New Test Methods to Determine the Shielding Effectiveness of Small Enclosures Defined in IEEE P299.1

Today’s end-use electronic equipment has a number of characteristics that require protection from the electromagnetic environment. These characteristics include the growing use of digital electronics (still with a layer of analog electronics); multiple inputs and outputs for power, data, controls and indicators; ventilation for air flow and thermal management; and small openings for accessories. Few pieces of equipment use only one microprocessor. Multiple digital packages (i.e., integrated circuits) are used for small and large amounts of memory, signal processing, and input/output control just to name a few. The days of having just one power cord and a few knobs for control have long since past.

A piece of consumer electronic equipment such as a DVD player has an average of 27 penetrations in its case. On the industrial side, a piece of instrumentation and control (I&C) equipment used in a power plant has an average of 38 penetrations. Components used on the surface of a metallic equipment enclosure such as a liquid crystal display (LCD) screen require fairly large penetrations. Universal serial bus (USB) connectors and Ethernet ports are two examples of input/output ports that are being used much more frequently today than just several years ago. While some equipment is getting more efficient and generating less heat, other types of equipment generate significant heat requiring increased air flow across the electronics.

Each of the above surface components requires a penetration, or aperture. The electronics and subcomponents inside equipment generate radiated emissions made up of electric and magnetic fields with not only low, mid, and high frequencies up to 1 GHz, but also frequencies above 1 GHz. From electromagnetic theory, we know that some of these fields will propagate through these apertures. Emissions that escape an enclosure add to the cluttering and energetic nature of the electromagnetic environment. Some emissions that escape will be the cause of EMI problems. The use of apertures in equipment enclosures degrades the shielding effectiveness that enclosures with no apertures can provide.

Electromagnetic compatibility (EMC) standards in place today specify various levels of emissions control based on product type, application, and frequency among other considerations. To maintain control over the emissions that do escape an equipment enclosure, the equipment designer must be able to determine just how much the shielding effectiveness is degraded by the presence and characteristics of the apertures. Components such as switches and indicators used on the surface of an enclosure can be fitted with EMC gaskets — a common practice used today. EMC gaskets are specially designed and manufactured materials that can provide some level of shielding. Gaskets essentially ‘seal up’ the small spaces between a surface component and the plane of the enclosure. If an aperture intended for the flow of heat does not require some type of EMC-grade air filter for dust control, then gaskets are not used. Most standard apertures intended for air flow do not use filters. The sizes of these apertures are bounded by the amount of air flow and heat that must pass through them. Thus, the equipment designer cannot size them small enough to limit the level of emissions escaping from the enclosure and still maintain the required air flow to maintain specific operating temperatures in various ambient environments.

The comprised shielding effectiveness for enclosures larger than two meters when apertures are used can easily be measured using test methods defined in IEEE Standard 299, Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures, first published in 1997 and then revised in 2005. Many enclosures larger than two meters are the large shielded rooms that EMC test houses commonly use. There are some other industry-specific enclosures larger than two meters where the IEEE 299 standard can be applied. Examples of these include medical imaging suites where magnetic resonance imaging (MRI) systems are used. (Patients typically don’t see the enclosures as they are behind sheet rock walls to provide a pleasing imaging suite.) Because the definition of shielding effectiveness is straightforward, the application of IEEE 299 is not a problem for these large enclosures. However, when the methods of IEEE 299 are applied to enclosures with dimensions smaller than two meters, problems arise in defining and measuring shielding effectiveness.

For enclosures smaller than 2 meters, applying IEEE 299 test and measurement methods will lead to erroneous shielding effectiveness numbers. Manufacturers of small shields with dimensions less than 2 meters who apply IEEE 299 to determine shielding effectiveness versus frequency will end up with misleading results. Some manufacturers do apply IEEE 299 to small enclosures and publish misleading results. Thus, one can see that there is a need to provide new test methods allowing manufacturers to accurately measure the shielding effectiveness of small enclosures.

The IEEE P299.1 draft standard, Draft Full-Use Std. Method for Measuring the Shielding Effectiveness of Enclosures and Boxes Having All Dimensions between 0.1 m and 2 m, addresses the measurement of shielding effectiveness for enclosures between the dimensions of 0.1 to 2 meters. A lower boundary of 0.1 meters was set for this standard as the new test methods are acceptable down to this dimension. In most cases, the test methods presented in this standard will be applied to square or rectangular enclosures. The development of this draft standard is a project within the IEEE Electromagnetic Compatibility Society.

