Military EMC design can be particularly vexing. Multiple environments combined with multiple threats lead to multiple requirements. The threat levels, and the resulting requirements, are usually more stringent than found in the commercial world.
As a result, commercial design techniques are often woefully inadequate for military applications. This can lead to frustration for those moving into military EMC from other areas. It can also lead to frustration to those wishing to use COTS (commercial off the shelf) equipment in military environments.
In this article, we’ll explore some of the unique EMC challenges presented by military electronics, and how they differ from those of the commercial world.
Multiple Environments with Multiple Threats
Unlike commercial equipment, military systems may need to work in a wide range of environments. These can range from the arctic to the desert, and from the bottom of the ocean to outer space. Fortunately, most systems only need to operate in selected environments, rather than in every potential situation. This leads to subsets of requirements, and even tailoring in select cases.
Furthermore, military systems are often subjected to multiple threats. These threats are typically more severe than in commercial environments. Here are some examples of five general environments and their associated threats, and how they contrast with nonmilitary environments.
Fixed Land Based – This environment includes residential and office buildings. For commercial electronics, these are considered relatively benign in terms of EMC. As an aside, this is the primary EMC environment for most commercial electronics.
The emissions concerns are moderate, and are aimed at protecting nearby television receivers. The susceptibility concerns are a bit more challenging, and include threats such as RF (radio frequency) energy from nearby hand held radio transmitters, human ESD (electrostatic discharge), and power disturbances such as lightning or EFT (electrical fast transients.)
These same buildings on a military base, however, may pose much more severe conditions, particularly for radiated emissions and susceptibility. Both field levels and frequency ranges can be much higher than commercial environments. Due to radar systems, those frequencies can extend to 40 GHz or more, well above the typical 1-5GHz upper limits for commercial equipment. Also, many military systems are designed to include protection against EMP (electromagnetic pulse) effects from nuclear weapons, which adds another level of complexity.
As such, commercial emissions requirements may not be adequate to protect nearby military communications receivers, which can be much more sensitive that a television receiver. Commercial susceptibility requirements may also be inadequate, due to radio and radar transmitters with higher radiated field levels, and EMP. The little bit of good news is that commercial levels for ESD and power disturbances are often still adequate.
Mobile Land Based – These environments include cars, trucks, buses, etc. Even for commercial vehicular electronics, these can be quite harsh. The emissions concerns are severe, and usually aimed at protecting entertainment radios (AM/FM),
with secondary concerns for protecting land mobile VHF/UHF radios. The susceptibility concerns are also severe, and include RF, ESD, and a range of power transients and other power disturbances unique to vehicles.
Military vehicles share these same concerns, but as with fixed systems, the frequencies and amplitudes may be well above commercial levels. Nevertheless, commercial vehicular electronics can be expected to do fairly well in military environments, but may need some additional protection for radar and EMP.
Due to their experience working with harsh environments, we’ve found that commercial vehicular EMC engineers often have a relatively easy time making the transition to military electronics.
Marine Based – These environments include large surface ships, submarines, and even smaller water craft. Ships with metal hulls have vastly different EMC concerns depending on whether the equipment is located above deck (outside) or below deck (inside.)
For both the military and commercial environment, emissions concerns are severe and are aimed at protecting communications and navigation receivers, including radar. Susceptibility concerns are also severe, and include RF and power disturbances. Since most military ships have multiple communications and radar transmitters, the levels and frequencies can be much higher than for commercial ships.
A classic tale of military EMC at sea was the sinking of the HMS Sheffield in the Falkland Islands War in 1982. It turns out there was a compatibility problem between the satellite communications and a defensive radar system. The “solution” was to disable the radar when communicating via satellite. Unfortunately, the launch of an enemy missile went undetected during one of these radar blackouts, and the ship was lost due to an EMC problem.
One bit of good news is that ESD is usually not a big concern for marine applications, due to high humidity conditions. A notable exception is helicopter ESD, which has resulted in special requirements for both helicopters and electronics equipment (and ordnance) that might be located near a helicopter landing pad. Lightning and EMP, of course, are major concerns for all military naval vessels.
Air based – These environments include all aircraft, and include small aircraft, helicopters, fighters, bombers, and more. Like ships, EMC concerns vary depending on whether the electronics are located inside or outside the aircraft. An emerging concern is the use of composite material rather than aluminum, which can affect overall shielding performance.
The commercial and military EMC environments are actually quite similar. In fact, the predominant commercial avionics requirements (RTCA DO-160) are derived from the military requirements (MIL-STD-461). The commercial requirements are even a bit more comprehensive, and include very specific lightning and power quality requirements.
Additional military concerns include HIRF (high intensity RF) and EMP. The former can come from radar exposure which may be quite high in a tactical situation, or as a weapons effect. ESD is also a big concern, particularly for helicopters transporting materials or munitions.
Magnetic field emissions are a unique concern for antisubmarine warfare (ASW) aircraft. One way of locating submarines is to look for low level magnetic field perturbations. The sub hunters need to maintain clean electronic environments so they can detect the perturbations.
