When a shielded room, enclosure, or SCIF is handed over, the number that matters is its shielding effectiveness — how many decibels of attenuation the barrier delivers across the frequencies it has to block. IEEE Std 299 is the document that defines how that number is measured. Without it, "100 dB of shielding" is a marketing claim; with it, the figure becomes a repeatable, defensible measurement that a specifier, an accrediting body, or a security authority can rely on.
For contractors and specifiers, the practical risk lives in the details of the method: which antennas are used in which frequency band, how the reference and shielded measurements are taken, and what the resulting dB figure actually represents. Misreading any of those leads to acceptance disputes, wasted re-tests, and rooms that look compliant on paper but fail in service. This guide explains how IEEE 299 works, the three measurement regimes it defines, and how it differs from the older MIL-STD-285 and the NSA standards it is often confused with.
What IEEE Std 299 Is
IEEE Std 299, "IEEE Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures," is the recognized standard method for measuring the shielding effectiveness (SE) of solid, walk-in electromagnetic enclosures. It establishes uniform procedures for measuring the attenuation a shielded enclosure provides to electric fields, magnetic fields, and plane waves over a frequency range from roughly 9 kHz to 18 GHz. The result is expressed as shielding effectiveness in decibels (dB) — the ratio, in dB, of the field strength measured without the shield in place to the field strength measured with it in place.
The standard applies to enclosures large enough to walk into — shielded rooms, RF/EMI test chambers, SCIFs, and MRI suites — where the measurement antennas can be positioned inside and outside the barrier. A higher dB value means better shielding: 100 dB of attenuation means the field on the protected side is reduced by a factor of 100,000 in amplitude. National Shielding measures and certifies enclosures to this standard as part of its RF/EMI testing services.
The Three Measurement Regimes
IEEE 299 divides the frequency spectrum into three regimes, because the physics of how a shield attenuates energy — and therefore how you measure it — changes with frequency. At low frequencies the dominant challenge is the magnetic field; at high frequencies it is the plane wave; and between them lies a resonant region where enclosure dimensions interact with the wavelength. Each regime uses its own antennas, geometry, and measurement procedure.
| Measurement Regime | Typical Frequency Range | Antenna / Method | What It Detects |
|---|---|---|---|
| Low range | ~9 kHz – 20 MHz | Magnetic-field (small loop) antennas, coaxial loops | Magnetic-field shielding effectiveness — the hardest to achieve at low frequency |
| Resonant range | ~20 MHz – 300 MHz | Tuned dipoles / biconical antennas; sampled at multiple points | Electric-field and transition-region performance, where enclosure resonances appear |
| High range | ~300 MHz – 18 GHz | Plane-wave (electric-field) antennas — horns, log-periodic dipole arrays | Plane-wave (far-field) shielding effectiveness at microwave frequencies |
Low range: magnetic field is the weak point
Below about 20 MHz, IEEE 299 calls for loop antennas to measure magnetic-field shielding effectiveness. This is the regime where most enclosures are weakest, because thin conductive walls attenuate low-frequency magnetic fields far less effectively than high-frequency electric fields. A welded steel or galvanized-panel room that delivers 100+ dB at microwave frequencies may provide only 30-70 dB of magnetic shielding at 10 kHz. For facilities where low-frequency magnetic performance is critical — such as MRI suites and some secure rooms — this regime drives the specification.
Resonant range: where dimensions matter
In the resonant region, the wavelength of the test signal becomes comparable to the dimensions of the enclosure and of apertures such as door gaps and seams. Standing waves can form, and shielding effectiveness can dip sharply at specific frequencies. IEEE 299 addresses this by sampling at multiple frequencies and antenna positions so that resonant dips are not missed. This is why a credible SE survey reports many frequency points, not a single headline number.
High range: plane-wave attenuation
Above roughly 300 MHz the test signal behaves as a plane wave in the far field, and electric-field antennas — horns and log-periodic arrays — are used. This regime exposes the integrity of doors, honeycomb vents, waveguides, and filtered penetrations, because microwave energy readily exploits small apertures. High-range performance is usually where the most demanding specifications (for TEMPEST or high-security work) concentrate their pass criteria.
Measurement Geometry: Reference and Insertion
Every IEEE 299 measurement is a comparison of two readings taken with a transmit antenna on one side of the barrier and a receive antenna on the other. The first reading establishes a reference; the second establishes how much the shield reduces it.
- Reference measurement — the transmit and receive antennas are set a fixed distance apart with no shield between them (or with the antennas positioned to characterize the source field), recording the unattenuated signal level.
- Shielded (insertion) measurement — the same separation and signal are reproduced with the enclosure wall, door, seam, or penetration between the antennas, recording the attenuated level.
- Shielding effectiveness — the difference in decibels between the reference and shielded readings: SE (dB) = reference level (dB) − shielded level (dB).