Enclosures used in equipment like computers and other electronic equipment offer a shielding effectiveness ranging from low to medium values. To determine the shielding effectiveness of enclosures with dimensions between 0.1 and 2 meters, one must distinguish between physically small but electrically large enclosures and those that are both physically and electrically small. The former successfully allows a reverberation chamber (RC) method to be applied for SE evaluation by means of the frequency stirring technique. For the latter case, the traditional SE definition is hard to apply because of the undermoded condition, the strong dependence of the internal field on probe positioning and orientation, and the dependence on the incoming field polarization. The frequency stirring technique was pioneered by EMC researchers at the National Institute of Standards and Technology (NIST). Other organizations such as Sapienza Univ. di Roma in Rome, Italy and the University of York in York, United Kingdom were also instrumental in developing the new test methods.

Application of the IEEE P299.1 Standard

Figure 1 illustrates a view of an enclosure with overall dimensions between 0.1 and 2 meters. This enclosure is used for a computer power supply on the market today. The actual dimensions of this enclosure are 0.15 meters in width, 0.14 meters in depth, and 0.084 meters in height. These dimensions fit within the requirements of IEEE P299.1. Measuring the shielding effectiveness of computer power supplies is important to the overall EMC performance of the computer. Emissions generated inside the enclosure must be kept under control while emissions generated by the computer hardware must be kept from causing power supply upsets. From the figure, one can see that there are a number of slots in the metal forming the enclosure. In fact, it is possible to notice apertures for the cooling fan, a multiple rectangular aperture for the power plug, a squared hole for switching cables, and a series of slots for heat transfer. These slots and apertures will degrade the shielding effectiveness of an enclosure of this size with no penetrations. This type of enclosure is exactly the enclosure size that is applicable to the new IEEE P299.1 standard when trying its shielding effectiveness.

Figure 1: Metallic enclosure for a computer power supply (dimensions are in centimeters).

There are millions of applications of the new test methods defined in the IEEE P299.1 draft standard. Use of these new test methods will help designers and manufacturers of metallic enclosures with dimension 0.1 to 2 meters better understand the shielding effectiveness of their enclosures and how they perform to provide shielding when used in end-use equipment.

Overview of the IEEE P299.1 Standard

The IEEE P299.1 draft standard is divided up into seven chapters and twelve annexes. Listed below are the chapter titles and a brief discussion of their contents:

• Chapter 1: Overview – This chapter presents the scope and purpose of the IEEE P299.1 along with discussion on application of test methods and use of the IEEE P299.1.
• Chapter 2: Definitions – This chapter presents some definitions of particular use and interest to those interested in learning about terminology used in applying the new test methods for determining the shielding effectiveness of enclosures with dimensions between 0.1 and 2 meters.
• Chapter 3: Preparing for shielding effectiveness measurements – preliminary procedures – This chapter is designed to provide technical assistance to the user on various subjects related to shielding effectiveness and the use of the new test methods. Subjects such as the test plan, calibration, reference level, dynamic range, preliminary procedures for checking a shield under measurement, reverberation qualification, pass/fail requirements, and usable frequency ranges and limits are provided. Some of these technical subjects apply to one of the two test methods or both.
• Chapter 4: Measurement instrumentation – This chapter provides guidance on what measurement instruments to consider in using the two new methods.
• Chapter 5: Measurement uncertainty – This chapter provides some guidance on measurement uncertainty when measuring shielding effectiveness. Measurement uncertainty is a parameter that can be associated with the result of a measurement of shielding effectiveness. It characterizes the dispersion of values that could reasonably be attributed to the measurements. There are many aspects of shielding effectiveness where measurement uncertainty can be estimated to gain the overall expanded measurement uncertainty of the shielding effectiveness process contained in the IEEE P299.1.
• Chapter 6 – Test procedures – This chapter presents the two new test methods. Part I deals with enclosures 0.75 to 2 meters. Part II deals with enclosures that are physically small and electrically large. Manufacturers and designers of enclosures that fit these categories will want to purchase the new standard once it is available and review these parts.
• Chapter 7 – Qualify assurance technical report – This chapter presents the format for developing a technical report when using either Part I or Part II (new test methods) of the IEEE P299.1. The investigator has the choice of developing an abbreviated test report or a full test report.

