IEEE 1302-2008 pdf free.IEEE Guide for the Electromagnetic Characterization of Conductive Gaskets in the Frequency Range of DC to 18 GHz.
The ideal electromagnetic shield is an infinitely conductive enclosure with no apertures or penetrations of any kind. Functional requirements and praciicalities of design and construction prevent this ideal from being realized. Penetrations for power. signals. and ventilation must be provided Access apertures for calibrations, controls, and adjustments must exist. The different pieces of chassis and enclosure must be joined for the final product.
Electromagnetic energy exits or enters the shield at apertures, along conductive penetrations, and through imperfect seams. To restrict this coupling of energy to levels sufficiently low to comply with regulations and to permit interference-free operation, these unwanted coupling paths must be closed, Filters are used on the 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 methods for making joints and seams. Many situations arisc. however, where these techniques cannot be used and direct metal-to-metal contact does not provide an adequate electromagnetic seal. In these cases, an electromagnetic interference (EMI) gasket should be installed in the joint.
EMI gaskets are conductive materials designed to conform to joint surfaces and provide a low-impedance path. EMT 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 spring fingers. spiraled bands, perforated sheets, knitted wire mesh, conductive fabric, reinforced foil, and oriented wires. 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 generic industrial definition of a gasket. The electromagnetic fields being shielded impinge on the conductive materials of the enclosure. The incident tield induces currents in the enclosure walls. Seams represent discontinuities in shield current paths with resulting voltage differentials across the scams. The purpose of the EMI gasket is to reduce the voltage differential across the seam 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 of electromagnetic environments (EMEs) in tenns of both intensity and frequency. The environments can vary from that of the home to the battlefield. Since there is no “one size/type fits all” gasket. the challenge facing equipment designers is that of choosing the most efficient and cost-ctTective gasket for their particular application.
An essential parameter in this selection process is the degree to which the gasket prevents electromagnetic energy impinging on one side of the metal joint containing the gasket from coupling through the joint to the other side (i.e., the gasket’s electromagnetic shielding capability). Many factors determine the electromagnetic seal provided by an EMI gasket, including the following:
a) Gasket material
b) Gasket construction and geometry
c) Geometry of the joint
d) Condition of the contact surfaces
e) Method of fastening
f) Closure pressure
g) Nature of the impinging field
The ideal EMI gasket measurement technique would reveal the full range of effects caused by mounting surface variations, aging. and fasteners and would provide results that indicate the behavior expected from the gasket as installed. Currently, no single measurement technique does this over the frequency range of direct current (dc) to IX GHz.IEEE 1302 pdf download.