IEEE 1302:2008 pdf free download – IEEE Guide for the Electromagnetic Characterization of Conductive Gaskets in the Frequency Range of DC to 18 GHz
1.1 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 frombeing realized.Penetrations for power,signals，and ventilation must be provided.Access apertures forcalibrations,controls, and adjustments must exist. The different pieces of chassis and enclosure must bejoined for the final product.
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 must 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 preferredmethods for making joints and seams.Many situations arisec, however,where these techniques cannot beused and direct metal-to-metal contact does not provide an adequate electromagnetic seal. In these cases, anelectromagnetic interference (EMI) gasket should be installed in the joint.
EMl gaskets are conductive materials designed to conform to joint surfaces and provide a low-impedancepath.EMl gaskets are made from a wide variety of materials: beryllium copper, galvanized steel, stainless steel，electroplated steel，aluminum, and conductively loaded polymers.Gasket types include springfingers, spiraled bands, perforated sheets, knitted wire mesh, conductive fabric, reinforced foil, and orientedwires. Materials added to polymeric binders to achieve conductivity include copper，silver，carbon,aluminum, and nickel as flakes, powders,wires, and coated spheres. The shapes available include sheets,strips, washers,tubes,and customized geometries.
The term“EMI gasket” is consistent with the gencric 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 resultingvoltage differentials across the seams. The purpose of the EMI gasket is to reduce the voltage differentialacross the seam because the strength of the field emanating from the seam is directly proportional to thisvoltage.
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. Since there is no “one size/type fits al” gasket, the challenge facingequipment designers is that of choosing the most efficient and cost-effective gasket for their particularapplication.
4.Factors affecting gasket performance
Figure 1 illustrates a typical application where the EMI gasket is placed underneath the lip of a cover toclose the gap between it and the base enclosure.Other similar applications are under connector shells,waveguide flanges, filter cans and meter flanges and in seams. The objective of the gasket is to provide anelectrical path across the gap in the shield such that the impedance of the path through the gasketapproaches that of a comparable span of the base shield.The impedance of the gasket is a function of itsmaterials and construction, its geometry, and the interface between it and the shield.