Can a TEM Waveguide Replace an EMC Chamber? Check the Equipment and Method Boundaries

A TEM waveguide can support EMC tests within its applicable scope, but a sample fitting inside it does not make it a universal substitute for an EMC chamber. Before choosing a TEM or GTEM arrangement, check the test method applicable to the product, the equipment configuration, the usable test space and the intended use of the results. This comparison concerns electromagnetic compatibility facilities, not acoustic anechoic rooms.

Start with the method, not the room dimensions

IEC 61000-4-20:2022 covers emission and immunity testing in TEM waveguides, including the equipment under test, waveguide validation and test arrangements. Its methods are distinct from IEC 61000-4-3.[1] IEC 61000-4-3:2020 addresses radiated RF electromagnetic field immunity. Relevant product committees determine whether products require this test and the appropriate levels and performance criteria. That standard also explicitly excludes substituting other test methods to claim compliance with IEC 61000-4-3.[2]

A procurement specification should therefore say more than “suitable for EMC testing”. It is useful to identify the product category, proposed standard editions, emission or immunity tests, and whether reports will support development comparisons or conformity assessment. The ability to perform electromagnetic tests in both environments is not evidence that their methods are interchangeable. If the applicable documents do not explicitly accept the proposed method, establish the basis before procuring the equipment, rather than looking for a justification after delivery.

Fitting inside is not the same as being testable by the method

The applicable frequency range and equipment size depend on the particular TEM waveguide and its validation conditions.[1] Internal enclosure dimensions should not automatically be specified as usable test dimensions. During a design review, show the operating sample in a layout drawing, including necessary accessories, supports and connections that must be maintained. Include every orientation required for the test. Review the complete test configuration, not just the bare product envelope.

This is a procurement review recommendation, not an additional standard requirement introduced by this article. If normal operation relies on external components, first decide which belong to the test configuration and how they will be arranged. Then assess whether sufficient test space is available. Do not arbitrarily alter the product configuration to fit the enclosure and subsequently use the result to judge the original configuration. Whether and how the sample must be rotated, positioned or oriented should follow the selected method and procedure, not a universal rule based on experience.

Similar emission plots do not establish interchangeability

NPL's GTEM guide discusses sample orientations in emission measurements and the correlation between GTEM and open-area test-site results.[3] Comparing results therefore requires attention to measurement and processing, not just the appearance of plots. Published in 2003, the guide is used here only to explain a measurement issue. Its historical standard provisions, dimensional ratios and frequency data are not adopted as current acceptance criteria.

If GTEM results will inform a product decision, identify the output quantity, processing method, applicable configuration and limitations in the test plan. When comparing results from another facility, retain records of the product version, operating state and arrangement. If results differ, check the comparison conditions first. Do not immediately attribute the difference to a product change, or apply an arbitrary fixed correction to force agreement.

Accept a defined testing capability

Acceptance documentation can be organised into three parts: the actual sample configurations; validation records for the corresponding test scope; and an example report explaining the method, conditions and intended use of its results. This is an engineering recommendation to help both parties review the deliverables. It does not mean that these three documents alone satisfy every compliance obligation.

The conclusion is neither “TEM can never replace a chamber” nor “a GTEM removes the need for a chamber”. A more defensible decision starts by establishing which method is accepted for these products and this test, then selecting a facility capable of implementing it. List uncovered test tasks separately in the capability statement rather than hiding them behind a broad claim of “complete EMC capability”.

References
[1] IEC, IEC 61000-4-20:2022, Electromagnetic compatibility (EMC) - Part 4-20: Testing and measurement techniques - Emission and immunity testing in transverse electromagnetic (TEM) waveguides, Edition 3. Official scope and edition information.
https://webstore.iec.ch/en/publication/34378

[2] IEC, IEC 61000-4-3:2020, Electromagnetic compatibility (EMC) - Part 4-3: Testing and measurement techniques - Radiated, radio-frequency, electromagnetic field immunity test, Edition 4. Official scope and edition information.
https://webstore.iec.ch/en/publication/59849

[3] NPL, The Use of GTEM Cells for EMC Measurements, Measurement Good Practice Guide No. 65, 2003, particularly Section 4. A historical technical guide, not a substitute for current standards.
https://eprintspublications.npl.co.uk/2771/1/mgpg65.pdf