (Wilson, Crawford, derived from the early 1990s) combines the three measurements into a total radiated power, then converts that power to OATS-equivalent field strength at a reference distance.
CISPR 32 informative annexes (Annex E or similar depending on the revision) describe the GTEM-to-OATS correlation. Two structural caveats limit its use as a final compliance environment:
- The conversion algorithm assumes a small EUT that radiates essentially as a single dipole. For an EUT with multiple emission centres (long cables, distributed clock buses), the correlation degrades.
- The CISPR reference site remains the OATS or its SAC equivalent. A GTEM result is generally not accepted by a notified body or by the FCC as the final report on its own. It is accepted for pre-compliance, for product-family screening, and for design iteration.
The GTEM remains the cost-effective bench-top tool for an in-house pre-compliance setup, with capex an order of magnitude below a SAC and a footprint of a few cubic metres. For the broader pre-compliance methodology, see pre-compliance TEM cell and near-field.
Reverberation chamber, the immunity option
Section titled “Reverberation chamber, the immunity option”The mode-stirred reverberation chamber is an entirely different paradigm. The chamber is a metallic shielded enclosure, oversized relative to the wavelength, in which a rotating mechanical stirrer (or stepped tuner) breaks the boundary conditions and generates many electromagnetic modes that combine into a statistically uniform field over the stirrer rotation.
IEC 61000-4-21 framework
Section titled “IEC 61000-4-21 framework”IEC 61000-4-21 (2011) standardises the use of reverberation chambers for radiated immunity. The chamber is validated by measuring the statistical field distribution over a complete stirrer rotation at multiple EUT-volume positions, and verifying that:
- the field is independent of probe orientation (isotropy),
- the field follows the expected statistical distribution (Rayleigh for the amplitude),
- the field uniformity over the test volume is within the standard's tolerance.
The lowest usable frequency (LUF) of the chamber is fixed by its volume: enough modes must overlap at the test frequency for the statistical assumption to hold. A typical commercial chamber has an LUF around 200 MHz to 400 MHz, with full usability above 1 GHz.
When reverberation makes sense
Section titled “When reverberation makes sense”| Test case | Anechoic chamber | Reverberation chamber |
|---|---|---|
| Directional immunity (antenna pointing) | Direct, controlled polarisation | Not directly possible, statistical |
| High field strength (200 V/m and above) | Requires high-power amplifier | Same field with 10 to 20 dB less amplifier power |
| Large EUT with multiple cables | Hard, cable routing affects field | Natural, field is statistical |
| Frequency range | 30 MHz to 18 GHz typical | LUF (200, 400 MHz) to 40 GHz |
| Time per frequency point | Tens of seconds | Stirrer rotation, can be minutes |
| Acceptance in IEC 61000-4-3 testing | Reference environment | Alternative per IEC 61000-4-21 |
Reverberation chambers are well suited to automotive (cable harness immunity at high field), aerospace (DO-160 RTCA HIRF radiated immunity), and any case where the EUT or its cabling dominates the system response and a single plane-wave incidence misses the worst-case coupling.
Reading the test environment in an EMC report
Section titled “Reading the test environment in an EMC report”A CISPR-compliant emissions report cites the test site, its validation status, and the measurement distance. The information to look for, beyond the limit-versus-measurement plot:
- Site type and identifier: "SAC, 3 m, EUT volume 1.5 m diameter x 1.5 m height" or equivalent.
- Validation method and date: "NSA per CISPR 16-1-4 clause 8, validated 2024-03, next due 2027-03; sVSWR per clause 9, validated 2024-03". An expired validation invalidates the report.
- Test distance and limit correction: "Measured at 3 m, limit per CISPR 32 Class B at 3 m, correction -10.5 dB applied from the 10 m reference limit".
- EUT placement in the test volume: turntable position, height above ground plane, cable routing.
- Ambient noise floor sweep: chamber baseline scan before EUT is energised, showing margin to the limit.
For immunity, the equivalent block includes the field uniformity calibration (IEC 61000-4-3), the calibration date and the verified test volume.
| Report block | What to look for | Pitfall |
|---|---|---|
| Site description | Type, dimensions, validation status | "Anechoic chamber" without NSA/sVSWR data is not a CISPR site |
| Antenna calibration | Antenna factor curve, calibration date, ISO/IEC 17025 traceability | Expired calibration |
| EUT layout | Photo, cable routing diagram, distances | Layout not reproducible from the report |
| Limits | Standard reference, class, distance correction | Wrong class (A used where B applies) |
| Detectors | Peak / QP / Average per band | QP not used where required |
| Ambient sweep | Noise floor at least 6 dB below limit | Unreported, masks possible non-conformities |
For the detector and measurement-flow details, see radiated emissions EMC test and calibration and measurement uncertainty per GUM.
OTA, SAR and CATR, related but distinct environments
Section titled “OTA, SAR and CATR, related but distinct environments”EMC chambers (SAC, FAR, reverberation) measure electromagnetic disturbance and immunity. Three related environments serve adjacent radio measurements and are sometimes confused with EMC chambers.
| Environment | Purpose | Standard reference |
|---|---|---|
| OTA chamber (Over The Air) | Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) for cellular, Wi-Fi radios | CTIA, 3GPP TS 38.521, typically multi-probe anechoic |
| CATR (Compact Antenna Test Range) | mmWave radio testing (FR2, 24, 71 GHz) with a quiet zone produced by a reflector | 3GPP TS 38.521 for FR2 |
| SAR chamber | Specific Absorption Rate measurement on phantom for handset and wearable | IEC 62209-1, IEC 62209-2, IEC 62209-3, FCC OET-65 |
| Reverberation for OTA | TRP and TIS in a statistical environment, especially for MIMO antennas | CTIA, IEC 61000-4-21 (cross-reference) |
A SAR chamber is partly absorber-lined but its purpose is dosimetry, not EMC compliance. A CATR uses a parabolic reflector to create a plane-wave quiet zone in a compact footprint and is the standard tool for FR2 mmWave radio characterisation. None of these substitute for a CISPR-validated emissions chamber.
