Military EMC: A New Ball Game

William D. Kimmel and Daryl D. Gerke

With a renewed focus on military EMC, engineers are finding themselves crossing over and applying commercial technologies to military uses.

This past year has produced a major redirection in the electromagnetic compatibility (EMC) business. The aftermath of 9/11 and continuing Middle East tensions, along with the slumping commercial economy, have resulted in a significant shift of emphasis back to the military and avionics business. Military programs (such as the JSF, or Joint Strike Fighter) are now being funded. Commercial avionics security systems also are being augmented.

For years, EMC efforts were dominated by the military and a few select industries. Military electronics was the driving force in the development of integrated circuits (ICs) up until the emergence of the microprocessor in the middle seventies, when commercial electronics started taking the lead. This put computers into the home, office, and factory, which ultimately led to the promulgation of commercial EMC standards and raised a crop of EMC engineers who had no experience in military electronics. The collapse of the Soviet Bloc led to a starvation diet for military programs (and, of course, military EMC), driving many EMC engineers to seek employment in the commercial arena.

The pendulum has swung back with a jerk, but with some new twists: military electronics is now adopting and adapting commercial technology. EMC people are being pressed into service. Some who grew up in the military EMC business and moved into the commercial world are now being called back to fill a need. Others who have spent their time in commercial EMC are seeing military EMC for the first time. Both types face a new way of doing business. This article looks at how the new ball game affects both the military retread and the commercial EMC engineer.

The Military EMC Engineer

The news to the returning military EMC engineer is COTS and cost (military referring to the business area, not to the engineer's enlistment status.)

COTS, or commercial-off-the-shelf, emphasizes the use of commercial equipment to the maximum extent feasible. The pace of innovation in the commercial world is such that there is no way that military funding can keep up. No matter how much money you throw at the project, you will always be scooped by a similar commercial venture. So, the goal is to adopt commercial technology to address the military need. This results in more-contemporary equipment, less development cost, and less time to completion. Of course, there is some downside, and that is the thrust of this article.

Cost overlaps COTS to a significant extent, but not completely. Even when engineers are not using COTS equipment, they are still under significant pressure to reduce the production cost. This means making design compromises. When you take compromise steps, you run risks, and when you run risks, sometimes problems occur. That means more engineering time.

We wonder about the wisdom of this approach, which no doubt was thought of by some bright politicians (an oxymoron). Certainly, there are cases where one can get by with less than maximum EMI control and reduce the build cost. Some requirements were likely overspecified, but the fact remains that a lot of EMC techniques were developed in the military EMC world to solve common real-world EMC problems, and those problems haven't gone away.

So if engineers think these tried-and-true methods are too expensive, they will need to be inventive. The needs will still be there, so the only hope is to find cheaper techniques— if they exist—and that takes engineering dollars. This approach is followed by the consumer industry, where production quantities may be in the millions. Such high production quantities can justify $100,000 or more in development costs to shave 10 cents from the unit production cost. But the military buy never reaches those high numbers (it might reach a thousand units), so the repro cost savings may never materialize.

That doesn't mean that engineers can't effect some significant savings by drawing from commercial equipment technology. There is no doubt the new military credo is cost, and we EMC engineers are going to have to do the best we can.

It is useful to spend a little time comparing the military environment and standards with the various commercial standards. One of the fond hopes of using COTS equipment is that engineers might be able to simply buy the equipment and use it, without making any modifications and without doing any additional testing.

There are reports that commercial vehicles and laptops were effectively employed during Desert Storm, so the hope is not without merit. If the equipment is built robustly, then one easy approach is to use the equipment and see what happens. In all probability, some fixing will be necessary (though ideally only a little), but that has always been the case with our military designs.

Military Standards

MIL-STD-461E is the current document for equipment requirements. MIL-STD-464 is the main systems EMC document, replacing MIL-E-6051D and several other documents as well.

