IEEE 1302:2019 pdf free download – IEEE Guide for the ElectromagneticCharacterization of Conductive Gasketsin the Frequency Range of DC to 40 GHz
1.4 Background
The ideal electromagnetic shield is an infinitely conductive enclosure with no apertures or penetrations ofany kind.Functional requirements and practicalities of design and construction prevent this ideal from beingrealized.Penetrations for power, signals, and ventilation as well as access apertures for calibrations, controls,and adjustments could be incorporated into an enclosure preventing it from being an ideal enclosure.Anenclosure should include all of the intended design features when evaluating it for its shielding effectiveness.
Electromagnetic energy exits or enters the shield at apertures,along conductive penetrations, and throughimperfect seams. To restrict this coupling of energy to levels sufficiently low to comply with regulationsand to permit interference-free operation, these unwanted coupling paths shall be closed.Filters are used onthe penetrations; screens and covers may be used over apertures. Seams and joints require special attention,however.For shielding,metal flow processes such as welding, brazing,and soldering are the preferred methodsfor making joints and seams.Many situations arise, however, where these techniques cannot be used and directmetal-to-metal contact does not provide an adequate electromagnetic seal. In these cases, an electromagneticinterference (EMI) gasket should be installed in the joint.
EMI gaskets are conductive materials designed to conform to joint surfaces and provide a low-impedancepath. EMIl gaskets are made from a wide variety of materials: beryllium copper, galvanized steel, stainlesssteel, electroplated steel, aluminum, and conductively loaded polymers.
Gasket types include spring fingers, spiraled bands, perforated sheets, knitted wire mesh, conductive fabric,reinforced foil, and oriented wires.Materials added to polymeric binders to achieve conductivity includecopper，silver,carbon, aluminum, and nickel as flakes， powders，wires, and coated spheres.The shapesavailable include sheets, strips, washers, tubes, and customized geometries.
The term“EMl gasket” is consistent with the generic industrial definition of a gasket. The electromagneticfields being shielded impinge on the conductive materials of the enclosure. The incident field inducescurrents in the enclosure walls. Seams represent discontinuities in shield current paths with resulting voltagedifferentials across the seams. The purpose of the EMl gasket is to reduce the voltage differential across theseam because the strength of the field emanating from the seam is directly proportional to this voltage.
Depending upon function and application, electronic equipment operates in an extremely wide range ofelectromagnetic environments (EMEs) in terms of both intensity and frequency.The environments can varyfrom that of the home to the battlefield.Because there is no“one size/type fits all”gasket, the challengefacing equipment designers is that of choosing the most efficient and cost-effective gasket for their particularapplication.
An essential parameter in this selection process is the degree to which the gasket prevents electromagneticenergy impinging on one side of the metal joint containing the gasket from coupling through the joint to theother side(i.e. the gasket ‘s electromagnetic shielding capability).Many factors determine the electromagneticseal provided by an EMl gasket, including the following:
4. Factors affecting gasket performance
Figure 1 illustrates a typical application where the EMl gasket is placed underneath the lip of a cover to closethe gap between it and the base enclosure.Other similar applications are under connector shells, waveguideflanges, filter cans and meter flanges, and in seams. The objective of the gasket is to provide an electricalpath across the gap in the shicld such that the impedance of the path through the gasket approaches that of acomparable span of the base shield. The impedance of the gasket is a function of its materials and construction,its gcometry, and the interface between it and the shicld.
All gasket matcrials have resistive,inductive, and capacitive propertics that may exhibit themselves overdifferent portions of the frequency band. The property，which determines the gasket’s predominantcharactcristic, depends on the matcrials and construction of the gasket as well as the geometry of the joint.