How Do You Select EMI Shielding Components to Pass MIL-STD-461G?

Military and aerospace electronics must pass MIL-STD-461G before they can ship—and most don't pass on the first attempt. The standard defines emission and susceptibility limits across frequencies up to 18 GHz, but passing comes down to physical components: which gasket material seals each enclosure seam, how cable shields terminate at connectors, and whether the galvanic couple between gasket filler and enclosure alloy will corrode over a 20-year service life. These are component selection decisions, and getting them right before the first pre-compliance scan is significantly less expensive than redesigning after a failed test.

What Does MIL-STD-461G Require for EMI Shielding?

MIL-STD-461G is the Department of Defense interface standard governing electromagnetic emissions and susceptibility for military electronic equipment, with test requirements that vary by platform type (ground, naval, airborne, space).

The tests most directly affected by shielding component selection are:

• RE102 (Radiated Emissions): Measures electric field emissions from 10 kHz to 18 GHz (platform-dependent). Rev G eliminated the previous allowance to stop testing at 1 GHz, requiring all platforms to test to 18 GHz. Limits are peak-detected—more stringent than the quasi-peak detection used in commercial standards like CISPR 25.

• CE102 (Conducted Emissions): Measures RF conducted emissions on power leads from 10 kHz to 10 MHz. The basic limit curve starts at approximately 94 dBμV at 10 kHz and decreases to approximately 60 dBμV from 500 kHz through 10 MHz.

• RS103 (Radiated Susceptibility): Tests immunity to electric fields from 2 MHz to 18 GHz at levels between 20 and 200 V/m, depending on the platform threat environment. Both horizontal and vertical polarization are required above 30 MHz.

The shielding components in an enclosure—gaskets, filtered connectors, ventilation panels, cable shields—collectively determine whether the equipment meets these limits. An enclosure with adequate wall shielding but poor gasket sealing at seams will fail RE102. An enclosure with proper gaskets but unfiltered cable penetrations will fail both RE102 and RS103.

Why Do Most Enclosures Fail RE102 on the First Attempt?

RE102 is the most commonly failed test in the MIL-STD-461 suite, with reported first-pass failure rates of 50–90%. The primary cause is not the enclosure walls—it is the cables.

Cables are efficient antennas. Even cables with 60–80 dB of shielding effectiveness can cause RE102 failures when shield terminations are imperfect, when pigtail ground connections are used instead of 360-degree termination, or when common-mode currents develop on cable shields. The cable routing within the test setup—maintained 5 cm above the ground plane per the standard—creates a transmission line structure that radiates efficiently at frequencies where the cable length approaches a quarter wavelength.

Inside the enclosure, failure modes include gasket discontinuities at seam interfaces, untreated apertures for ventilation or indicators, and poor bonding between the enclosure and system ground. MIL-STD-461G requires 2.5 milliohm or less DC resistance between the equipment under test and the ground plane—but low DC resistance alone does not guarantee adequate RF bonding performance.

Pre-compliance testing should target at least 10 dB of margin below the limit. Simulations during the design phase should target 20 dB to account for model uncertainties and manufacturing variation.

How Do You Select EMI Gaskets for MIL-STD-461G Enclosures?

MIL-DTL-83528 defines the conductive elastomer materials used in military EMI gaskets. The current revision (Rev J, October 2023) specifies eight material types, each with different filler metals, base elastomers, and performance characteristics.

Galvanic Compatibility

The filler metal in the gasket must be compatible with the enclosure alloy to prevent galvanic corrosion over the system's service life. Per MIL-STD-889D, silver-plated copper fillers (Types A, C, K) create a strong galvanic couple with aluminum enclosures—silver sits far from aluminum on the galvanic series. In wet or salt-spray environments, this couple accelerates corrosion at the gasket-to-flange interface, degrading both shielding and environmental sealing.

Silver-plated aluminum fillers (Types B, D) offer dramatically better compatibility with aluminum housings because the core filler is galvanically similar to the enclosure material. For aluminum enclosures in marine or high-humidity environments, Types B or D should be the default selection unless the higher shielding of Type A is specifically required and corrosion protection measures (chromate conversion coatings, conductive topcoats) are specified.

Compression Design

Conductive elastomer gaskets require 10–25% compression for solid profiles to achieve consistent filler particle contact and low contact resistance. Under-compression creates micro-gaps that break electromagnetic continuity. Over-compression causes permanent deformation, compression set, and accelerated gasket degradation.

Mechanical compression limiters—metal standoffs or integral stops in the flange design—prevent over-compression during assembly and maintain the target compression range across the thermal cycling and vibration profiles that military equipment experiences. Flange flatness directly affects compression uniformity: if the mating surface has more than 0.1 mm of waviness across the gasket run, the gasket cross-section height must accommodate that variation while still maintaining minimum compression.

What Other Shielding Components Support MIL-STD-461G Compliance?