It is important to remember that MIL-STD-461E (and earlier versions) is really a set of EMC requirements, intended to serve a wide range of applications, from trucks to ships to aircraft to fixed installations, not to mention the different requirements within an application (e.g., above deck and below deck on a Navy ship). And, there is a continuing trend to tailor the requirements to particular applications. Although the most modest EMC requirements are not much different from commercial requirements, most applications are decidedly in the harsh environment category.

Another thing to remember is that MIL-STD-461E doesn't directly cover a number of EMI situations, including lightning and ESD, nor does it cover power quality. So, for the EMC requirements in a military project, a number of related requirements are added to the list, some from commercial standards. Occasionally, even some DEFSTAN (UK) or STANAG (NATO) requirements may be sprinkled in.

The following sections present a comparison of MIL-STD-461E with common commercial standards. (An in-depth comparison can be found at http://www.dsp.dla.mil, Select Library, Standardization, Opportunities, Engineering Practice Study.

MIL-STD-461E versus RTCA-DO-160D

DO-160 is the commercial avionics specification and is closest to the military standards, at least for aircraft. In a military avionics application, it is not unreasonable to hope that a commercial avionics device can be installed in a military aircraft without modification. The commercial standards are pretty tough. In fact, the lightning requirement specified in DO-160 may very likely be called out in a military avionics application.

The most obvious difference is that MIL-STD-461E requirements cover a wider frequency range than DO-160. The consequences of this are simply unknown—it must be tested. But it is known that commercial avionics equipment is not designed for high-intensity radar fields, or for EMP. There is no way to know how the equipment will be affected until it is tested. With any luck, necessary modifications are minimal.

MIL-STD-461E versus Automotive Applications

In many ways, automobiles are exposed to environments not too different from military ground vehicles. A private passenger vehicle can be expected to encounter high temperatures in places such as Death Valley, or extremely low temperatures in central Alaska. Commercial vehicles may be exposed to radio fields from powerful onboard transmitters and may carry sensitive onboard receiving devices. Accordingly, such vehicles are designed and subjected to high-level EMI in accordance with Society of Automotive Engineers (SAE) and internal corporate standards.

So, it is not unreasonable to hope that equipment designed for automobiles might be adequate for use in a military ground vehicle. As with the avionics, note that the automotive standards do not test to as wide a frequency range, notably the radar frequencies and EMP. Testing will usually uncover problems.

MIL-STD-461E versus Commercial Standards

The primary commercial standards include FCC Part 15 and the European Union requirements. The telecom standards (not commonly seen in recent years) and medical standards are a little tougher, but not notably so. With a few medical exceptions (pacemaker standards are limited, but tough, and aren't terribly relevant to military applications), the commercial requirements are all quite moderate compared with MIL-STD-461E. Equipment designed and tested to these commercial requirements is quite unlikely to work satisfactorily in military environments. In these cases, the COTS equipment must be shored up to meet the more rigorous military needs. (Note that this aspect goes beyond EMC and includes other environmental requirements, such as wide temperature range, shock, vibration, salt spray, and others.)

But there are still benefits to using COTS equipment, even when the equipment can't be used directly. The key is the ability to use commercial components and assemblies, not to mention software, whenever possible. Availability of military-grade ICs is sharply limited, so it is likely that commercial parts must be used, regardless of cost. Fortunately, the difference between military components and commercial components is not as great as it used to be, so there is a reasonable chance of finding the appropriate component. The good news is that commercial parts are much cheaper.

It is also important to consider subassemblies (an LCD, for example) that are readily available for commercial purposes, but not designed for military applications. Using such subassemblies, however, often requires finding a way to shore it up. It is good to get test data on existing equipment before starting modifications. However, it may not be worth the effort here. The equipment will probably fail miserably.

The Commercial EMC Engineer

The typical commercial EMC engineer (avionics and automotive engineers excepted to some extent) will find a major shift in emphasis when stepping into the military EMC arena. Here are some of the key differences.

First, commercial EMC is basically box oriented—the box is designed to commercial standards. In most cases, the equipment works as intended when installed in the field or, if not, you call an EMC consultant to isolate the problem and prescribe a fix.

