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Absorber for Electromagnetic Waves

IP.com Disclosure Number: IPCOM000061418D
Original Publication Date: 1986-Jun-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Stanley, CB: AUTHOR

Abstract

A method and means are described for providing a shielded anechoic chamber having wave-propagation characteristics approaching those of free space without its ambient electromagneti noise.

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Absorber for Electromagnetic Waves

A method and means are described for providing a shielded anechoic chamber having wave-propagation characteristics approaching those of free space without its ambient electromagneti noise.

Shielded "anechoic" chambers are widely used to provide isolated environments for tests involving electromagnetic waves. To minimize reflections, with consequent formation of standing waves, the conductive walls and ceiling of chamber shielding enclosures are lined with lossy dielectric media, usually in the form of pyramids or wedges, intended to absorb incident-wave power. A requirement for complete absorption is that the impedance of the absorbers match the intrinsic impedance of the propagating medium (air).

The intrinsic impedance is equal to the square root of u/e, where u is the permeability and e is the permitivity (dielectric constant) of the medium. The wave impedance is equal to the ratio of the total electric field to the total magnetic field. The impedance of a plane wave in free space is 120 pi (377 ohms). To satisfy the impedance-matching requirement, and thus ensure minimum reflections, the ratio of absorber permeability to permitivity must be controlled to mimic the intrinsic impedance of free space.

Presently, absorbers are made from dielectric foam, typically polyurethane, impregnated with carbon to decrease its vol resistivity. Nothing is done to alter its permeability. These absorbers selectively attenuate the electric fie...