Space – This is probably the most unique and varied of military environments. There has been very little commercial space electronics, although this may be starting to change. Nevertheless, we expect to see the commercial space designers closely follow military design practices.
Due to the expense of launching hardware into space, the EMC requirements are often highly tailored. Extensive engineering efforts are made to optimize (and not over design) for EMC. Extensive testing is performed to assure EMC is achieved. After all, if something doesn’t work, it is almost impossible to fix (the Hubble telescope being one very expensive exception.)
Space electronics are subjected to several environments that must be considered. For example, during pre-launch, precautions must be taken to prevent damage due to human ESD. During launch, precautions must be taken to prevent damage due to triboelectric charging and also due to high RF levels from tracking radar, etc. In a tactical situation, the RF may also include antimissile efforts. Once on-orbit, space electronics are subjected to “space charging,” and also cumulative degradation from ionizing radiation present in space.
Another unique space requirement is “magnetic cleanliness.” This is often a requirement for satellites that employ magnetometers for navigation. Even small magnetic fields, from either permanent magnetization or from power electronics, can interfere with the on orbit navigation. Of course, nuclear weapons effects (such as EMP and ionizing radiation) are a also a major concern for military space electronics.
Military EMC Requirements
These various environments and threats have resulted in specific EMC requirements. Although these have evolved over the years, we now have two major military EMC requirements, MIL-STD-461 and MIL-STD-464.
MIL-STD-461 is applied at the module (box) level. The current revision level is MIL-STD-461F, and should be applied to new procurements. Existing equipment may use earlier versions, so it is important to be sure you are using the on-correct version when dealing with updates or legacy systems.
MIL-STD-461F provides both recommended test levels and the test procedures for a number of different tests. These are divided into four broad categories:
CE – Conducted Emissions
CS – Conducted Susceptibility
RE – Radiated Emissions
RS – Radiated Susceptibility
These are further subdivided into specific tests, with a three number designator, such as RE101. As an aside, older versions of MIL-STD-461 (A,B, and C) used the same nomenclature but with two number designators, such as CS06. This distinction is important, as legacy systems may still be using the older versions of MIL-STD-461 for qualification purposes. For more details, see MIL-STD-461F, Table IV.
Source: MIL-STD-461F
Note that not all tests are required for all equipment. Rather, different tests and different levels are recommended for various situations. These recommendations are based on anticipated environments and threats. For more details, see MIL-STD-461F, Table V.
Source: MIL-STD-461F
Note that requirements may vary among the different services for similar equipment. For example, the electric field radiated emissions (RE102) differ for Army, Air Force, and some Navy aircraft. Since Air Force and most Navy aircraft rarely use radios below the 2 MHz, they have no recommended requirements at the lower frequencies, while the Army goes down to 10 kHz.
Special cases may deserve special attention. For example, Navy aircraft used for antisubmarine warfare extend their electric field emissions (RE102) down to 10 kHz. They also include magnetic field emission requirements (RE101) that are not recommended for other Navy aircraft. The reason is that hunting for submarines often means detecting low level magnetic fields at low frequencies. In order to detect these fields, the local environment must be clean at those low frequencies.
There are two important philosophical differences between MIL-STD-461 and commercial requirements. First, MIL-STD-461 can be tailored as needed. Second, test failures can be waived. Of course, both require the customer to agree. We feel both of these options should be considered as needed, as they often yield good EMC systems engineering solutions.
One caveat on MIL-STD-461. It is not a guarantee of ultimate EMC, but rather it increases the overall probability of success. You still need to plug everything together and see if it works.
MIL-STD-464, the second common EMC requirement, is applied at the systems or platform level. This document supersedes a number of older documents, and addresses grounding, bonding, lightning, EMP, HIRF, and more. Since this requirement applies to the platform level, it is often of secondary concern to the box/module designer.
Unlike MIL-STD-461, the actual test methods are not well defined in MIL-STD-464. This makes sense, as these are platform requirements, and platforms can vary widely. But as a result, these requirements can be difficult if not impossible to validate at the box level.
In spite of the system emphasis, we have seen increasing attempts by the platform designers to “flow down” their system requirements to the box designer. Since systems level testing is not appropriate at the box level, the result is often a request for engineering analysis. This is certainly prudent early in the design, but should not be a substitute for testing later at the full system/platform level.
Design Solutions – Systems Engineering over Circuit Boards
This is an area where commercial and military systems differ in their EMC approaches. Most commercial designs focus on circuit board design, and then apply shielding as needed. Military systems, however, take the opposite approach, emphasizing shielding (and other systems design issues) over the circuit boards.
We’ve seen this subtle difference cause frustration for designers moving from commercial to military electronics. We recall one young EMC engineer who was questioning why his new company even hired him. As he said, “All they worry about here is grounding, shielding, and cables. They aren’t even using my circuit board experience.” He felt much better after we assured him that his EMC experience was indeed very valuable – only the focus was different.
