What Is MIL-STD-461?

MIL-STD-461 controls electromagnetic interference (EMI) for electronic, electrical and electromechanical equipment used by the US military. It is the equipment-level electromagnetic compatibility (EMC) standard: it defines both how much interference a device may emit and how much external interference it must withstand without malfunctioning or losing performance.

The current revision is MIL-STD-461G, issued in December 2015. It replaced 461F (2007), which had replaced 461E (1999). Each revision refines test methods and adds requirements; procurement contracts name the revision to be used, so older equipment is often still qualified to F or E.

The standard is written in two parts: the requirements and limits, and the test procedures used to measure them, such as the line impedance stabilisation network (LISN) that sets a known impedance on power leads during conducted tests. Not every requirement applies to every product. The procuring activity selects requirements by platform and installation, and can tailor limits to suit.

Each requirement has a two-letter prefix that tells you what it tests and how the energy travels:

The table lists the requirements most often applied to computers and electronics. Frequency ranges are typical and can be tailored by the procuring activity.

Which of these apply depends on where the equipment goes. The applicability matrix in the standard is different for surface ships, submarines, aircraft, space systems and ground vehicles, and for Army, Navy and Air Force installations.

461G kept the structure of 461F and the main emission and susceptibility tests. The notable changes were three new requirements, CS106 for transients on submarine power and CS117 and CS118 for lightning and electrostatic discharge, together with refined test set-ups and calibration, and updated guidance on tailoring. In practice, a design that passed 461F needs reviewing against the new requirements rather than a full redesign.

Some limits also moved. RE102 and RS103 test set-ups were clarified, and the applicability tables were updated for each platform type. Because the contract fixes the revision, a supplier may hold 461F data for one programme and need 461G testing for the next, even on the same hardware.

Passing MIL-STD-461 is mostly a matter of design rather than added parts. Common measures in rugged computers include:

Filtered power input. An EMI filter on the DC input controls CE101 and CE102 and protects against CS101, CS114 and CS116. Many military power supplies combine the filter with MIL-STD-1275 or MIL-STD-704 transient protection.

A continuous metal enclosure. Conductive gaskets, tight seams and filtered or shielded connectors limit RE102 and RS103.

Shielded cables and good bonding. Cable shields terminated through 360 degrees at the connector stop cables from acting as antennas.

Careful board layout. Clock and switching-regulator noise is cheaper to control at its source than to filter at the enclosure wall.

Results come from a test report by an EMC laboratory against the selected requirements; there is no single pass certificate. Our overview of US military standards shows how 461 fits with MIL-STD-810 and the power standards, and our guide to MIL-STD-464 covers the platform-level requirements.

Military platforms are electromagnetically dense: radios, radar, motors and other computers share a confined space. A computer that meets MIL-STD-461 is less likely to disrupt, or be disrupted by, the systems around it. Many of the systems in our military computers and rugged mission computers ranges list MIL-STD-461 test results, usually alongside MIL-STD-810 environmental and MIL-STD-1275 or 704 power data. Our guide to how a military Ethernet switch is built shows the same design measures in a network switch.

We supply military and rugged mission computers with MIL-STD-461 test data from the manufacturer, and integrate and test each system before it ships. Tell us the platform and the requirements your programme calls up, and we will shortlist systems with matching evidence.