For the DoD, specifying a computer’s electromagnetic interference (EMI) and its electromagnetic compatibility (EMC) is an important judge on how well the computer will operate within many “noisy” environments. MIL-STD 461 documents the EMI requirements for a wide range of applications, from trucks to ships to aircraft to fixed installations, not to mention the different requirements within an application—above deck and below deck on a Navy ship. There is also a trend to tailor the requirements to particular applications. Although the most modest EMC requirements are not much different from commercial requirements, most military applications are decidedly harsh.

The Evolution of MIL-STD 461

The military’s concern for EMI began as far back as the installation of the first radio in a vehicle before World War I. Because the application of radio technology and understanding its effects seem to parallel each other, it wasn’t until 1934 that the U.S. Army Signal Corps published its first EMI standard: SCL-49, “Electrical Shielding and Radio Power Supply in Vehicles.” This document “protected” radio receivers by requiring vehicle operations to not “disturb” radio reception through shielding the ignition system, regulator and generator.

From this simple beginning, military EMI evolved and changed as the complexity of the systems increased, frequencies jumped, and the threat from EMP (electromagnetic pulses) was documented and quantified. As the specifications evolved, each branch of the service defined requirements specifically for their departments or platforms. This forced manufacturers to comply with significantly different specifications for each branch as well as different specifications for specific programs within each branch.

As a result, the DoD formed a working group to consolidate and replace approximately 20 requirements into the initial MIL-STD 461 (the requirements), MIL-STD 462 (the measurement methodology) and MIL-STD 463 (definitions and acronyms) that were published in 1967. As with any general standard, especially the initial standard, revisions were required resulting in MIL-STD 461A being issued in August 1968. Although mandated, many programs made revisions and exceptions to the standard (even as 461 went from revision A to C).

In 1993, the Tri-Service EMC Committee issued an updated MIL-STD 461 and MIL-STD 462, revision D. MIL-STD 463 was dropped and its definitions referenced to the American National Standards Institute (ANSI) C63.14 “Standard Dictionary for Technologies of Electromagnetic Compatibility (EMC), Electromagnetic Pulse (EMP) and Electrostatic Discharge (ESD).” In 1999, 461 and 462 were combined, requirements updated and published in the currently enforced standard: MIL-STD 461E.

Applying MIL-STD 461E

MIL-STD 461E is a set of EMC requirements intended to serve a wide range of applications, from trucks to ships to aircraft to fixed installations (Figure 1). It specifically provides the opportunity to tailor the requirements for each application without having to issue exceptions to the standard. Although the more modest sections are not much different from the common IEC and FCC commercial requirements, most sections are decidedly harsh. Table 1 and Table 2 are adapted from the MIL-STD 461E standard and identify the emissions and susceptibility requirements. It is important to remember that MIL-STD 461E doesn’t directly cover a number of commercial EMI situations, including lightning and ESD. Thus, a number of related requirements are added to the list—some from commercial standards and some from DEFSTAN (UK) or STANAG (NATO) requirements.

Applicable sections are summarized in Table 3 and cross-referenced as to how and where the equipment and subsystems are intended to be installed in, on, or launched from various military platforms or installations. If the equipment or subsystem may be installed on more than one platform, the standard requires that it comply with the more stringent requirement.

How stringent these requirements are enforced on laptop and other portable computers depends solely on the application and procurement office. Many DoD purchases require only commercial units; but if 461 is required, it is safe to say that most commercial-grade computers will fail without serious modifications. This is primarily due to the RS103 and RE102 requirements. RS103 requires that a computer maintain normal operation when exposed to a 20V per meter field (double the IEC 61000-4-3 requirements of 3 to 10V per meter).