Critically, the receive antenna is moved to probe the suspected weak points — door perimeters, gasket and fingerstock seams, panel joints, filter plates, waveguide entries, and pipe penetrations — rather than only the centers of flat panels. Apertures, not the bulk wall, are almost always where real enclosures lose performance, which is why the geometry and the choice of probe locations are as important as the antennas. National Shielding's laboratory and testing shielding practice plans these survey points against the way each enclosure is actually built.
IEEE 299.1 for Small Enclosures
IEEE Std 299 was written for walk-in enclosures, so it does not apply cleanly to small shielded boxes, cabinets, and equipment housings where antennas cannot be placed inside. IEEE Std 299.1 extends the methodology to enclosures with linear dimensions below the range 299 covers (roughly 0.1 m to 2 m), using alternative measurement techniques suited to compact volumes. Specifiers procuring small shielded enclosures or shielded cabinets should reference 299.1 rather than the parent 299, while large rooms, chambers, and SCIFs remain squarely under IEEE 299. Our overview of shielded rooms for EMC compliance laboratories covers how room-scale enclosures are specified and validated.
IEEE 299 vs MIL-STD-285 and the NSA Standards
IEEE 299 is frequently confused with two other families of shielding documents, and getting the distinction right matters for any defense, intelligence, or medical procurement.
- MIL-STD-285 (superseded) — the original military method for measuring the attenuation of shielded enclosures. It was officially canceled, and IEEE 299 is its civilian successor and the method most specifications now cite. A specification that still calls out MIL-STD-285 is referencing an obsolete document; the modern equivalent is IEEE Std 299.
- NSA standards (e.g., NSA 65-6, NSTISSAM TEMPEST/1-92) — these define shielding performance and attenuation requirements for RF-shielded enclosures used in national-security and TEMPEST contexts. They set the required dB limits and construction criteria for those environments; IEEE 299 is often the measurement method used to verify the attenuation those standards demand.
- IEEE 299 — a measurement method, not a performance requirement. It tells you how to measure shielding effectiveness; it does not tell you how many dB your enclosure must achieve. The required value comes from the application or the governing standard (MRI vendor specs, EMC test needs, TEMPEST or SCIF criteria).
The clean mental model: IEEE 299 (and 299.1) is the ruler, while NSA, MIL-STD, ICD 705, and MRI-vendor documents set the target the ruler measures against. For medical work, the relationship between method and required performance is detailed in our guide to MRI shielding certification and testing requirements, and the underlying barrier choices are compared in comparing RF shielding materials.
Frequently Asked Questions About IEEE 299 Shielding Effectiveness Testing
What is IEEE Std 299?
IEEE Std 299 is the standard method for measuring the shielding effectiveness of walk-in electromagnetic shielding enclosures. It defines uniform procedures and antenna setups for measuring the attenuation an enclosure provides to magnetic fields, electric fields, and plane waves from about 9 kHz to 18 GHz, with the result expressed as shielding effectiveness in decibels.
What frequency range does the IEEE 299 test cover?
IEEE 299 covers roughly 9 kHz to 18 GHz, divided into three regimes: a low range (about 9 kHz–20 MHz) measured with magnetic-loop antennas, a resonant range (about 20–300 MHz) measured with dipole or biconical antennas, and a high range (about 300 MHz–18 GHz) measured with plane-wave horn and log-periodic antennas. Each regime uses different equipment because shielding physics changes with frequency.
How is shielding effectiveness calculated in IEEE 299?
Shielding effectiveness is the difference, in decibels, between a reference measurement taken without the shield and an insertion measurement taken with the shield in place: SE (dB) equals the reference level minus the shielded level. A higher dB value means better shielding, so 100 dB represents a 100,000-to-1 reduction in field amplitude.
What is the difference between IEEE 299 and IEEE 299.1?
IEEE 299 applies to large, walk-in enclosures such as shielded rooms, chambers, and SCIFs, where antennas can be placed inside and outside the barrier. IEEE 299.1 extends the methodology to small enclosures, cabinets, and equipment housings that are too compact for the standard antenna geometry, using alternative techniques suited to those volumes.
Is MIL-STD-285 still used for shielding effectiveness testing?
No. MIL-STD-285 was the original military method for measuring shielded-enclosure attenuation, but it has been canceled and superseded. IEEE Std 299 is its modern successor and the method most current specifications cite. A specification still referencing MIL-STD-285 should be updated to IEEE 299.
How does IEEE 299 relate to NSA and TEMPEST standards?
IEEE 299 is a measurement method, while NSA standards such as NSA 65-6 and TEMPEST documents set the required shielding-performance levels for national-security enclosures. In practice IEEE 299 is frequently the procedure used to verify that an enclosure meets the dB attenuation those security standards require, so the two are complementary rather than alternatives.
Where are shielded enclosures most likely to fail an IEEE 299 test?
Enclosures rarely fail at the center of a flat wall; they fail at apertures. The common weak points are door perimeters and their gaskets or fingerstock, panel seams and joints, filter plates, waveguide and pipe penetrations, and any unsealed conductive gap. IEEE 299 surveys deliberately probe these locations because they govern real-world shielding effectiveness far more than the bulk barrier does.