In the military world, the equipment is one element in a complex platform, coexisting alongside other equipment, often a powerful transmitter or sensitive receiver. For all new developments, the actual needs may not be well defined in the early design stages. However, they will inevitably be much tougher than commercial requirements.

In the commercial world, engineers like to do as much at the circuit board level as possible, often eliminating the need for shielding entirely. In the military world, however, shielding and gasketing will most likely be needed no matter how well the circuit board is designed. Thus, you will find much less emphasis on circuit board design, and more emphasis on shielding, cabling, connectors, etc. Good PCB design techniques will help to moderate shielding effectiveness requirements, but not eliminate them. Extraordinary PCB design techniques are of little value. In the commercial world, requirements are mandated by law. They must be met to sell equipment.

In the military world, the requirements as initially defined are subject to negotiation. When the EMC requirements are initially levied on the supplier, they are generally pretty tough and are often simply a flow-down of platform requirements without any relaxation for criticality or shielding afforded by the platform. As the design progresses and requirements firm up, some threat requirements may be relaxed. As a matter of practicality, it is always better to start conservatively and relax the requirements later wherever possible.

Inevitably, some part of the design may need some relaxation of a requirement; perhaps vulnerability to some frequencies is too high. If those frequencies are not used in the platform or expected to be encountered in the environment, then a relaxation of requirements may be possible. This is accomplished by waivers wherein the contracting agency grants a deviation for a specific piece of equipment.

This approach does not lead to a defective product, as one might think. It merely recognizes that as the development continues, design trade-offs are made (possibly trading off a weight savings in an aircraft against a requirement that poses no real threat). After all, the goal is to get a satisfactorily functioning system, not to win a regulatory contest. It is important to remember that a waiver won't be granted if it degrades the performance of the platform.

When meeting military requirements, much greater emphasis is placed on planning and testing. Engineers may talk about EMC planning in the commercial world, but even the best efforts pale compared with those on a military project. All military projects require three companion documents to be prepared: the EMC Control Plan, the EMC Test Plan, and the EMC Test Report.

The EMC Control Plan is prepared early in the project. Its purpose is to assess the threat requirements, identify risk areas and the method of quantifying the risk (test, analysis), and design techniques to meet the requirements.

The substance of this document is presented at a preliminary design review (PDR), where project participants (contracting office, prime contractor, other subcontractors) review and offer criticism of the plan. This is not to be taken as an adversarial process. PDR is a discovery process, whereby potential problem areas are brought to the table for discussion. The idea is to determine whether anyone on the project has overlooked anything or whether there is an unexpected vulnerability with the equipment, so that timely action can be taken.

The initial EMC Control Plan is necessarily incomplete. Key information is not available at this stage, but is expected to be updated as needed to reflect emerging information.

The EMC Test Plan identifies how equipment will be tested, in a manner that best reflects the actual use. Although there are standard methods of testing, many methods contain ambiguities, and methods vary from standard to standard. The best approach is to select an optimal method and include it in the plan. This will ensure that disagreements can be resolved in a timely fashion, certainly before testing starts.

The EMC Test Report documents the results. This not only is needed for contractual reasons, but is also valuable in future applications, so that retesting may not be required.

Some final advice to commercial EMC engineers: Military product life is measured in tens of years, not months. Parts availability must be considered accordingly.

Conclusion

Military EMC is a new ball game both for old-timers returning to the business and for commercial EMC engineers being pressed (dare we say drafted?) into service. The new twist for the old-timer is COTS and cost. The new approach for the commercial EMC engineer is the emphasis on systems rather than circuit boards.

William D. Kimmel, PE, and Daryl D. Gerke, PE, are cofounders of the engineering consulting firm Kimmel Gerke Associates Ltd., with offices in St. Paul, MN, and Phoenix, AZ. They share more than 60 years in the EMC arena and publish and lecture widely on the subject. They can be reached at 888-EMI-GURU or at http://www.emiguru.com. They can also be contacted by e-mail at bkimmel@emiguru.com or dgerke@emiguru.com.