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A NOVEL METHOD TO MEASURE THE PROPERTIES OF RF ABSORBING MATERIALS

IP.com Disclosure Number: IPCOM000009484D
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2002-Aug-28
Document File: 4 page(s) / 223K

Publishing Venue

Motorola

Related People

Andrew T. Gobien: AUTHOR [+2]

Abstract

The ideal testing environment for RF equipment is non-reflective, non-reverberant, and provides suf- ficient isolation from the ambient RF environment. A conducting enclosure provides a very high degree of isolation from the environment but is, by defini- tion, a reflective and reverberant environment because the reflection coefficient at the boundary is near one. Therefore, for ideal RF testing, some means is necessary to reduce the reflection coeffi- cient at the boundary of a conducting enclosure while maintaining a given level of attenuation from outside fields.

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MO-LA Technical Developments

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A NOVEL METHOD TO MEASURE THE PROPERTIES OF RF ABSORBING MATERIALS

by Andrew i. Gobien and Robert B. Schneider

INTRODUCTION

  The ideal testing environment for RF equipment is non-reflective, non-reverberant, and provides suf- ficient isolation from the ambient RF environment. A conducting enclosure provides a very high degree of isolation from the environment but is, by defini- tion, a reflective and reverberant environment because the reflection coefficient at the boundary is near one. Therefore, for ideal RF testing, some means is necessary to reduce the reflection coeffi- cient at the boundary of a conducting enclosure while maintaining a given level of attenuation from outside fields.

  An RF absorbing material at the boundary of the test enclosure reduces the reflection coefficient by absorbing the power that would be reflected. The most commonly used RF absorbing material is car- bon impregnated foam. Although carbon impreg- nated foam is proven in practice, its absorption coef- ficient (which describes the energy absorbed by the material compared to a flat, conducting plate) is strongly dependent on thickness in proportion to operating wavelength. Even at 900 MHz, 3 or 4 inches of material is required to provide 20dB of absorption. At VHF and UHF the required thick- ness is greater than one foot.

  Practical RF test enclosures require sufficient space for fixtures, electronics packages, and large Devices Under Test (DUTs) within relatively com- pact enclosures. Therefore, carbon impregnated foam is not the best material for use in test fixtures, and thinner absorbing materials are preferred.

  A large variety of thin absorbing materials are available as radar absorbers in the microwave and millimeter wave frequency range above 2 GHz. Due to limitations on measurement procedures. the absorption coefficients of these materials are seldom specified below 1GHz. However, many commercial

test applications require specification of RF absorp- tion levels into the VHF region. To date, materials have been tested in an empirical hit-or-miss fashion. This report provides a novel and highly intuitive method for testing RF materials at low frequencies in commercial test enclosures. In Section 2, the Physics of the problem are derived. Section 3 dis- cusses the methodology used to apply the technique in practice. Concluding Remarks are found in Section 4.

DERIVATION OF THEORY'

  When a constant amplitude RF source is operat- ed in an enclosure, the resulting steady-state energy level depends on the energy supplied by the source and the absorption at the boundary of the enclosure. If the dimensions of the enclosure are large, absorp- tion may also occur in the medium. This discussion assumes a small enclosure so absorption in the medium is neglected. Consider an enclosure with highly absorbing boundaries.

  The received power at any point in the enclosure is dominated by the direct path. However, if the boundaries of th...