Most military systems are already in metal enclosures. Thus, shielding becomes a key EMC design approach. Furthermore, many military systems use embedded controllers, and don’t need the latest and greatest speeds and raw performance. As a result, there is more emphasis on systems design, and less on circuit board design. (We still recommend good EMC circuit board design practices for military electronics.)
The systems design solutions often revolve around interfaces. These include the following:
Power – This is an energy interface. Design protection of this interface typically combines passive circuits (filters and transient protection) with active power supply circuits. The goal is to provide clean regulated output power under varying input conditions. Since the bandwidth for power is low, the input power wiring is often unshielded.
Signal – This is an information interface. Design protection of this interface typically includes a combination of passive circuits (filters and transient protection) with active I/O circuit design. Due to bandwidth requirements, filtering is often traded off with external cable shielding or even fiber optics. Thus, cables and connectors also become an important part of this interface, along with the specific I/O circuits.
Grounding – This is primarily a safety interface, but it also affects the power and signal interfaces. The primary strategy here is topology control. Single point grounds are preferred for low frequency circuits, such as analog sensors and input power. Multi-point grounds are preferred for high frequency circuits, such as digital and RF circuits. Hybrid grounding approaches (using capacitors and inductors to make grounding paths and connections frequency dependent) are often used when both types of circuits or threats are present.
Shielding – This is an electromagnetic field interface. This is usually bi-directional, and designed to contain internal electromagnetic fields (emissions) while providing protection against external electromagnetic fields (susceptibility.) Design strategies include metallic enclosures, and then sealing any penetrations or discontinuities with gasket, screening, and filters.
In addition to interfaces, risk management is an important aspect for EMC systems design. This is accomplished
several ways:
Design reviews – Most military programs follow a detailed design procedure that includes formal design reviews at critical junctures. Additional design checkpoints may also be employed. We often recommend dedicated EMC reviews. These can be brief, yet can be helpful in uncovering potential EMC problems early in the design process.
Engineering tests and analysis – Many military programs depend on test and analysis throughout the design process to validate design approaches. We certainly encourage this.
Documentation – Most military programs have mandatory documentation requirements. These typically include an EMC Control Plan, and EMC Test Plan, and an EMC Test Report. All three are used to document the process, and as communications tools between the contractor and customers. Yes, we know that most engineers don’t like documentation, but this is a very important part of the EMC systems design process.
Mission Success Trumps Cost
All this design effort, analysis, test, and documentation costs money, which can lead to complaints about $100 hammers or $400 toilet seats. In spite of carping by politicians, the extra costs are usually justified. Furthermore, since most military systems have relatively low volumes, there are fewer units over which to amortize the extra engineering and test costs.
Military equipment must operate as designed and when needed. Reliability is crucial. For example, you can’t power down or push the reset button on a missile or torpedo after it has been launched. Furthermore, you don’t want them turning around and coming back home.
The true bottom line is not cost, but mission success. Remember, lives are often at stake. Our servicemen and women who go in harm’s way deserve the absolute best engineering we can deliver – EMC and otherwise!
Conclusions
- Military EMC is different from commercial EMC. There are multiple environments to consider, with multiple threats. Those are usually much more severe than commercial threats.
- Complex military systems require systems engineering approach. The focus is often on interfaces, rather than on circuit boards. Design reviews and documentation are critical to keep everyone in the loop and on schedule.
- Mission success trumps costs, and reliability is key.
Bibliography
Daryl Gerke and William Kimmel,”Focus on Military EMC,” Kimmel Gerke Bullets, Spring 2002.
Daryl Gerke and William Kimmel, “Military EMC and the Revival of EMC Systems Engineering,” Interference Technology, 2003 Annual Guide.
William Kimmel and Daryl Gerke, “Military EMC: A New Ball Game,” Compliance Engineering, 2003 Annual Guide.
Daryl Gerke and William Kimmel, “EMC in Space,” Interference Technology, 2005 Annual Guide.
William Kimmel and Daryl Gerke, “Addressing EMC in Harsh Environments,” Compliance Engineering, 2005 Annual Guide.
About the Authors
Daryl Gerke, PE and William (Bill) Kimmel, PE, are principals and cofounders of Kimmel Gerke Associates, Ltd., an EMC consulting and training firm. They share over 80 years of industry experience and have served clients in a wide range of industries (military, medical, commercial, industrial, vehicular, avionics, telecommunications, and more.) They have trained over 10,000 engineers and technicians through their public and private EMC seminars. They have also written three books on EMC, and over 100 technical articles.
Daryl and Bill got started in military EMC over 40 years ago, and have been full time EMC consulting engineers since 1987. Daryl lives in Mesa, AZ, and can be reached at dgerke@emiguru.com. Bill lives in South St. Paul, MN, and can be reached at bkimmel@emiguru.com. Their newly updated web site, www.emiguru.com, contains numerous EMC resources, including a 20 years archive of their popular free EMC newsletter, Kimmel Gerke Bullets.
