Choosing the right shielded enclosure need not be a difficult procedure if you apply some of the methodical skills you learned in engineering school. For starters, deduce which enclosure material and design provide the appropriate shielding level. Some steps to take are outlined by Al Domeshek, engineering manager at Optima Electronic Packaging Systems:
Verify attenuation levels required at the frequencies of interest. Since the enclosure is passive, the internal and external environments determine the levels of EMI/RFI emissions and the frequencies at which they need to be attenuated.
Evaluate the design. Understand the construction details and shielding provisions to determine what works.
Appraise the limitations of existing enclosure designs and determine if improvements are possible with gaskets or heavier materials such as steel instead of aluminum.
Consider the need for access and cable entry to minimize enclosure openings.
Determine the impact of cost, weight, shielding deterioration, and environment on the performance and heat dissipation of the enclosure material.
The ideal shielded cabinet is a metal box with no seams or openings because any opening creates a potential EMI/RFI path, said Dan Estes at Emcor Products. This, however, is not a realistic expectation because openings are needed for maintenance, ventilation of equipment, and electrical connections. In addition, no single enclosure works in all situations, and tests performed on different equipment using the same enclosure will not pass automatically.
Meeting the requirements of an EMI shielding standard depends on the operating frequency of the equipment, its susceptibility to EMI/RFI signals, the amount of shielding on the equipment, and its power source, said Mr. Estes. The enclosure design you select will be dictated in part by the level of shielding needed. The shielding effectiveness (SE) of the enclosure is determined by its material, the construction of the frame and panel-joint design. You can verify the SE by performing tests to assess an enclosure’s suitability.
SE is measured by establishing the reference intensity of an incident magnetic, electric, or plane-wave electromagnetic field, said George Ross, director of marketing at Vero Electronics. The test is done in a screen room by transmitting RF energy to a radio receiver and measuring the field intensity at the receiver antenna terminals.
The transmitting antenna is placed in the equipment under test, and the field strength is measured. The SE is defined as the ratio of electric or magnetic field strengths before and after the placement of the shield around the equipment under test:
SE = 20 log(Fb/Fa)
where: Fb = the field strength before shielding.
Fa = the field strength after shielding.
The field-strength values can be the intensity of a high-impedance electric field (E) measured in volts-per-meter, the intensity of a low-impedance magnetic field (H) measured in amperes-per-meter, or the intensity of a plane wave. The ratio of E and H components in an electromagnetic wave is known as the wave impedance. The magnitude of wave impedance affects how the interference signal is attenuated at the shield.
The E-field wave impedance diminishes exponentially from the source of radiation to a distance of approximately 1/6th of a wavelength from the source. The H-field wave impedance increases over the same distance. At more than 1/6th of a wavelength, the wave impedance remains constant at 377 W , the characteristic impedance of free space.
Reflection and absorption of a signal limit the transmission of electromagnetic waves through a shield, said Vero’s Mr. Ross. In the E-field, attenuation by reflection improves with conductivity. It is adversely affected by increases in frequency, permeability, and, within the near field, distance from the signal.
In the H-field, increasing conductivity, frequency, and distance from the source are beneficial. Decreasing the permeability improves the attenuation.
For a plane wave, the more conductive the material, the better the attenuation. Reductions in frequency and permeability also can improve attenuation.
Attenuation by absorption, said Mr. Ross, is improved as conductivity, permeability, material thickness, and frequency increase. For conductive materials ³ 1 mm-thick and frequencies above 1 kHz, attenuation due to material properties can exceed 200 dB. For very thin shields, such as conductive coatings on plastic enclosures, attenuation is inherently low. Therefore, material properties become an important part of the equation to attain the desired shielding.
Once you know what level of shielding is required, review the available enclosure materials. Although a variety of materials such as stainless steel, aluminum, and plastics are used, cold rolled steel generally is considered the most cost-effective, said Emcor’s Mr. Estes. Whichever material is used, it should be thick enough to provide adequate absorption as well as assist in maintaining the structural rigidity of the cabinet.
