Sunday, April 1st, 2012
In an article for COTS Journal, Joel Young, engineer at Crenlo, discusses why shielding of military electronics from electromagnetic interference (EMI) often necessitates the use of a custom enclosure.
There’s a special art to designing the proper EMI shielding for a system. Because every scenario is unique, getting the adequate protection means a lot of custom modifications to enclosures.
Today’s military is more reliant than ever on electronics. Along with that, the need to protect those electronics from potential threats has become all the more vital. One such threat—electromagnetic interference (EMI)—has the ability to interrupt or even destroy the functionality of unprotected electronics. Meanwhile the EMI radiated by military electronics can distribute an unwanted signal that could put covert military personnel at risk of being located. By properly shielding electronics, however, susceptibility to these threats can be drastically reduced. Because every scenario is unique, getting the adequate protection more often than not necessitates the development of custom or modified enclosures rather than the use of an off-the-shelf product.
For purchasing agents and systems integrators serving the military market, it’s essential to have a good understanding of what to look for in an enclosure in order to ensure electronics will be adequately protected. Specifically, a buyer should be educated in three key areas: standards governing EMI, enclosure testing procedures and enclosure design considerations.
Standards Governing EMI
The military has long been aware of the effects of electromagnetic interference and has taken proper precautions to shield electronics. Often bases use shielded rooms to house the bulk of their network equipment. In mobile settings, smaller, transportable, shielded enclosures have been used to protect individual components for nearly 80 years. Since the first military standard governing EMI shielding was introduced in the 1930s, new standards—both commercial and military—have arisen at nearly the same rate at which technology has evolved during this time.
Today, the list of industry and military-related standards governing EMI shielding requirements and testing procedures is long enough to fill an entire library. The military alone has developed regulations and revisions to those regulations hundreds of times, and in addition to the military, organizations such as the Federal Communications Commission (FCC), the American National Standards Institute (ANSI), the International Electrotechnical Commission (IEC), the National Security Agency (NSA) and the Institute of Electrical and Electronics Engineers (IEEE) have all created their own standards.
The abundance of these commercial and military EMI standards gives little meaning to an off-the-shelf enclosure being marketed in generalities such as “military-grade” or “shielded.” These terms could mean many different things, and often, the shielded enclosures of yesteryear are no longer adequate to protect against today’s higher level of EMI threats. Not only is this true from one standard to another, but also between enclosures in compliance with different versions of the same standard.
Let’s take MIL-STD-461 as an example, because it is one of the most widely used and far-reaching EMI military standards in use today. The standard was originally published in 1967 as a way of consolidating dozens of other standards governing EMI shielding of electronics. As technology has evolved and created even more installation and technology scenarios, the standard has also evolved, going through a series of revisions from A through F, most recently coming to be known as MIL-STD-461F in 2007.
To complicate matters further, attenuation level requirements and testing procedures can vary greatly within the same version of the same standard based on where the electronics will be installed. For example, an enclosure marketed as “shielded” may meet the MIL-STD-461F requirements for a ground-based Air Force installation, but may not meet the requirements for a Navy ship installation. Furthermore, necessary testing requirements for compliance are often specified by the person procuring the enclosure, and therefore, are very application-specific. Let’s further explore the testing procedures behind MIL-STD-461F to demonstrate these points.
The goal of any shielding is to protect from one or both of two things—emitted EMI and susceptibility to outside EMI. EMI can be either conducted through materials, or it can be radiated from a source and distributed through the air. As such, the goal of MIL-STD-461 testing is to measure an enclosure’s ability to shield from all four scenarios: conducted emissions (CE), conducted susceptibility (CS), radiated emissions (RE) and radiated susceptibility (RS). MIL-STD-461F outlines several tests for each of these concerns, shown in Figure 1.
MIL-STD-461 testing involves measuring an enclosure’s ability to shield from all four scenarios: conducted emissions (CE), conducted susceptibility (CS), radiated emissions (RE) and radiated susceptibility (RS). Those tests are summarized here.
