The Evolution of MIL-STD 461

The military’s concern for EMI began with the installation of the first radio in a vehicle before World War I. However, since the application of radio technology and understanding its effects seem to parallel each other, it wasn’t until 1934 that the US 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. It specifically provides the opportunity to tailor the requirements for each application (e.g., above deck or below deck on a Navy ship) 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 is adapted from the MIL-STD 461E standard and identifies the emissions and susceptibility requirements. Each test is defined using the following syntax:

C = Conducted	CExxx = Conducted emissions test
R = Radiated	RExxx = Radiated emissions test
E = Emission	CSxxx = ? Conducted susceptibility test
S = Susceptibility	RSxxx = Radiated susceptibility test

The table also summarizes how the tests apply to computers with the text lifted and edited directly from the applicability sections of the standard. Major changes from revision D are also shown and summarized.

Requirement	Application	Frequency Range	Description	Changes From MIL-STD 461E
Conducted Emissions
CE101	Power & Interconnecting Leads	30Hz – 10kHz		No longer applicable to shipboard equip.
CE102	Power & Interconnecting Leads	10kHz – 10MHz
CE106	Antenna Terminals	10kHz – 40GHz
Conducted Susceptibility
CS101		30Hz – 150kHz	The requirement is applicable to power input leads that obtain power from other sources not part of the computer including those that are rechargeable. There is no requirement for power output leads. This test ensures that performance is not degraded from the ripple voltages associated with generators, airborne, ship-borne, and vehicle-borne power sources.	Applicability and limits extended to 150kHz
CS103	Antenna Port , Intermodulation	15kHz – 10GHz	Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like.)
CS104	Antenna Port, Signal Rejection	30Hz – 20GHz	Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like.)
CS105	Antenna Port, Cross modulation	30Hz – 20GHz	Not applicable to laptop, portable, or rack computers. (Applies only to receivers, transceivers, amplifiers, and the like, which extract information from the amplitude modulation of a carrier.)
CS109	Structure current	60Hz – 100kHz	Not applicable to computers unless it is directly attached to very sensitive equipment (sensitive to 1�V or better) such as tuned receivers operating over the frequency range of the test.	Measurement procedures revised
CS114	Bulk Cable Injection	10kHz – 200MHz	The requirements are applicable to all electrical cables interfacing with the computer. It simulates currents that may be developed on the platform cabling from electromagnetic fields generated by antenna transmissions both on and off the platform.	Limits reduced to 200MHz
CS115	Bulk Cable Injection, Impulse Excitation	Transients – 2nS x 30nS	The requirements are applicable to all electrical cables interfacing with the computer. It stimulates the fast rise and fall time transients that may be present due to platform switching operations and external transients such as lightning and electromagnetic pulses.	Applicability revised
CS116	Damped Sinusoid Transients – I/O & Power Cables	10kHz – 100MHz	The requirements are applicable to all electrical cables interfacing with the computer. It simulates electrical current and voltage waveforms occurring in platforms from the excitation of natural resonance. In contrast to fast transients of CS115, this test uses dampened sine waveforms to simulate the occurrence on platforms resulting from lightning, electromagnetic pulses, and the electrical switching phenomena of the platform.	Measurement procedures & applicability revised
Radiated Emissions
RE101	Magnetic Field	30Hz – 100kHz	This is applicable to computers and is intended primarily to control magnetic fields for applications where other equipment is sensitive to magnetic induction at lower frequencies. The most common example is a tuned receiver. (RS101 is a complementary requirement imposed on equipment to ensure compatibility with the anticipated magnetic fields.)	50cm requirement deleted; limits more stringent
RE102	Electric Field	10kHz – 18GHz	The requirements are applicable to electric field emissions from the computer and associated cables. The intent is to protect sensitive receivers from interference radiated from the computer and coupled through the antennas associated with the receiver (many receivers have sensitivities on the order of one microvolt).	Limits revised for submarine equipment
RE103	Antenna Spurious & Harmonic Outputs	10kHz – 40GHz	Not applicable to computers, laptops, and notebooks.
Radiated Susceptibility
RS101	Magnetic Field	30Hz – 100kHz	This requirement is applicable to computers and ensures that performance is not degraded when subjected to low-frequency magnetic fields.	Limits revised for Navy applications; Added Helmholtz coil test
RS103	Electric Field	2MHz – 40GHz		Added use of mode-tuned reverberation chambers above 200MHz
RS105	Electromagnetic Pulse Field Transient		This requirement is primarily intended for computers that could be subject to the fast rise time, free-field, transient environment of an electromagnetic pulse (EMP). It applies only to that computer equipment and enclosures that are directly exposed to the incident field outside of the platform structure, or for equipment inside poorly shielded or unshielded platforms. This requirement is applicable only for EUT enclosures (electrical interface cabling should be protected in a shielded conduit). Potential equipment responses due to cable coupling are controlled under CS116.	Limits revised for consistency with IEC Standards