The level of shielding that suits the application should be considered at the component, board, system, and multiple-system levels for the best results, said Mr. Ross of Vero.
At the component level, consider using metal screens to enclose devices such as inductors, transformers, and small modules such as switching power supplies, continued Mr. Ross. For board-level protection, think about enclosing the board in a screening module or in a treated small plastic enclosure. These modules can house individual plug-in boards and offer a high level of attenuation, and they can screen particularly noisy or sensitive boards. If you use these modules, make sure that you use connectors that maintain the shielding properties of the screen.
Enclosing multiple boards at the system level is achieved by using subracks, desk-mounting enclosures, and treated, plastic enclosures, said Mr. Ross. Subracks can be retrofitted with EMC covers in ventilated or unventilated forms. Desktop versions also are available in ventilated and unventilated versions and include a wraparound cover. The treated cases offer a high level of attenuation.
Enclosures for multiple systems requires that they be individually shielded in large, screened cabinets, continued Mr. Ross. This method offers good EMC performance, and many manufacturers supply fully assembled enclosures with an extensive range of sizes, accessories, and options. EMC performance is maintained with finger stock around the doors and covers.
Also ensure that your enclosure has a rigid structure. An enclosure that does not have structural rigidity will not maintain flat surfaces between the frame and its component parts, continued Mr. Estes of Emcor. It makes it more difficult to seal and more susceptible to leaks.
To enhance the stiffness, the frame should be constructed of tubing or multiformed frame channels. The corner posts should be fully welded to the top and bottom pieces. Welded joints strengthen the frame and minimize the areas of potential EMI/RFI leakage.
Rigidity and flatness of mating surfaces, such as panels or doors with the frame, also must be addressed. These best surfaces are those using heavier gauge materials in addition to the multiformed corner posts. For the best shielding performance, a conductive surface is needed on these mating surfaces. The surfaces also must be galvanically compatible to prevent corrosion.
The attachment methods of the enclosure panels and doors are important for attenuating radiated signals, said Mr. Estes. Using fasteners to attach the closure panels will ensure that gasket compression is adequate to seal the opening.
When you become familiar with the available materials and know your shielding-level requirements, then review design techniques and construction principles with your chosen manufacturer, said Mr. Ross of Vero. Seek more information from your manufacturer about other details such as slots, ventilation, grilles, honeycomb vent materials, and windows.
Slots or apertures such as doors, panel joints, and ventilation grilles can limit the actual shielding level to well below the achievable shielding level of the enclosure material. The shielding effect on a slot in an enclosure can vary according to its length, cross-sectional area, and orientation. It can interrupt the current flow by setting up electric charges along the slot edges and becoming a waveguide slot antenna.
The slot size should be kept to the smallest opening in all directions to minimize electric charges, said Mr. Ross. The higher the frequency encountered, the smaller the slot size should be.
Allowing for ventilation and heat exchange through convection requires an opening in the shield, said Kris Holla, vice president at Elma Electronic. Usually, the opening is a panel with slots. The optimum opening is circular because it keeps the critical dimension, the length, to a minimum and therefore provides better attenuation than long thin openings.
The openings should be kept shorter than 1/20th of a wavelength at the highest frequency of concern, to achieve approximately 20 dB of attenuation, said Mr. Holla. For example, if your equipment has frequencies in the 300 MHz range, the slots should be kept shorter than 2″ in length.
A vent made out of honeycomb material provides a good dimensional aspect ratio and satisfactory attenuation properties. It is made from very thin sheet metal and offers excellent ventilation characteristics. Honeycomb material does have its disadvantages, including high cost, fragility, and bulkiness.
Windows are used in enclosures for shielding and viewing. Some window constructions may have a metallic wire mesh or a thin, perforated sheet bonded between transparent sheets of glass, acrylic, or polycarbonate. These materials are rigid and heavy, prone to shattering, and reduce the transparency of the window.
Another type of window construction uses a conductive coating of vacuum-deposited gold or indium oxide and provides good transparency. It has lower attenuation than the wire mesh or perforated sheets.
