Controlled Measurements of Shielding Effectiveness of Materials, Conducting Gaskets and Contact Strips

A special fixture has been developed which enables carefully controlled measurements of shielding effectiveness to be made on materials (two-dimensional), conducting gaskets, shielding O-rings and caulking compounds, and peripheral contact finger-strips. Special features of this fixture are described. The method is basically that of MIL-STD-285, with variations. The authors present a justification, on theoretical and practical grounds, for this experimental method, which is based fundamentally on thorough physical exploration, with frequency and field as fixed parameters. The importance of a controlled fixture and experimental setup in enabling the relative measurements of shielding effectiveness of a group of samples is discussed, together with the additional requirements for absolute measurement of any one sample. In the latter connection, several definitions are possible, for example, of the shielding effectiveness of a conducting gasket, and it is of utmost importance that agreement be established among manufacturers on definitions of basic terms, so that comparative analyses of statements of experimentally measured shielding effectiveness can be intelligently made by the engineering community and, especially, the potential user of the products being measured. This point is discussed at length in the paper, and arguments advanced pro and con the various possible definitions. Typical results of fairly extensive completed programs of shielding measurements involving varieties of test samples are presented, to demonstrate the use of the fixture and the methodology employed, and the orders of magnitudes of results obtained.