In general, these tests are conducted on an enclosure with the use of a signal generator. When testing susceptibility, the signal generator is located outside the enclosure, and measurements are taken within the enclosure to determine its shielding effectiveness from outside interference. On the other hand, when testing for emissions, the signal generator is located inside the enclosure, and measurements are taken outside the enclosure to measure its shielding effectiveness of emissions.
Not all of these tests are necessary for each installation scenario, which is why an enclosure could be in compliance with MIL-STD-461F in one installation, but if installed in another location, may not be. Figure 2 shows which installation scenarios require each of the tests. As shown, there is a fair amount of variation in the required testing procedures based on the location of installation, so it is safe to say that shielding requirements are application-specific, and thus, enclosure design should also be application-specific. Regardless of application, however, there are basic design characteristics that differentiate a standard, off-the-shelf enclosure from a custom, highly shielded enclosure. Understanding these design characteristics is important in selecting an enclosure manufacturer with adequate design and manufacturing capabilities.
The table shows which installation scenarios require whichof the tests. There’s a lot of variation in the required testing procedures based on the location of installation, so shielding requirements are basically application-specific.
Enclosure Design Considerations
Any closed box could be considered “shielded” to some extent—even a lunch box—but to achieve the level of shielding needed to meet the stringent requirements of standards such as MIL-STD-461F, an enclosure must be sealed to the extent that signal leakage is nil. The ideal EMI cabinet would be a metal box with no seams or openings, but unfortunately, that’s not possible due to the need for access to the equipment inside. Thus, a very high level of attention must be given to welding and materials selection.
The typical welding methods associated with the fabrication of an off-the-shelf enclosure—spot and arc welding—are not adequate to form a sealed EMI cabinet. These methods leave the potential for gaps where unwanted frequencies can escape or enter the enclosure. A manufacturer chosen to develop a highly shielded enclosure should have extensive welding capabilities to form a fully welded seal. This sometimes necessitates chill-blocking to maintain the structural integrity of the enclosure from the excessive heat on the frame associated with a continuous weld.
Importance of Materials
Equally important are the materials used in the enclosure. All mating materials, such as the doors and gaskets to the frame, must be connected electrically in order to maintain conductivity. An effectively designed EMI enclosure will form a Gaussian sphere, in which all the unwanted energy from EMI is absorbed and conducted throughout the materials of the enclosure, effectively shielding the outside from the inside. These can be the same materials or different, but if different conductive materials are used, they must be galvanically compatible in order to protect against galvanic corrosion and the resulting loss of conductivity.
The frame of the enclosure, along with all of the optional bolt-on components, should be plated per specification ASTM-B633, which calls for electrodeposited zinc plating with a clear chromate conversion coating. All surfaces that mate with the EMI gasket should be masked prior to painting, and a highly conductive, galvanically compatible wire mesh gasket should be used with the optional components to provide necessary EMI shielding around the access openings (Figure 3).
A highly conductive, galvanically compatible wire mesh gasket is used to provide necessary EMI shielding around the access openings.
As with testing procedures, materials selection should be application-specific, because while one material may be well-suited for a certain operating environment, it may not be for another. For instance, beryllium copper fingerstock may provide great shielding for office requirements, but would not hold up well to dust or dripping liquids in an industrial or military environment without a secondary gasket.
Custom Design Issues
More often than not, an enclosure needs to be built per scenario, to fit the exact technology, the footprint of installation and meet any necessary military or commercial standards. This generally goes far beyond EMI standards, often requiring a number of other harsh-environment considerations, including shock and vibration, corrosion protection and more.
MIL-STD-461F is merely one example of how there can be significant variations in testing requirements between one application and the next. Many other military EMI standards exist and are in use today to address threats such as electromagnetic pulses (EMP) and high-altitude electromagnetic pulses (HEMP). Standards such as MIL-STD-188 and MIL-STD-464 offer guidelines for highly shielded enclosures, and as with MIL-STD-461F, testing procedures and necessary attenuation levels differ by type of electronics and area in which they are installed.
No matter which standards must be met, selecting an enclosure manufacturer with the ability to modify existing off-the-shelf products or build completely custom solutions is the key to ensuring military electronics are adequately shielded from EMI and other harsh environmental factors.