To supplement the two new test methods presented in the main body of the IEEE P299.1, twelve annexes are included. Listed below are the annex titles and a brief discussion of their contents:

• Annex A (informative) – Bibliography – Annex A presents a list of all the technical papers used in developing the IEEE P299.1. Users may desire to refer to them for more detailed information and when interested in applying these new test methods to irregularly-shaped enclosures.
• Annex B (informative) – Rational (for Part I – 0.75 to 2 meter enclosures) – Annex B presents the basis for this new test method, considerations pertinent to the objectives, cavity resonances, measurement locations, and measurement equipment.
• Annex C (informative) – Mathematical formulas (for Part I – 0.75 to 2 meter enclosures) – Annex C presents specific mathematical formulations, low range (50 Hz to 200 MHz) shielding effectiveness, and high range (300 MHz to 100 GHz) shielding effectiveness, non-linear (logarithmic) calculations, and dynamic range considerations.
• Annex D (normative) – Miscellaneous supporting information (for Part I – 0.75 to 2 meter enclosures) – Annex D presents discussion on coplanar versus coaxial loops, non-linearity of high-permeability ferromagnetic enclosures, and selecting measurement frequencies.
• Annex E (informative) – Guidelines for the selection of measurement techniques (for Part I – 0.75 to 2 meter enclosures) – Annex E presents discussion on types of enclosures, performance requirements, equipment requirements, and regulatory agency conflicts.
• Annex F (informative) – Preliminary measurements and repairs (for Part I – 0.75 to 2 meter enclosures) – Annex F presents discussion on background related to this subject, frequencies for preliminary checks, and preliminary check procedures.
• Annex G (informative) – Rationale for wall-mounted monopoles – Annex G presents some technical discussion and understanding of wall-mounted monopoles that are used in carrying out the new test methods.
• Annex H (informative) – Impedance mismatchcorrection – Annex H presents discussion on impedance mismatch issues that can be encountered when using antennas with the new test methods.
• Annex I (informative) – Using isolated monopoles in outer reverberation chambers – Annex I presents discussion on the use of isolated monopoles when using reverberation chambers to carryout the new test methods.
• Annex J (informative) – Measurement the shielding effectiveness of physically small and electrically small enclosures using magnetic field measurements (≤ 300 MHz) – Annex J presents technical discussion on determining the shielding effectiveness for physically small and electrically small enclosures. This annex is provided should this need arise with shielded enclosures. Test methods for use in determining the shielding effectiveness for enclosures that are physically and electrically small are still under investigation by EMC researchers who study shielding effectiveness and enclosures. This material may spawn the development of further new test methods in future revisions of IEEE 299.1 (when approved).
• Annex K (informative) – Electrically small enclosures in reverberation chambers – Annex K presents technical discussion on the background, measurement procedure, formula to be applied, and internal probe type and positioning when setting out to measure the shielding effectiveness of electrically small enclosures using reverberation chambers.
• Annex L (informative) – Utilization of absorbing (dissipative) materials in equipment enclosures for the measurement of shielding properties – Annex L presents technical discussion on the use of absorbing materials in enclosures when setting out to apply the new test methods to determine shielding effectiveness.

Upcoming Balloting of the IEEE P299.1

The IEEE P299.1 document has been completed and is ready for balloting in 2011. Once it has been balloted and approved by IEEE, then it will be available for purchase from the IEEE.

Volunteering for IEEE

IEEE depends upon many volunteers to provide the many services, such as the development of new standards, it offers to its members. Volunteering for work on new standards or the revision of existing standards is just one important role that volunteers may play. Volunteers have the opportunity to meet new people and learn about new developments in the technical community, such as how the shielding effectiveness of small enclosures can be achieved with new test methods. Serving as editor of this new standard has been a challenging but rewarding experience. The author of this paper encourages all IEEE members, especially those of the new generation, to take part in IEEE activities such as standards development. When a project is completed, you’ll find out that it was well worth the time spent.