Your 3 m chamber has a size limit, and it is not the quiet zone

Ask a laboratory how large a product its 3 m chamber can take and the answer is usually the quiet zone: the volume over which normalised site attenuation was validated, often 2 m across. For a long time that was the only answer the standards gave. Since 2019 the basic standard for radiated measurements gives a second one, and it is smaller. It depends on the distance to the antenna, not on how good the chamber is.

What the amendment says

The amendment's own introduction states its purpose in one line: EUT volume specifications for radiated disturbance measurements, depending on test method and on measurement distance. It adds a definition, 3.1.35, of a small EUT: equipment, including its cables, on a table or standing on the floor, that fits in a cylindrical volume of 1.5 m in diameter and 1.5 m in height at a 3 m measurement distance, or 2.0 m by 2.0 m at 5 m, on an OATS or in a SAC. It then replaces the old introductory clause with a new 7.1.2, an overview of maximum EUT volumes by method, frequency range and distance.

Three details in the footnotes matter as much as the numbers.

• 3 m and 5 m are limits; 10 m and 30 m are not. The footnote to the 10 m and 30 m entries says they are a recommendation only, because those distances may be regarded as protection distances, and any EUT volume encompassed by the receive antenna's beamwidth is accepted provided the test volume meets the validation criteria. No such footnote is attached to 3 m or 5 m.
• The reason is the near field. The amendment defines the near-field effect as the deviation from far-field propagation close to the EUT. A large source at a short distance is not seen by the antenna as a point, and the rays from its top, centre and bottom do not add as the 10 m limit assumes. CISPR 16-4-2 puts a number on it in its own comments: for an EUT 1.5 m high at 3 m, the field reduction at 1 GHz is about 4 dB, it is larger for larger EUTs, and it is not included in the published uncertainty.
• Above 1 GHz the same sizes are recommended "for reduced near-field uncertainty", together with minimum antenna beamwidths of 28° at 3 m and 22.6° at 5 m and 10 m, so that the antenna actually sees the whole volume.

The table's note also says that work is in progress to define conditions for medium-sized EUT volumes. The standard already expects a class of equipment that is neither small nor suited to a short distance.

Where it is cited

CISPR 16-2-3 is the basic standard for radiated disturbance measurement, and every CISPR product standard calls it up for the method. Whether the size table applies to a given test depends on which edition the product standard names, because the references are dated. Reading the normative references of the current editions gives a clear picture: two product standards already name the amendment, and the largest one has deliberately kept it out for now.

The editions these replaced, which many laboratories are still accredited to, all name the 2010 text. They show why the amendment was written, because each had the idea in a different form.

One standard had a number, one had a ratio but only for a FAR, and two had a warning with no number at all. Amendment 1 puts one table in the basic method that all of them share, gives the OATS and the SAC the limit the FAR already had, and extends it to 5 m.

What it changes today

• Under CISPR 11:2024 and CISPR 14-1:2020 the table already applies. A laboratory accredited to those editions has 1.5 m by 1.5 m at 3 m and 2.0 m by 2.0 m at 5 m in its referenced method today. Where the product standard sets a tighter limit of its own, as CISPR 11 has long done with its 1.2 m small size equipment definition for 3 m, the tighter one governs, and the basic standard adds what the product standard does not cover, such as the 5 m entry.
• Under CISPR 32 the binding rule in a SAC is still the validated test volume. A chamber with a 2 m quiet zone at 3 m may take a product 2 m across, because the amendment to CISPR 32 sets aside additional requirements of the basic standard. In a FAR the half-distance rule already gives 1.5 m at 3 m.
• Laboratories still accredited to the previous editions work to the rules in the second table until their scope moves on, which for most is the next transition date set by the regulator or the accreditation body.
• Below 30 MHz, the link is already made through CISPR 16-1-4:2025, which limits the test volume by the amendment's table as well as by the NSIL validation.
• For the measurement result, the near-field error does not depend on which reference is in force. A 1.9 m rack measured at 3 m carries an error the uncertainty budget does not contain, and the same rack measured at 10 m in another laboratory will not give the same margin. That shows up as disagreement between laboratories long before it shows up as a non-conformity.
• For a laboratory that tests to several product standards in one chamber, the strictest reference on the scope sets the practice. Once CISPR 11:2024 or CISPR 14-1:2020 is on the scope, checking EUT size against distance becomes part of the procedure for every job.

What is foreseeable

The pattern is already visible. Each product standard revised since 2019 has named the amended text, and the one that has not, CISPR 32, excluded it by a sentence in an amendment rather than by a technical argument. Its next edition is the foreseeable change: when multimedia equipment follows CISPR 11 and CISPR 14-1, 1.5 m by 1.5 m becomes the limit at 3 m for the largest product family of all. The products affected are easy to list because the cylinder includes the cables and is measured across the widest dimension.

Two further developments are signalled in the text itself. The medium-sized EUT category is under development, and conditions for it are more likely to mean additional requirements at short distances than a relaxation of them. And the sizes above 1 GHz, which are recommendations today, sit in the same clause and follow the same reasoning, so a future edition can harden them without a new argument. A laboratory planning a chamber that has to last twenty years should plan for the table, not for the dated reference it happens to work to this year.

Why 5 m is the sensible step

For equipment that no longer fits 1.5 m, the choice is between 5 m and 10 m. The table makes the case for 5 m clearly. It takes the volume from 1.5 m to 2.0 m in both directions, which is where most of the products in the list above sit. The conversion from a 10 m limit is 6 dB at 5 m against more than 10 dB at 3 m, so less of the result rests on an assumed fall-off. And a 5 m chamber stays in the height class of a 3 m chamber, which means an ordinary industrial building rather than a purpose-built hall.

Semi-anechoic chamber, 5 m and 3 m

Frankonia SAC-5 Plus

A dome-roof chamber with two measurement distances in one room: 5 m for equipment up to the 2.0 m by 2.0 m volume of the table, and 3 m for everything that still qualifies as a small EUT.

• Quiet zone of 2.0 m or 3.0 m, so the validated volume is never the limiting factor at 5 m, with room for the cables that count towards the EUT volume
• NSA within ±3.5 dB and site VSWR within 5.5 dB, guaranteed, against the 4 dB and 6 dB of CISPR 16-1-4
• External size 12.7 m by 8.2 m by 6.3 m for the L variant, the same height class as a 3 m chamber
• Turntable load up to 5 t, for cabinets, racks and floor-standing machines
• One lining for CISPR 11, 14, 15, 25 and 32 and for MIL-STD-461, with the Frankosorb hybrid absorbers keeping the walls thin

Equipment larger than 2.0 m belongs at 10 m, where Frankonia's SAC-10 range applies. Quotations on request.

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What to check this year