Beyond gaskets, three additional shielding component categories contribute to MIL-STD-461G compliance.

Filtered connectors address the cable emission problem at the enclosure boundary. Connectors with integrated feedthrough capacitors or pi-filter elements suppress conducted emissions on signal and power pins before they can radiate from external cable runs. For RE102 compliance, filtered connectors at every cable penetration point are often more effective than improving enclosure shielding.

Waveguide-below-cutoff ventilation panels allow airflow through enclosure walls without creating EMI apertures. Honeycomb panels with cell dimensions sized below the cutoff frequency of the highest frequency of concern provide 40–80 dB of attenuation while maintaining adequate airflow for thermal management.

Board-level shield cans isolate high-frequency circuits within the enclosure. For equipment with multiple clock frequencies or mixed analog/digital circuits, localized shielding prevents internal coupling that would otherwise require the enclosure to attenuate emissions that originate from the PCB. For guidance on board-level shield design, see our EMI Shielding Guide.

What Should Procurement Managers Specify When Sourcing MIL-STD-461 Shielding Components?

Procurement specifications for defense EMI shielding components must be more detailed than commercial equivalents to ensure compliance through production and sustainment.

Material specification: Call out the specific MIL-DTL-83528 type (A, B, C, D, or K), not just "conductive elastomer." Include the shielding effectiveness requirement at the frequency of interest, operating temperature range, and durometer.

QPL certification: For defense contracts requiring qualified products, verify that the supplier and specific material formulation appear on QPL-83528, maintained by the Defense Logistics Agency. QPL-listed manufacturers include Specialty Silicone Products (SSP), Parker Chomerics, and Laird Technologies. The current QPL can be accessed through ASSIST Online (assist.dla.mil).

Galvanic compatibility: Specify the enclosure alloy and surface treatment in the RFQ so the gasket supplier can verify filler compatibility per MIL-STD-889D. Include the expected environmental exposure (salt spray hours, humidity, temperature cycling profile).

Compression and dimensional tolerances: Define the nominal compression percentage, the compressed height, and the tolerance on uncompressed gasket cross-section dimensions. Include the flange flatness tolerance and fastener spacing that determine compression uniformity.

Environmental and qualification testing: Specify compression set requirements (per ASTM D395 Method B), salt fog exposure hours, and thermal cycling profiles that the gasket must survive while maintaining shielding performance. These requirements should trace to the system-level environmental qualification standard (typically MIL-STD-810).

Frequently Asked Questions

What is the most commonly failed test in MIL-STD-461G?

RE102 radiated emissions is the most commonly failed MIL-STD-461 test, with first-pass failure rates reported between 50% and 90%. The primary cause is cable emissions rather than enclosure leakage—cables act as efficient antennas even when the enclosure itself provides adequate shielding. Rev G extended the mandatory test range to 18 GHz for all platforms, making the test more demanding than previous revisions.

Which MIL-DTL-83528 gasket type provides the highest shielding effectiveness?

MIL-DTL-83528 Type A (silver-plated copper in silicone) and Type K (silver-plated copper in high-durometer silicone) both achieve approximately 110 dB shielding effectiveness at 10 GHz. Type A operates from -55°C to +125°C at 65 Shore A durometer, while Type K uses 85 Shore A for applications requiring greater compression resistance. For environments with fuel or solvent exposure, Type C (fluorosilicone) provides 100 dB with chemical resistance.

How does galvanic compatibility affect EMI gasket selection?

Galvanic corrosion between conductive gasket filler metals and enclosure materials can degrade both shielding effectiveness and structural integrity over time. Silver-plated copper fillers create a strong galvanic couple with aluminum enclosures, leading to corrosion in wet environments. Silver-plated aluminum fillers are dramatically more compatible with aluminum housings. MIL-STD-889D provides the galvanic compatibility classification system for selecting appropriate material combinations.

What compression percentage should be specified for MIL-DTL-83528 gaskets?

The recommended compression range for solid-profile conductive elastomer gaskets is 10–25%. Compression within this range ensures uniform filler particle contact for consistent conductivity. Over-compression causes permanent deformation and accelerated compression set, while under-compression creates micro-gaps that break the EMI seal. Mechanical compression limiters should be specified to prevent over-compression during assembly.

What bonding resistance does MIL-STD-461G require?

MIL-STD-461G specifies 2.5 milliohm or less DC resistance between the equipment under test and the ground plane during testing. For actual installations, MIL-STD-464C specifies 10 milliohm for equipment enclosure to system structure bonds and 2.5 milliohm for individual faying interfaces. Note that low DC bond resistance does not reliably predict RF bonding performance—separate high-frequency verification may be needed.

---

Sourcing EMI shielding components for a MIL-STD-461 program? Contact POCONS USA to discuss MIL-DTL-83528 gasket selection, enclosure shielding design, and qualification testing with our engineering team.