Field uniformity for immunity testing
Section titled “Field uniformity for immunity testing”IEC 61000-4-3 (2020) defines field uniformity in a way that is independent of the chamber type, as long as the chamber supports a plane-wave incidence at the EUT.
The 1.5 m by 1.5 m grid
Section titled “The 1.5 m by 1.5 m grid”The test field is calibrated over a square of 1.5 m by 1.5 m, located at the EUT position, perpendicular to the propagation axis. The grid has 16 points (4 x 4). For each frequency in the test range (typically 80 MHz to 6 GHz), the field is measured at the 16 points and the test is valid if the field at 12 of the 16 points (75 percent) lies in the range 0 dB to +6 dB relative to the nominal value. The four worst points may be outside the range, which accommodates corner effects.
Implications for EUT size
Section titled “Implications for EUT size”| EUT footprint | Compatible setup |
|---|---|
| Less than 0.5 m x 0.5 m | Centred in the 1.5 m grid, no issue |
| 0.5 m to 1.5 m wide | Fits in the validated grid, central placement |
| Greater than 1.5 m wide | Does not fit, two options: increase chamber and validated grid (e.g. 2 m grid), or switch to reverberation chamber |
A large EUT placed partially outside the validated uniform field area is non-compliant by construction, regardless of the field amplitude. For the radiated immunity methodology in detail, see IEC 61000-4-3, radiated RF immunity.
Choosing a lab and reading its accreditation scope
Section titled “Choosing a lab and reading its accreditation scope”An EMC lab's value is its validated chamber, the calibration of its instruments, and its ISO/IEC 17025 accreditation scope. When selecting a lab:
- Verify the accreditation scope lists the relevant standards (CISPR 32, IEC 61000-4-3, FCC Part 15, ETSI EN 300 328, and so on) with the corresponding frequency ranges. An accreditation that does not cover the test the report claims to perform is invalid.
- Verify the validation status of the chamber: NSA up to 1 GHz, sVSWR above 1 GHz, both within the three-year window.
- Verify the calibration certificates of the antennas, receivers, signal generators, field probes, with ISO/IEC 17025 traceability to a national metrology institute.
- Verify the test volume: dimensions, EUT mass capacity, cabinet height clearance.
- Verify the ambient noise floor sweep at the chamber baseline.
For the audit logic that applies to ISO/IEC 17025 labs and notified bodies, the reasoning is symmetric to the calibration uncertainty discipline; see calibration and measurement uncertainty per GUM.
Frequent pitfalls
Section titled “Frequent pitfalls”| Pitfall | Consequence |
|---|---|
| Choosing a 3 m SAC for an EUT that does not fit the validated test volume | Test invalid, full re-test in a larger chamber, schedule slip |
| Using a SAC above 1 GHz without sVSWR validation | Report rejected by the certification body |
| Running RED above-1 GHz emissions in a SAC when the schedule requires a FAR | Re-test in a FAR, additional cost |
| Missing chamber validation re-cal (three-year window expired) | All reports from the expired period potentially invalidated |
| Ambient noise floor not measured or above the limit -6 dB | Hidden non-conformity, potential market surveillance issue |
| Test volume not verified on the day of test, EUT placed outside | Field uniformity not guaranteed, immunity result not defensible |
| Antenna polarisation only measured in one orientation | Up to 20 dB error on the worst-case emission |
| Using a GTEM result as the final report instead of for pre-compliance | Final dossier rejected by the certification body |
| Confusing OTA, SAR or CATR chambers with EMC chambers | Wrong test campaign scoped, project delay |
| Reverberation result interpreted as directional immunity | Worst-case directional coupling missed |
Going further
Section titled “Going further”- Pre-compliance TEM cell and near-field: bench-top tooling for the design phase
- Radiated emissions EMC test: emissions test flow, detectors, distances
- IEC 61000-4-3, radiated RF immunity: immunity test setup and uniform field area
- Calibration and measurement uncertainty per GUM: the metrology backbone of any EMC report
- Glossary: definitions of SAC, FAR, OATS, GTEM, NSA, sVSWR, LUF
See also
Section titled “See also”- Radio: RX blocking, selectivity and intermodulation tests
- Calibration and measurement uncertainty (GUM, CISPR)
- PCB design for EMC: return paths, decoupling, stackup
- Antenna design and impedance matching for IoT
Sources & references
- CISPR 16-1-4:2019, Antennas and test sites for radiated disturbance measurements , IEC webstore.iec.ch/publication/36296
- CISPR 32:2015+A1:2019, Multimedia equipment, emission requirements , IEC webstore.iec.ch/publication/26241
- IEC 61000-4-3:2020, Radiated radio-frequency electromagnetic field immunity test , IEC webstore.iec.ch/publication/59849
- IEC 61000-4-21:2011, Reverberation chamber test methods , IEC webstore.iec.ch/publication/4191
- IEC 62209-1:2016, Procedure to determine SAR for hand-held devices, frequency range 300 MHz to 6 GHz , IEC webstore.iec.ch/publication/25336
- FCC OET Bulletin 65 (2001), Evaluating compliance with FCC guidelines for human exposure to RF fields , FCC www.fcc.gov/general/oet-bulletins-line
- ANSI C63.4-2014, American National Standard for methods of measurement of radio-noise emissions , ANSI / IEEE standards.ieee.org/ieee/C63.4/5715/