After you have gathered all the data regarding the material, construction, and shielding, you can judge which enclosure is best for your application.
Shielded Enclosures
Enclosure Provides
68-dB Shielding @ 100 MHz
The Type 15 Smart Enclosure Series is designed for tabletop and rack-mount use. Shielding effectiveness is 46 dB @ 100 MHz with an option kit extending it to 68 dB @ 100 MHz. The enclosure ranges in size from 3.5″ to 7.0″ high, 9.3″ to 17.7″ wide, and 9.6″ to 15.6″ deep. Elma Electronic, (510) 656-3400.
Modular Enclosure Meets
Military Shielding Levels
The Emission Control Plus line of enclosures provides the level of shielding attenuation required by the military. Tests performed at an FCC-recognized and Tempest-capable facility meet the requirements of MIL-STD-285. The line features a rigid 12-gauge frame, a patented door and latching system, and modular options. The enclosure is nickel-plated with beryllium-copper spring-finger gasketing. Shielded doors are available in plain or honeycomb-filtered styles. Emcor Products, (507) 289-3371.
Shielded Cabinets Provide
50-dB Attenuation @ 1 GHz
With the addition of a metallizing process and EMI gaskets to limit emissions and interference, the FCC/VDE Line of shielded enclosures meets the European Union EMC requirements. The enclosures provide 50-dB attenuation @ 1 GHz. Built to the specifications of the company’s cold-rolled steel Heavy-Duty Line, the FCC/VDE model holds more than 3,000 lb of equipment. It also meets requirements of MIL-STD 810 for shock and vibration and MIL-STD 901 for shock. The cabinet is available in panel widths of 19″, 24″, and 30″ and 17″, 24″, 29″, and 36″ deep. It is offered in 10 heights with vertical and sloped front consoles. Equipto Electronics, (800) 204-RACK.
Modular Chassis Meets
Basic to MIL-STD Requirements
Primus is the 19″ modular chassis suitable for applications ranging from basic EMI/RFI needs to MIL-STD 810E shockproof requirements. Retrofitable chassis components are available to meet changing EMI/RFI shielding needs. The system includes an enclosure, a backplane, a power supply, a cooling apparatus, cabling, and shielding. Knurr USA, (805) 526-7733.
Anechoic Chamber Helps Meet
EU, ANSI and IEC Requirements
The FACT 5 Free-Space Anechoic Chamber Test Site helps perform on-site full-compliance EMI testing at 3 m and 10 m. The chamber dimensions are 59’ L × 39’ D × 26’ H. It meets ANSI C63.4, prEN 50147, and IEC 1000-4-3 testing requirements. Lindgren RF Enclosures, (630) 307-7200.
Monitor Enclosure Eliminates
Magnetic-Field Disturbances
The Image Guard™ II Enclosure eliminates monitor-screen disturbances caused by magnetic fields from power lines, medical equipment, and computers. It prevents smeared colors or jitter in the image. The shield uses CO-NETIC AA, a magnetic shielding alloy. Enclosure finishes include metal, wood veneer, or Formica. Magnetic Shield, (630) 766-7800.
Enclosure Top/Sidewalls
Made of One Piece
The ES 5000 and PS 4000 EMC Enclosures are made of sheet steel with zinc/aluminum surfaces. Continuous bonding between the housing and flat enclosure components is maintained via a combination of EMC/NEMA gasketing. A zinc-galvanized mounting panel provides a large surface for equipotential bonding of interference sources. Enclosure roof and sidewalls are made of one piece. Rittal, (800) 477-4000.
Cabinet Is Used
For ETSI Applications
The ETRAK VE-7900 Networking Cabinet is available with accessories to accommodate European telecommunications as well as EMC requirements. The cabinet offers inter-rack cabling that installs without threading. Doors and side panels have a quick-release design. Accessories include vented panels, ducting trays, and monitored fan trays. The cabinets are supplied fully assembled and configured. Vero Electronics, (800) 642-VERO.
Copyright 1997 Nelson Publishing Inc.
September 1997